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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: January 6, 2020 Apple Inc. 6 July 5, 2019 8 DNS Push Notifications 9 draft-ietf-dnssd-push-21 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 January 6, 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 2. Motivation . . . . . . . . . . . . . . . . . . . . . . . . . 4 59 3. Overview . . . . . . . . . . . . . . . . . . . . . . . . . . 5 60 4. Transport . . . . . . . . . . . . . . . . . . . . . . . . . . 6 61 5. State Considerations . . . . . . . . . . . . . . . . . . . . 7 62 6. Protocol Operation . . . . . . . . . . . . . . . . . . . . . 8 63 6.1. Discovery . . . . . . . . . . . . . . . . . . . . . . . . 9 64 6.2. DNS Push Notification SUBSCRIBE . . . . . . . . . . . . . 13 65 6.2.1. SUBSCRIBE Request . . . . . . . . . . . . . . . . . . 13 66 6.2.2. SUBSCRIBE Response . . . . . . . . . . . . . . . . . 16 67 6.3. DNS Push Notification Updates . . . . . . . . . . . . . . 20 68 6.3.1. PUSH Message . . . . . . . . . . . . . . . . . . . . 20 69 6.4. DNS Push Notification UNSUBSCRIBE . . . . . . . . . . . . 25 70 6.4.1. UNSUBSCRIBE Message . . . . . . . . . . . . . . . . . 25 71 6.5. DNS Push Notification RECONFIRM . . . . . . . . . . . . . 27 72 6.5.1. RECONFIRM Message . . . . . . . . . . . . . . . . . . 28 73 6.6. DNS Stateful Operations TLV Context Summary . . . . . . . 30 74 6.7. Client-Initiated Termination . . . . . . . . . . . . . . 31 75 7. Security Considerations . . . . . . . . . . . . . . . . . . . 31 76 7.1. Security Services . . . . . . . . . . . . . . . . . . . . 32 77 7.2. TLS Name Authentication . . . . . . . . . . . . . . . . . 33 78 7.3. TLS Session Resumption . . . . . . . . . . . . . . . . . 33 79 8. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 34 80 9. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . 34 81 10. References . . . . . . . . . . . . . . . . . . . . . . . . . 35 82 10.1. Normative References . . . . . . . . . . . . . . . . . . 35 83 10.2. Informative References . . . . . . . . . . . . . . . . . 36 84 Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 39 86 1. Introduction 88 Domain Name System (DNS) records may be updated using DNS Update 89 [RFC2136]. Other mechanisms such as a Discovery Proxy [DisProx] can 90 also generate changes to a DNS zone. This document specifies a 91 protocol for DNS clients to subscribe to receive asynchronous 92 notifications of changes to RRSets of interest. It is immediately 93 relevant in the case of DNS Service Discovery [RFC6763] but is not 94 limited to that use case, and provides a general DNS mechanism for 95 DNS record change notifications. Familiarity with the DNS protocol 96 and DNS packet formats is assumed [RFC1034] [RFC1035] [RFC6895]. 98 1.1. Requirements Language 100 The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", 101 "SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and 102 "OPTIONAL" in this document are to be interpreted as described in 103 BCP 14 [RFC2119] [RFC8174] when, and only when, they appear in all 104 capitals, as shown here. These words may also appear in this 105 document in lower case as plain English words, absent their normative 106 meanings. 108 2. Motivation 110 As the domain name system continues to adapt to new uses and changes 111 in deployment, polling has the potential to burden DNS servers at 112 many levels throughout the network. Other network protocols have 113 successfully deployed a publish/subscribe model following the 114 Observer design pattern [obs]. XMPP Publish-Subscribe [XEP0060] and 115 Atom [RFC4287] are examples. While DNS servers are generally highly 116 tuned and capable of a high rate of query/response traffic, adding a 117 publish/subscribe model for tracking changes to DNS records can 118 deliver more timely notification of changes with reduced CPU usage 119 and lower network traffic. 121 Multicast DNS [RFC6762] implementations always listen on a well known 122 link-local IP multicast group, and record changes are sent to that 123 multicast group address for all group members to receive. Therefore, 124 Multicast DNS already has asynchronous change notification 125 capability. However, when DNS Service Discovery [RFC6763] is used 126 across a wide area network using Unicast DNS (possibly facilitated 127 via a Discovery Proxy [DisProx]) it would be beneficial to have an 128 equivalent capability for Unicast DNS, to allow clients to learn 129 about DNS record changes in a timely manner without polling. 131 The DNS Long-Lived Queries (LLQ) mechanism [LLQ] is an existing 132 deployed solution to provide asynchronous change notifications, used 133 by Apple's Back to My Mac [RFC6281] service introduced in Mac OS X 134 10.5 Leopard in 2007. Back to My Mac was designed in an era when the 135 data center operations staff asserted that it was impossible for a 136 server to handle large numbers of mostly-idle TCP connections, so LLQ 137 was defined as a UDP-based protocol, effectively replicating much of 138 TCP's connection state management logic in user space, and creating 139 its own poor imitations of existing TCP features like the three-way 140 handshake, flow control, and reliability. 142 This document builds on experience gained with the LLQ protocol, with 143 an improved design. Instead of using UDP, this specification uses 144 DNS Stateful Operations (DSO) [RFC8490] running over TLS over TCP, 145 and therefore doesn't need to reinvent existing TCP functionality. 146 Using TCP also gives long-lived low-traffic connections better 147 longevity through NAT gateways without depending on the gateway to 148 support NAT Port Mapping Protocol (NAT-PMP) [RFC6886] or Port Control 149 Protocol (PCP) [RFC6887], or resorting to excessive keepalive 150 traffic. 152 3. Overview 154 A DNS Push Notification client subscribes for Push Notifications for 155 a particular RRSet by connecting to the appropriate Push Notification 156 server for that RRSet, and sending DSO message(s) indicating the 157 RRSet(s) of interest. When the client loses interest in receiving 158 further updates to these records, it unsubscribes. 160 The DNS Push Notification server for a DNS zone is any server capable 161 of generating the correct change notifications for a name. It may be 162 a primary, secondary, or stealth name server [RFC7719]. 163 Consequently, the "_dns-push._tcp." SRV record for a zone MAY 164 reference the same target host and port as that zone's 165 "_dns-update._tcp." SRV record. When the same target host and 166 port is offered for both DNS Updates and DNS Push Notifications, a 167 client MAY use a single TCP connection to that server for both DNS 168 Updates and DNS Push Notification Subscriptions. 170 Supporting DNS Updates and DNS Push Notifications on the same server 171 is OPTIONAL. A DNS Push Notification server is not required to 172 support DNS Update. 174 DNS Updates and DNS Push Notifications may be handled on different 175 ports on the same target host, in which case they are not considered 176 to be the "same server" for the purposes of this specification, and 177 communications with these two ports are handled independently. 179 Standard DNS Queries MAY be sent over a DNS Push Notification (i.e., 180 DSO) session. For any zone for which the server is authoritative, it 181 MUST respond authoritatively for queries on names falling within that 182 zone (e.g., the in the "_dns-push._tcp." SRV record) 183 both for normal DNS queries and for DNS Push Notification 184 subscriptions. For names for which the server is acting as a 185 recursive resolver, e.g. when the server is the local recursive 186 resolver, for any query for which it supports DNS Push Notification 187 subscriptions, it MUST also support standard queries. 189 DNS Push Notifications impose less load on the responding server than 190 rapid polling would, but Push Notifications do still have a cost, so 191 DNS Push Notification clients MUST NOT recklessly create an excessive 192 number of Push Notification subscriptions. Specifically: 194 (a) A subscription should only be active when there is a valid reason 195 to need live data (for example, an on-screen display is currently 196 showing the results to the user) and the subscription SHOULD be 197 cancelled as soon as the need for that data ends (for example, when 198 the user dismisses that display). In the case of a device like a 199 smartphone which, after some period of inactivity, goes to sleep or 200 otherwise darkens its screen, it should cancel its subscriptions when 201 darkening the screen (since the user cannot see any changes in the 202 display anyway) and reinstate its subscriptions when re-awakening 203 from display sleep. 205 (b) A DNS Push Notification client SHOULD NOT routinely keep a DNS 206 Push Notification subscription active 24 hours a day, 7 days a week, 207 just to keep a list in memory up to date so that if the user does 208 choose to bring up an on-screen display of that data, it can be 209 displayed really fast. DNS Push Notifications are designed to be 210 fast enough that there is no need to pre-load a "warm" list in memory 211 just in case it might be needed later. 213 Generally, as described in the DNS Stateful Operations specification 214 [RFC8490], a client must not keep a session to a server open 215 indefinitely if it has no subscriptions (or other operations) active 216 on that session. A client MAY close a session as soon as it becomes 217 idle, and then if needed in the future, open a new session when 218 required. Alternatively, a client MAY speculatively keep an idle 219 session open for some time, subject to the constraint that it MUST 220 NOT keep a session open that has been idle for more than the 221 session's idle timeout (15 seconds by default) [RFC8490]. 223 4. Transport 225 Other DNS operations like DNS Update [RFC2136] MAY use either User 226 Datagram Protocol (UDP) [RFC0768] or Transmission Control Protocol 227 (TCP) [RFC0793] as the transport protocol, in keeping with the 228 historical precedent that DNS queries must first be sent over UDP 229 [RFC1123]. This requirement to use UDP has subsequently been relaxed 230 [RFC7766]. 232 In keeping with the more recent precedent, DNS Push Notification is 233 defined only for TCP. DNS Push Notification clients MUST use DNS 234 Stateful Operations [RFC8490] running over TLS over TCP [RFC7858]. 236 Connection setup over TCP ensures return reachability and alleviates 237 concerns of state overload at the server which is a potential problem 238 with connection-less protocols using spoofed source addresses. All 239 subscribers are guaranteed to be reachable by the server by virtue of 240 the TCP three-way handshake. Flooding attacks are possible with any 241 protocol, and a benefit of TCP is that there are already established 242 industry best practices to guard against SYN flooding and similar 243 attacks [SYN] [RFC4953]. 245 Use of TCP also allows DNS Push Notifications to take advantage of 246 current and future developments in TCP, such as Multipath TCP (MPTCP) 248 [RFC6824], TCP Fast Open (TFO) [RFC7413], Tail Loss Probe (TLP) 249 [I-D.dukkipati-tcpm-tcp-loss-probe], and so on. 251 Transport Layer Security (TLS) [RFC8446] is well understood and 252 deployed across many protocols running over TCP. It is designed to 253 prevent eavesdropping, tampering, and message forgery. TLS is 254 REQUIRED for every connection between a client subscriber and server 255 in this protocol specification. Additional security measures such as 256 client authentication during TLS negotiation MAY also be employed to 257 increase the trust relationship between client and server. 259 5. State Considerations 261 Each DNS Push Notification server is capable of handling some finite 262 number of Push Notification subscriptions. This number will vary 263 from server to server and is based on physical machine 264 characteristics, network bandwidth, and operating system resource 265 allocation. After a client establishes a session to a DNS server, 266 each subscription is individually accepted or rejected. Servers may 267 employ various techniques to limit subscriptions to a manageable 268 level. Correspondingly, the client is free to establish simultaneous 269 sessions to alternate DNS servers that support DNS Push Notifications 270 for the zone and distribute subscriptions at the client's discretion. 271 In this way, both clients and servers can react to resource 272 constraints. 274 6. Protocol Operation 276 The DNS Push Notification protocol is a session-oriented protocol, 277 and makes use of DNS Stateful Operations (DSO) [RFC8490]. 279 For details of the DSO message format refer to the DNS Stateful Oper- 280 ations specification [RFC8490]. Those details are not repeated here. 282 DNS Push Notification clients and servers MUST support DSO. A single 283 server can support DNS Queries, DNS Updates, and DNS Push 284 Notifications (using DSO) on the same TCP port. 286 A DNS Push Notification exchange begins with the client discovering 287 the appropriate server, using the procedure described in Section 6.1, 288 and then making a TLS/TCP connection to it. 290 A typical DNS Push Notification client will immediately issue a DSO 291 Keepalive operation to request a session timeout and/or keepalive 292 interval longer than the the 15-second default values, but this is 293 not required. A DNS Push Notification client MAY issue other 294 requests on the session first, and only issue a DSO Keepalive 295 operation later if it determines that to be necessary. Sending 296 either a DSO Keepalive operation or a Push Notification subscription 297 over the TLS/TCP connection to the server signals the client's 298 support of DSO and serves to establish a DSO session. 300 In accordance with the current set of active subscriptions, the 301 server sends relevant asynchronous Push Notifications to the client. 302 Note that a client MUST be prepared to receive (and silently ignore) 303 Push Notifications for subscriptions it has previously removed, since 304 there is no way to prevent the situation where a Push Notification is 305 in flight from server to client while the client's UNSUBSCRIBE 306 message cancelling that subscription is simultaneously in flight from 307 client to server. 309 6.1. Discovery 311 The first step in a DNS Push Notification subscription is to discover 312 an appropriate DNS server that supports DNS Push Notifications for 313 the desired zone. 315 The client begins by opening a DSO Session to its normal configured 316 DNS recursive resolver and requesting a Push Notification 317 subscription. This connection is made to TCP port 853, the default 318 port for DNS-over-TLS [RFC7858]. If the request for a Push 319 Notification subscription is successful, then the recursive resolver 320 will make a corresponding Push Notification subscription on the 321 client's behalf (if the recursive resolver doesn't already have an 322 active subscription for that name, type, and class). This is closely 323 analogous to how a client sends normal DNS queries to its configured 324 DNS recursive resolver, which issues queries on the client's behalf 325 (if the recursive resolver doesn't already have appropriate answer(s) 326 in its cache). 328 In many contexts, the recursive resolver will be able to handle Push 329 Notifications for all names that the client may need to follow. Use 330 of VPN tunnels and split-view DNS can create some additional 331 complexity in the client software here; the techniques to handle VPN 332 tunnels and split-view DNS for DNS Push Notifications are the same as 333 those already used to handle this for normal DNS queries. 335 If the recursive resolver does not support DNS over TLS, or does 336 support DNS over TLS but is not listening on TCP port 853, or does 337 support DNS over TLS on TCP port 853 but does not support DSO on that 338 port, then the DSO Session session establishment will fail [RFC8490]. 340 If the recursive resolver does support DSO but not Push Notification 341 subscriptions, then it will return the DSO error code, DSOTYPENI 342 (11). 344 In some cases, the recursive resolver may support DSO and Push 345 Notification subscriptions, but may not be able to subscribe for Push 346 Notifications for a particular name. In this case, the recursive 347 resolver should return SERVFAIL to the client. This includes being 348 unable to establish a connection to the zone's DNS Push Notification 349 server or establishing a connection but receiving a non success 350 response code. In some cases, where the client has a pre-established 351 trust relationship with the owner of the zone (that is not handled 352 via the usual mechanisms for VPN software) the client may handle 353 these failures by contacting the zone's DNS Push server directly. 355 In any of the cases described above where the client fails to 356 establish a DNS Push Notification subscription via its configured 357 recursive resolver, the client should proceed to discover the 358 appropriate server for direct communication. The client MUST also 359 determine which TCP port on the server is listening for connections, 360 which need not be (and often is not) the typical TCP port 53 used for 361 conventional DNS, or TCP port 853 used for DNS over TLS. 363 The discovery algorithm described here is an iterative algorithm, 364 which starts with the full name of the record to which the client 365 wishes to subscribe. Successive SOA queries are then issued, 366 trimming one label each time, until the closest enclosing 367 authoritative server is discovered. There is also an optimization to 368 enable the client to take a "short cut" directly to the SOA record of 369 the closest enclosing authoritative server in many cases. 371 1. The client begins the discovery by sending a DNS query to its 372 local resolver, with record type SOA [RFC1035] for the record 373 name to which it wishes to subscribe. As an example, suppose the 374 client wishes to subscribe to PTR records with the name 375 _ipp._tcp.foo.example.com (to discover Internet Printing Protocol 376 (IPP) printers [RFC8010] [RFC8011] being advertised at 377 "foo.example.com"). The client begins by sending an SOA query 378 for _ipp._tcp.foo.example.com to the local recursive resolver. 379 The goal is to determine the server authoritative for the name 380 _ipp._tcp.foo.example.com. The closest enclosing DNS zone 381 containing the name _ipp._tcp.foo.example.com could be 382 example.com, or foo.example.com, or _tcp.foo.example.com, or even 383 _ipp._tcp.foo.example.com. The client does not know in advance 384 where the closest enclosing zone cut occurs, which is why it uses 385 the iterative procedure described here to discover this 386 information. 388 2. If the requested SOA record exists, it will be returned in the 389 Answer section with a NOERROR response code, and the client has 390 succeeded in discovering the information it needs. 391 (This language is not placing any new requirements on DNS 392 recursive resolvers. This text merely describes the existing 393 operation of the DNS protocol [RFC1034] [RFC1035].) 395 3. If the requested SOA record does not exist, the client will get 396 back a NOERROR/NODATA response or an NXDOMAIN/Name Error 397 response. In either case, the local resolver would normally 398 include the SOA record for the closest enclosing zone of the 399 requested name in the Authority Section. If the SOA record is 400 received in the Authority Section, then the client has succeeded 401 in discovering the information it needs. 402 (This language is not placing any new requirements on DNS 403 recursive resolvers. This text merely describes the existing 404 operation of the DNS protocol regarding negative responses 405 [RFC2308].) 407 4. If the client receives a response containing no SOA record, then 408 it proceeds with the iterative approach. The client strips the 409 leading label from the current query name and if the resulting 410 name has at least one label in it, the client sends an SOA query 411 for that new name, and processing continues at step 2 above, 412 repeating the iterative search until either an SOA is received, 413 or the query name consists of a single label, i.e., a Top Level 414 Domain (TLD). In the case of a single-label TLD, this is a 415 network configuration error which should not happen and the 416 client gives up. The client may retry the operation at a later 417 time, of the client's choosing, such after a change in network 418 attachment. 420 5. Once the SOA is known (either by virtue of being seen in the 421 Answer Section, or in the Authority Section), the client sends a 422 DNS query with type SRV [RFC2782] for the record name 423 "_dns-push._tcp.", where is the owner name of the 424 discovered SOA record. 426 6. If the zone in question is set up to offer DNS Push Notifications 427 then this SRV record MUST exist. (If this SRV record does not 428 exist then the zone is not correctly configured for DNS Push 429 Notifications as specified in this document.) The SRV "target" 430 contains the name of the server providing DNS Push Notifications 431 for the zone. The port number on which to contact the server is 432 in the SRV record "port" field. The address(es) of the target 433 host MAY be included in the Additional Section, however, the 434 address records SHOULD be authenticated before use as described 435 below in Section 7.2 and in the specification for using DANE TLSA 436 Records with SRV Records [RFC7673], if applicable. 438 7. More than one SRV record may be returned. In this case, the 439 "priority" and "weight" values in the returned SRV records are 440 used to determine the order in which to contact the servers for 441 subscription requests. As described in the SRV specification 442 [RFC2782], the server with the lowest "priority" is first 443 contacted. If more than one server has the same "priority", the 444 "weight" indicates the weighted probability that the client 445 should contact that server. Higher weights have higher 446 probabilities of being selected. If a server is not willing to 447 accept a subscription request, or is not reachable within a 448 reasonable time, as determined by the client, then a subsequent 449 server is to be contacted. 451 Each time a client makes a new DNS Push Notification subscription 452 session, it SHOULD repeat the discovery process in order to determine 453 the preferred DNS server for subscriptions at that time. However, 454 the client device MUST respect the DNS TTL values on records it 455 receives, and store them in its local cache with this lifetime. This 456 means that, as long as the DNS TTL values on the authoritative 457 records were set to reasonable values, repeated application of this 458 discovery process can be completed nearly instantaneously by the 459 client, using only locally-stored cached data. 461 6.2. DNS Push Notification SUBSCRIBE 463 After connecting, and requesting a longer idle timeout and/or 464 keepalive interval if necessary, a DNS Push Notification client 465 then indicates its desire to receive DNS Push Notifications for 466 a given domain name by sending a SUBSCRIBE request to the server. 467 A SUBSCRIBE request is encoded in a DSO message [RFC8490]. 468 This specification defines a primary DSO TLV for DNS Push 469 Notification SUBSCRIBE Requests (tentatively DSO Type Code 0x40). 471 DSO messages with the SUBSCRIBE TLV as the Primary TLV are not 472 permitted in early data. 474 The entity that initiates a SUBSCRIBE request is by definition the 475 client. A server MUST NOT send a SUBSCRIBE request over an existing 476 session from a client. If a server does send a SUBSCRIBE request 477 over a DSO session initiated by a client, this is a fatal error and 478 the client should immediately abort the connection with a TLS 479 close_notify alert. See Section 6.1 of [RFC8446]. 481 6.2.1. SUBSCRIBE Request 483 A SUBSCRIBE request begins with the standard DSO 12-byte header 484 [RFC8490], followed by the SUBSCRIBE primary TLV. A SUBSCRIBE 485 request message is illustrated in Figure 1. 487 The MESSAGE ID field MUST be set to a unique value, that the client 488 is not using for any other active operation on this DSO session. For 489 the purposes here, a MESSAGE ID is in use on this session if the 490 client has used it in a request for which it has not yet received a 491 response, or if the client has used it for a subscription which it 492 has not yet cancelled using UNSUBSCRIBE. In the SUBSCRIBE response 493 the server MUST echo back the MESSAGE ID value unchanged. 495 The other header fields MUST be set as described in the DSO spec- 496 ification [RFC8490]. The DNS OPCODE field contains the OPCODE value 497 for DNS Stateful Operations (6). The four count fields MUST be zero, 498 and the corresponding four sections MUST be empty (i.e., absent). 500 The DSO-TYPE is SUBSCRIBE (tentatively 0x40). 502 The DSO-LENGTH is the length of the DSO-DATA that follows, which 503 specifies the name, type, and class of the record(s) being sought. 505 1 1 1 1 1 1 506 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 507 +--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+ \ 508 | MESSAGE ID | \ 509 +--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+ | 510 |QR| OPCODE(6) | Z | RCODE | | 511 +--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+ | 512 | QDCOUNT (MUST BE ZERO) | | 513 +--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+ > HEADER 514 | ANCOUNT (MUST BE ZERO) | | 515 +--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+ | 516 | NSCOUNT (MUST BE ZERO) | | 517 +--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+ | 518 | ARCOUNT (MUST BE ZERO) | / 519 +--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+ / 520 | DSO-TYPE = SUBSCRIBE (tentatively 0x40) | 521 +--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+ 522 | DSO-LENGTH (number of octets in DSO-DATA) | 523 +--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+ \ 524 | | \ 525 \ NAME \ | 526 \ \ | 527 +--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+ > DSO-DATA 528 | TYPE | | 529 +--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+ | 530 | CLASS | / 531 +--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+ / 533 Figure 1: SUBSCRIBE Request 535 The DSO-DATA for a SUBSCRIBE request MUST contain exactly one NAME, 536 TYPE, and CLASS. Since SUBSCRIBE requests are sent over TCP, 537 multiple SUBSCRIBE DSO request messages can be concatenated in a 538 single TCP stream and packed efficiently into TCP segments. 540 If accepted, the subscription will stay in effect until the client 541 cancels the subscription using UNSUBSCRIBE or until the DSO session 542 between the client and the server is closed. 544 SUBSCRIBE requests on a given session MUST be unique. A client MUST 545 NOT send a SUBSCRIBE message that duplicates the NAME, TYPE and CLASS 546 of an existing active subscription on that DSO session. For the 547 purpose of this matching, the established DNS case-insensitivity for 548 US-ASCII letters applies (e.g., "example.com" and "Example.com" are 549 the same). If a server receives such a duplicate SUBSCRIBE message 550 this is an error and the server MUST immediately terminate the 551 connection with a TLS close_notify alert. 553 DNS wildcarding is not supported. That is, a wildcard ("*") in a 554 SUBSCRIBE message matches only a literal wildcard character ("*") in 555 the zone, and nothing else. 557 Aliasing is not supported. That is, a CNAME in a SUBSCRIBE message 558 matches only a literal CNAME record in the zone, and not to any 559 records referenced by the owner name. 561 A client may SUBSCRIBE to records that are unknown to the server at 562 the time of the request (providing that the name falls within one of 563 the zone(s) the server is responsible for) and this is not an error. 564 The server MUST NOT return NXDOMAIN in this case. The server MUST 565 accept these requests and send Push Notifications if and when 566 matching records are found in the future. 568 If neither TYPE nor CLASS are ANY (255) then this is a specific 569 subscription to changes for the given NAME, TYPE and CLASS. If one 570 or both of TYPE or CLASS are ANY (255) then this subscription matches 571 any type and/or any class, as appropriate. 573 NOTE: A little-known quirk of DNS is that in DNS QUERY requests, 574 QTYPE and QCLASS 255 mean "ANY" not "ALL". They indicate that the 575 server should respond with ANY matching records of its choosing, not 576 necessarily ALL matching records. This can lead to some surprising 577 and unexpected results, where a query returns some valid answers but 578 not all of them, and makes QTYPE=ANY queries less useful than people 579 sometimes imagine. 581 When used in conjunction with SUBSCRIBE, TYPE and CLASS 255 should be 582 interpreted to mean "ALL", not "ANY". After accepting a subscription 583 where one or both of TYPE or CLASS are 255, the server MUST send Push 584 Notification Updates for ALL record changes that match the 585 subscription, not just some of them. 587 6.2.2. SUBSCRIBE Response 589 Each SUBSCRIBE request generates exactly one SUBSCRIBE response from 590 the server. 592 A SUBSCRIBE response begins with the standard DSO 12-byte header 593 [RFC8490]. The QR bit in the header is set indicating it is a 594 response. The header MAY be followed by one or more optional TLVs, 595 such as a Retry Delay TLV. 597 The MESSAGE ID field MUST echo the value given in the Message ID 598 field of the SUBSCRIBE request. This is how the client knows which 599 request is being responded to. 601 A SUBSCRIBE response message MUST NOT include a SUBSCRIBE TLV. If a 602 client receives a SUBSCRIBE response message containing a SUBSCRIBE 603 TLV then the response message is processed but the SUBSCRIBE TLV MUST 604 be silently ignored. 606 1 1 1 1 1 1 607 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 608 +--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+ \ 609 | MESSAGE ID | \ 610 +--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+ | 611 |QR| OPCODE(6) | Z | RCODE | | 612 +--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+ | 613 | QDCOUNT (MUST BE ZERO) | | 614 +--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+ > HEADER 615 | ANCOUNT (MUST BE ZERO) | | 616 +--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+ | 617 | NSCOUNT (MUST BE ZERO) | | 618 +--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+ | 619 | ARCOUNT (MUST BE ZERO) | / 620 +--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+ / 622 Figure 2: SUBSCRIBE Response Message 624 In the SUBSCRIBE response the RCODE indicates whether or not the 625 subscription was accepted. Supported RCODEs are as follows: 627 +-----------+-------+-----------------------------------------------+ 628 | Mnemonic | Value | Description | 629 +-----------+-------+-----------------------------------------------+ 630 | NOERROR | 0 | SUBSCRIBE successful. | 631 | FORMERR | 1 | Server failed to process request due to a | 632 | | | malformed request. | 633 | SERVFAIL | 2 | Server failed to process request due to a | 634 | | | problem with the server. | 635 | NOTIMP | 4 | Server does not implement DSO. | 636 | REFUSED | 5 | Server refuses to process request for policy | 637 | | | or security reasons. | 638 | NOTAUTH | 9 | Server is not authoritative for the requested | 639 | | | name. | 640 | DSOTYPENI | 11 | SUBSCRIBE operation not supported. | 641 +-----------+-------+-----------------------------------------------+ 643 Table 1: SUBSCRIBE Response codes 645 This document specifies only these RCODE values for SUBSCRIBE 646 Responses. Servers sending SUBSCRIBE Responses SHOULD use one of 647 these values. Note that NXDOMAIN is not a valid RCODE in response to 648 a SUBSCRIBE Request. However, future circumstances may create 649 situations where other RCODE values are appropriate in SUBSCRIBE 650 Responses, so clients MUST be prepared to accept SUBSCRIBE Responses 651 with any other RCODE value. 653 If the server sends a nonzero RCODE in the SUBSCRIBE response, that 654 means: 656 a. the client is (at least partially) misconfigured, 657 b. the server resources are exhausted, or 658 c. there is some other unknown failure on the server. 660 In any case, the client shouldn't retry the subscription to this 661 server right away. If multiple SRV records were returned as 662 described in Section 6.1, Paragraph 7, a subsequent server can be 663 tried immediately. 665 If the client has other successful subscriptions to this server, 666 these subscriptions remain even though additional subscriptions may 667 be refused. Neither the client nor the server are required to close 668 the connection, although, either end may choose to do so. 670 If the server sends a nonzero RCODE then it SHOULD append a Retry 671 Delay TLV [RFC8490] to the response specifying a delay before the 672 client attempts this operation again. Recommended values for the 673 delay for different RCODE values are given below. These recommended 674 values apply both to the default values a server should place in the 675 Retry Delay TLV, and the default values a client should assume if the 676 server provides no Retry Delay TLV. 678 For RCODE = 1 (FORMERR) the delay may be any value selected by the 679 implementer. A value of five minutes is RECOMMENDED, to reduce 680 the risk of high load from defective clients. 682 For RCODE = 2 (SERVFAIL) the delay should be chosen according to 683 the level of server overload and the anticipated duration of that 684 overload. By default, a value of one minute is RECOMMENDED. If a 685 more serious server failure occurs, the delay may be longer in 686 accordance with the specific problem encountered. 688 For RCODE = 4 (NOTIMP), which occurs on a server that doesn't 689 implement DNS Stateful Operations [RFC8490], it is unlikely that 690 the server will begin supporting DSO in the next few minutes, so 691 the retry delay SHOULD be one hour. Note that in such a case, a 692 server that doesn't implement DSO is unlikely to place a Retry 693 Delay TLV in its response, so this recommended value in particular 694 applies to what a client should assume by default. 696 For RCODE = 5 (REFUSED), which occurs on a server that implements 697 DNS Push Notifications, but is currently configured to disallow 698 DNS Push Notifications, the retry delay may be any value selected 699 by the implementer and/or configured by the operator. 701 If the server being queried is listed in a "_dns-push._tcp." 702 SRV record for the zone, then this is a misconfiguration, since 703 this server is being advertised as supporting DNS Push 704 Notifications for this zone, but the server itself is not 705 currently configured to perform that task. Since it is possible 706 that the misconfiguration may be repaired at any time, the retry 707 delay should not be set too high. By default, a value of 5 708 minutes is RECOMMENDED. 710 For RCODE = 9 (NOTAUTH), which occurs on a server that implements 711 DNS Push Notifications, but is not configured to be authoritative 712 for the requested name, the retry delay may be any value selected 713 by the implementer and/or configured by the operator. 715 If the server being queried is listed in a "_dns-push._tcp." 716 SRV record for the zone, then this is a misconfiguration, since 717 this server is being advertised as supporting DNS Push 718 Notifications for this zone, but the server itself is not 719 currently configured to perform that task. Since it is possible 720 that the misconfiguration may be repaired at any time, the retry 721 delay should not be set too high. By default, a value of 5 722 minutes is RECOMMENDED. 724 For RCODE = 11 (DSOTYPENI), which occurs on a server that 725 implements DSO but doesn't implement DNS Push Notifications, it is 726 unlikely that the server will begin supporting DNS Push 727 Notifications in the next few minutes, so the retry delay SHOULD 728 be one hour. 730 For other RCODE values, the retry delay should be set by the 731 server as appropriate for that error condition. By default, a 732 value of 5 minutes is RECOMMENDED. 734 For RCODE = 9 (NOTAUTH), the time delay applies to requests for other 735 names falling within the same zone. Requests for names falling 736 within other zones are not subject to the delay. For all other 737 RCODEs the time delay applies to all subsequent requests to this 738 server. 740 After sending an error response the server MAY allow the session to 741 remain open, or MAY send a DNS Push Notification Retry Delay 742 Operation TLV instructing the client to close the session, as 743 described in the DSO specification [RFC8490]. Clients MUST correctly 744 handle both cases. 746 6.3. DNS Push Notification Updates 748 Once a subscription has been successfully established, the server 749 generates PUSH messages to send to the client as appropriate. In the 750 case that the answer set was already non-empty at the moment the 751 subscription was established, an initial PUSH message will be sent 752 immediately following the SUBSCRIBE Response. Subsequent changes to 753 the answer set are then communicated to the client in subsequent PUSH 754 messages. 756 6.3.1. PUSH Message 758 A PUSH unidirectional message begins with the standard DSO 12-byte 759 header [RFC8490], followed by the PUSH primary TLV. A PUSH message 760 is illustrated in Figure 3. 762 In accordance with the definition of DSO unidirectional messages, the 763 MESSAGE ID field MUST be zero. There is no client response to a PUSH 764 message. 766 The other header fields MUST be set as described in the DSO spec- 767 ification [RFC8490]. The DNS OPCODE field contains the OPCODE value 768 for DNS Stateful Operations (6). The four count fields MUST be zero, 769 and the corresponding four sections MUST be empty (i.e., absent). 771 The DSO-TYPE is PUSH (tentatively 0x41). 773 The DSO-LENGTH is the length of the DSO-DATA that follows, which 774 specifies the changes being communicated. 776 The DSO-DATA contains one or more change notifications. A PUSH 777 Message MUST contain at least one change notification. If a PUSH 778 Message is received that contains no change notifications, this is a 779 fatal error, and the receiver MUST immediately terminate the 780 connection with a TLS close_notify alert. 782 The change notification records are formatted similarly to how DNS 783 Resource Records are conventionally expressed in DNS messages, as 784 illustrated in Figure 3, and are interpreted as described below. 786 The TTL field holds an unsigned 32-bit integer [RFC2181]. If the TTL 787 is in the range 0 to 2,147,483,647 seconds (2^31 - 1, or 0x7FFFFFFF), 788 then a new DNS Resource Record with the given name, type, class and 789 RDATA is added. A TTL of 0 means that this record should be retained 790 for as long as the subscription is active, and should be discarded 791 immediately the moment the subscription is cancelled. 793 If the TTL has the value 0xFFFFFFFF, then the DNS Resource Record 794 with the given name, type, class and RDATA is removed. 796 If the TTL has the value 0xFFFFFFFE, then this is a 'collective' 797 remove notification. For collective remove notifications RDLEN MUST 798 be zero and consequently the RDATA MUST be empty. If a change 799 notification is received where TTL = 0xFFFFFFFE and RDLEN is not 800 zero, this is a fatal error, and the receiver MUST immediately 801 terminate the connection with a TLS close_notify alert. There are 802 three types of collective remove notification: 804 For collective remove notifications, if CLASS is 255 (ANY), then for 805 the given name this deletes all records of all types in all classes. 806 In this case TYPE MUST be set to zero on transmission, and MUST be 807 silently ignored on reception. 809 For collective remove notifications, if CLASS is not 255 (ANY) and 810 TYPE is 255 (ANY) then for the given name this deletes all records of 811 all types in the specified class. 813 For collective remove notifications, if CLASS is not 255 (ANY) and 814 TYPE is not 255 (ANY) then for the given name this deletes all 815 records of the specified type in the specified class. 817 Summary of change notification types: 819 Delete all RRsets from a name, in all classes 820 TTL=0xFFFFFFFE, RDLENGTH=0, CLASS=255 (ANY) 822 Delete all RRsets from a name, in given class: 823 TTL=0xFFFFFFFE, RDLENGTH=0, CLASS specifies class, TYPE=255 (ANY) 825 Delete specified RRset from a name, in given class: 826 TTL=0xFFFFFFFE, RDLENGTH=0 827 CLASS and TYPE specify the RRset being deleted 829 Delete an individual RR from a name: 830 TTL=0xFFFFFFFF 831 CLASS, TYPE, RDLENGTH and RDATA specify the RR being deleted. 833 Add individual RR to a name 834 TTL>=0 835 CLASS, TYPE, RDLENGTH, RDATA and TTL specify the RR being added. 837 Note that it is valid for the RDATA of an added or removed DNS 838 Resource Record to be empty (zero length). For example, an Address 839 Prefix List Resource Record [RFC3123] may have empty RDATA. 840 Therefore, a change notification with RDLEN=0 does not automatically 841 indicate a remove notification. If RDLEN=0 and TTL is the in the 842 range 0 - 0x7FFFFFFF, this change notification signals the addition 843 of a record with the given name, type, class, and empty RDATA. If 844 RDLEN=0 and TTL = 0xFFFFFFFF, this change notification signals the 845 removal specifically of that single record with the given name, type, 846 class, and empty RDATA. 848 If the TTL is any value other than 0xFFFFFFFF, 0xFFFFFFFE, or a value 849 in the range 0 - 0x7FFFFFFF, then the receiver SHOULD silently ignore 850 this particular change notification record. The connection is not 851 terminated and other valid change notification records within this 852 PUSH message are processed as usual. 854 For efficiency, when generating a PUSH message, a server SHOULD 855 include as many change notifications as it has immediately available 856 to send, rather than sending each change notification as a separate 857 DSO message. Once it has exhausted the list of change notifications 858 immediately available to send, a server SHOULD then send the PUSH 859 message immediately, rather than waiting to see if additional change 860 notifications become available. 862 For efficiency, when generating a PUSH message, a server SHOULD use 863 standard DNS name compression, with offsets relative to the beginning 864 of the DNS message [RFC1035]. When multiple change notifications in 865 a single PUSH message have the same owner name, this name compression 866 can yield significant savings. Name compression should be performed 867 as specified in Section 18.14 of the Multicast DNS specification 868 [RFC6762], namely, owner names should always be compressed, and names 869 appearing within RDATA should be compressed for only the RR types 870 listed below: 872 NS, CNAME, PTR, DNAME, SOA, MX, AFSDB, RT, KX, RP, PX, SRV, NSEC 874 Servers may generate PUSH messages up to a maximum DNS message length 875 of 16,382 bytes, counting from the start of the DSO 12-byte header. 876 Including the two-byte length prefix that is used to frame DNS over a 877 byte stream like TLS, this makes a total of 16,384 bytes. Servers 878 MUST NOT generate PUSH messages larger than this. Where the 879 immediately available change notifications are sufficient to exceed a 880 DNS message length of 16,382 bytes, the change notifications MUST be 881 communicated in separate PUSH messages of up to 16,382 bytes each. 882 DNS name compression becomes less effective for messages larger than 883 16,384 bytes, so little efficiency benefit is gained by sending 884 messages larger than this. 886 If a client receives a PUSH message with a DNS message length larger 887 than 16,382 bytes, the this is a fatal error, and the receiver MUST 888 immediately terminate the connection with a TLS close_notify alert. 890 1 1 1 1 1 1 891 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 892 +--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+ \ 893 | MESSAGE ID (MUST BE ZERO) | \ 894 +--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+ | 895 |QR| OPCODE(6) | Z | RCODE | | 896 +--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+ | 897 | QDCOUNT (MUST BE ZERO) | | 898 +--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+ > HEADER 899 | ANCOUNT (MUST BE ZERO) | | 900 +--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+ | 901 | NSCOUNT (MUST BE ZERO) | | 902 +--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+ | 903 | ARCOUNT (MUST BE ZERO) | / 904 +--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+ / 905 | DSO-TYPE = PUSH (tentatively 0x41) | 906 +--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+ 907 | DSO-LENGTH (number of octets in DSO-DATA) | 908 +--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+ \ 909 \ NAME \ \ 910 +--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+ | 911 | TYPE | | 912 +--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+ | 913 | CLASS | | 914 +--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+ | 915 | TTL | | 916 | (32-bit unsigned big-endian integer) | > DSO-DATA 917 +--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+ | 918 | RDLEN | | 919 +--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+ | 920 \ RDATA (sized as necessary) \ | 921 +--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+ | 922 : NAME, TYPE, CLASS, TTL, RDLEN, RDATA : | 923 : Repeated As Necessary : / 924 +--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+ / 926 Figure 3: PUSH Message 928 When processing the records received in a PUSH Message, the receiving 929 client MUST validate that the records being added or deleted 930 correspond with at least one currently active subscription on that 931 session. Specifically, the record name MUST match the name given in 932 a SUBSCRIBE request, subject to the usual established DNS case- 933 insensitivity for US-ASCII letters. If the TYPE in the SUBSCRIBE 934 request was not ANY (255) then the TYPE of the record must match the 935 TYPE given in the SUBSCRIBE request. If the CLASS in the SUBSCRIBE 936 request was not ANY (255) then the CLASS of the record must match the 937 CLASS given in the SUBSCRIBE request. If a matching active 938 subscription on that session is not found, then that individual 939 record addition/deletion is silently ignored. Processing of other 940 additions and deletions in this message is not affected. The DSO 941 session is not closed. This is to allow for the unavoidable race 942 condition where a client sends an outbound UNSUBSCRIBE while inbound 943 PUSH messages for that subscription from the server are still in 944 flight. 946 In the case where a single change affects more than one active 947 subscription, only one PUSH message is sent. For example, a PUSH 948 message adding a given record may match both a SUBSCRIBE request with 949 the same TYPE and a different SUBSCRIBE request with TYPE=ANY (255). 950 It is not the case that two PUSH messages are sent because the new 951 record matches two active subscriptions. 953 The server SHOULD encode change notifications in the most efficient 954 manner possible. For example, when three AAAA records are deleted 955 from a given name, and no other AAAA records exist for that name, the 956 server SHOULD send a "delete an RRset from a name" PUSH message, not 957 three separate "delete an individual RR from a name" PUSH messages. 958 Similarly, when both an SRV and a TXT record are deleted from a given 959 name, and no other records of any kind exist for that name, the 960 server SHOULD send a "delete all RRsets from a name" PUSH message, 961 not two separate "delete an RRset from a name" PUSH messages. 963 A server SHOULD combine multiple change notifications in a single 964 PUSH message when possible, even if those change notifications apply 965 to different subscriptions. Conceptually, a PUSH message is a 966 session-level mechanism, not a subscription-level mechanism. 968 The TTL of an added record is stored by the client. While the 969 subscription is active, the TTL is not decremented, because a change 970 to the TTL would produce a new update. For as long as a relevant 971 subscription remains active, the client SHOULD assume that when a 972 record goes away the server will notify it of that fact. 973 Consequently, a client does not have to poll to verify that the 974 record is still there. Once a subscription is cancelled 975 (individually, or as a result of the DSO session being closed) record 976 aging for records covered by the subscription resumes and records are 977 removed from the local cache when their TTL reaches zero. 979 6.4. DNS Push Notification UNSUBSCRIBE 981 To cancel an individual subscription without closing the entire DSO 982 session, the client sends an UNSUBSCRIBE message over the established 983 DSO session to the server. The UNSUBSCRIBE message is encoded as a 984 DSO unidirectional message [RFC8490]. This specification defines a 985 primary unidirectional DSO TLV for DNS Push Notification UNSUBSCRIBE 986 Messages (tentatively DSO Type Code 0x42). 988 A server MUST NOT initiate an UNSUBSCRIBE message. If a server does 989 send an UNSUBSCRIBE message over a DSO session initiated by a client, 990 this is a fatal error and the client should immediately abort the 991 connection with a TLS close_notify alert. 993 6.4.1. UNSUBSCRIBE Message 995 An UNSUBSCRIBE unidirectional message begins with the standard DSO 996 12-byte header [RFC8490], followed by the UNSUBSCRIBE primary TLV. 997 An UNSUBSCRIBE message is illustrated in Figure 4. 999 In accordance with the definition of DSO unidirectional messages, the 1000 MESSAGE ID field MUST be zero. There is no server response to an 1001 UNSUBSCRIBE message. 1003 The other header fields MUST be set as described in the DSO spec- 1004 ification [RFC8490]. The DNS OPCODE field contains the OPCODE value 1005 for DNS Stateful Operations (6). The four count fields MUST be zero, 1006 and the corresponding four sections MUST be empty (i.e., absent). 1008 The DSO-TYPE is UNSUBSCRIBE (tentatively 0x42). 1010 The DSO-LENGTH field contains the value 2, the length of the 2-octet 1011 MESSAGE ID contained in the DSO-DATA. 1013 The DSO-DATA contains the value given in the MESSAGE ID field of an 1014 active SUBSCRIBE request. This is how the server knows which 1015 SUBSCRIBE request is being cancelled. After receipt of the 1016 UNSUBSCRIBE message, the SUBSCRIBE request is no longer active. 1018 It is allowable for the client to issue an UNSUBSCRIBE message for a 1019 previous SUBSCRIBE request for which the client has not yet received 1020 a SUBSCRIBE response. This is to allow for the case where a client 1021 starts and stops a subscription in less than the round-trip time to 1022 the server. The client is NOT required to wait for the SUBSCRIBE 1023 response before issuing the UNSUBSCRIBE message. 1025 Consequently, it is possible for a server to receive an UNSUBSCRIBE 1026 message that does not match any currently active subscription. This 1027 can occur when a client sends a SUBSCRIBE request, which subsequently 1028 fails and returns an error code, but the client sent an UNSUBSCRIBE 1029 message before it became aware that the SUBSCRIBE request had failed. 1030 Because of this, servers MUST silently ignore UNSUBSCRIBE messages 1031 that do not match any currently active subscription. 1033 1 1 1 1 1 1 1034 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 1035 +--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+ \ 1036 | MESSAGE ID (MUST BE ZERO) | \ 1037 +--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+ | 1038 |QR| OPCODE(6) | Z | RCODE | | 1039 +--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+ | 1040 | QDCOUNT (MUST BE ZERO) | | 1041 +--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+ > HEADER 1042 | ANCOUNT (MUST BE ZERO) | | 1043 +--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+ | 1044 | NSCOUNT (MUST BE ZERO) | | 1045 +--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+ | 1046 | ARCOUNT (MUST BE ZERO) | / 1047 +--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+ / 1048 | DSO-TYPE = UNSUBSCRIBE (tentatively 0x42) | 1049 +--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+ 1050 | DSO-LENGTH (2) | 1051 +--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+ \ 1052 | SUBSCRIBE MESSAGE ID | > DSO-DATA 1053 +--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+ / 1055 Figure 4: UNSUBSCRIBE Message 1057 6.5. DNS Push Notification RECONFIRM 1059 Sometimes, particularly when used with a Discovery Proxy [DisProx], a 1060 DNS Zone may contain stale data. When a client encounters data that 1061 it believes may be stale (e.g., an SRV record referencing a target 1062 host+port that is not responding to connection requests) the client 1063 can send a RECONFIRM message to ask the server to re-verify that the 1064 data is still valid. For a Discovery Proxy, this causes it to issue 1065 new Multicast DNS queries to ascertain whether the target device is 1066 still present. How the Discovery Proxy causes these new Multicast 1067 DNS queries to be issued depends on the details of the underlying 1068 Multicast DNS implementation being used. For example, a Discovery 1069 Proxy built on Apple's dns_sd.h API responds to a DNS Push 1070 Notification RECONFIRM message by calling the underlying API's 1071 DNSServiceReconfirmRecord() routine. 1073 For other types of DNS server, the RECONFIRM operation is currently 1074 undefined, and SHOULD result in a NOERROR response, but otherwise 1075 need not cause any action to occur. 1077 Frequent use of RECONFIRM operations may be a sign of network 1078 unreliability, or some kind of misconfiguration, so RECONFIRM 1079 operations MAY be logged or otherwise communicated to a human 1080 administrator to assist in detecting, and remedying, such network 1081 problems. 1083 If, after receiving a valid RECONFIRM message, the server determines 1084 that the disputed records are in fact no longer valid, then 1085 subsequent DNS PUSH Messages will be generated to inform interested 1086 clients. Thus, one client discovering that a previously-advertised 1087 device (like a network printer) is no longer present has the side 1088 effect of informing all other interested clients that the device in 1089 question is now gone. 1091 6.5.1. RECONFIRM Message 1093 A RECONFIRM unidirectional message begins with the standard DSO 1094 12-byte header [RFC8490], followed by the RECONFIRM primary TLV. 1095 A RECONFIRM message is illustrated in Figure 5. 1097 In accordance with the definition of DSO unidirectional messages, the 1098 MESSAGE ID field MUST be zero. There is no server response to a 1099 RECONFIRM message. 1101 The other header fields MUST be set as described in the DSO spec- 1102 ification [RFC8490]. The DNS OPCODE field contains the OPCODE value 1103 for DNS Stateful Operations (6). The four count fields MUST be zero, 1104 and the corresponding four sections MUST be empty (i.e., absent). 1106 The DSO-TYPE is RECONFIRM (tentatively 0x43). 1108 The DSO-LENGTH is the length of the data that follows, which 1109 specifies the name, type, class, and content of the record being 1110 disputed. 1112 1 1 1 1 1 1 1113 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 1114 +--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+ \ 1115 | MESSAGE ID | \ 1116 +--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+ | 1117 |QR| OPCODE(6) | Z | RCODE | | 1118 +--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+ | 1119 | QDCOUNT (MUST BE ZERO) | | 1120 +--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+ > HEADER 1121 | ANCOUNT (MUST BE ZERO) | | 1122 +--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+ | 1123 | NSCOUNT (MUST BE ZERO) | | 1124 +--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+ | 1125 | ARCOUNT (MUST BE ZERO) | / 1126 +--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+ / 1127 | DSO-TYPE = RECONFIRM (tentatively 0x43) | 1128 +--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+ 1129 | DSO-LENGTH (number of octets in DSO-DATA) | 1130 +--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+ \ 1131 \ NAME \ \ 1132 +--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+ | 1133 | TYPE | | 1134 +--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+ > DSO-DATA 1135 | CLASS | | 1136 +--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+ | 1137 \ RDATA \ / 1138 +--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+ / 1140 Figure 5: RECONFIRM Message 1142 The DSO-DATA for a RECONFIRM message MUST contain exactly one record. 1143 The DSO-DATA for a RECONFIRM message has no count field to specify 1144 more than one record. Since RECONFIRM messages are sent over TCP, 1145 multiple RECONFIRM messages can be concatenated in a single TCP 1146 stream and packed efficiently into TCP segments. 1148 TYPE MUST NOT be the value ANY (255) and CLASS MUST NOT be the value 1149 ANY (255). 1151 DNS wildcarding is not supported. That is, a wildcard ("*") in a 1152 RECONFIRM message matches only a literal wildcard character ("*") in 1153 the zone, and nothing else. 1155 Aliasing is not supported. That is, a CNAME in a RECONFIRM message 1156 matches only a literal CNAME record in the zone, and nothing else. 1158 6.6. DNS Stateful Operations TLV Context Summary 1160 This document defines four new DSO TLVs. As suggested in Section 8.2 1161 of the DNS Stateful Operations specification [RFC8490], the valid 1162 contexts of these new TLV types are summarized below. 1164 The client TLV contexts are: 1166 C-P: Client request message, primary TLV 1167 C-U: Client unidirectional message, primary TLV 1168 C-A: Client request or unidirectional message, additional TLV 1169 CRP: Response back to client, primary TLV 1170 CRA: Response back to client, additional TLV 1172 +-------------+-----+-----+-----+-----+-----+ 1173 | TLV Type | C-P | C-U | C-A | CRP | CRA | 1174 +-------------+-----+-----+-----+-----+-----+ 1175 | SUBSCRIBE | X | | | | | 1176 | PUSH | | | | | | 1177 | UNSUBSCRIBE | | X | | | | 1178 | RECONFIRM | | X | | | | 1179 +-------------+-----+-----+-----+-----+-----+ 1181 Table 2: DSO TLV Client Context Summary 1183 The server TLV contexts are: 1185 S-P: Server request message, primary TLV 1186 S-U: Server unidirectional message, primary TLV 1187 S-A: Server request or unidirectional message, additional TLV 1188 SRP: Response back to server, primary TLV 1189 SRA: Response back to server, additional TLV 1191 +-------------+-----+-----+-----+-----+-----+ 1192 | TLV Type | S-P | S-U | S-A | SRP | SRA | 1193 +-------------+-----+-----+-----+-----+-----+ 1194 | SUBSCRIBE | | | | | | 1195 | PUSH | | X | | | | 1196 | UNSUBSCRIBE | | | | | | 1197 | RECONFIRM | | | | | | 1198 +-------------+-----+-----+-----+-----+-----+ 1200 Table 3: DSO TLV Server Context Summary 1202 6.7. Client-Initiated Termination 1204 An individual subscription is terminated by sending an UNSUBSCRIBE 1205 TLV for that specific subscription, or all subscriptions can be 1206 cancelled at once by the client closing the DSO session. When a 1207 client terminates an individual subscription (via UNSUBSCRIBE) or all 1208 subscriptions on that DSO session (by ending the session) it is 1209 signaling to the server that it is longer interested in receiving 1210 those particular updates. It is informing the server that the server 1211 may release any state information it has been keeping with regards to 1212 these particular subscriptions. 1214 After terminating its last subscription on a session via UNSUBSCRIBE, 1215 a client MAY close the session immediately, or it may keep it open if 1216 it anticipates performing further operations on that session in the 1217 future. If a client wishes to keep an idle session open, it MUST 1218 respect the maximum idle time required by the server [RFC8490]. 1220 If a client plans to terminate one or more subscriptions on a session 1221 and doesn't intend to keep that session open, then as an efficiency 1222 optimization it MAY instead choose to simply close the session, which 1223 implicitly terminates all subscriptions on that session. This may 1224 occur because the client computer is being shut down, is going to 1225 sleep, the application requiring the subscriptions has terminated, or 1226 simply because the last active subscription on that session has been 1227 cancelled. 1229 When closing a session, a client should perform an orderly close of 1230 the TLS session in order to allow for future TLS session resumption 1231 with the server (if available). See Section 7.3 below. Typical APIs 1232 will provide a session close method that will send a TLS close_notify 1233 alert. This instructs the recipient that the sender will not send 1234 any more data over the session. Any pending writes on the server 1235 will be discarded when a close_notify is received. 1237 If the session is forcibly closed at the TCP level by sending a RST 1238 from either end of the connection, data may be lost and TLS session 1239 resumption of this session will not be possible. 1241 7. Security Considerations 1243 The Strict Privacy Usage Profile for DNS over TLS is REQUIRED for DNS 1244 Push Notifications [RFC8310]. Cleartext connections for DNS Push 1245 Notifications are not permissible. Since this is a new protocol, 1246 transition mechanisms from the Opportunistic Privacy profile are 1247 unnecessary. 1249 Also, see Section 9 of the DNS over (D)TLS Usage Profiles document 1250 [RFC8310] for additional recommendations for various versions of TLS 1251 usage. 1253 As a consequence of requiring TLS, client certificate authentication 1254 and verification may also be enforced by the server for stronger 1255 client-server security or end-to-end security. However, 1256 recommendations for security in particular deployment scenarios are 1257 outside the scope of this document. 1259 DNSSEC is RECOMMENDED for the authentication of DNS Push Notification 1260 servers. TLS alone does not provide complete security. TLS 1261 certificate verification can provide reasonable assurance that the 1262 client is really talking to the server associated with the desired 1263 host name, but since the desired host name is learned via a DNS SRV 1264 query, if the SRV query is subverted then the client may have a 1265 secure connection to a rogue server. DNSSEC can provided added 1266 confidence that the SRV query has not been subverted. 1268 7.1. Security Services 1270 It is the goal of using TLS to provide the following security 1271 services: 1273 Confidentiality: All application-layer communication is encrypted 1274 with the goal that no party should be able to decrypt it except 1275 the intended receiver. 1277 Data integrity protection: Any changes made to the communication in 1278 transit are detectable by the receiver. 1280 Authentication: An end-point of the TLS communication is 1281 authenticated as the intended entity to communicate with. 1283 Anti-replay protection: TLS provides for the detection of and 1284 prevention against messages sent previously over a TLS connection 1285 (such as DNS Push Notifications). Prior messages cannot be re- 1286 sent at a later time as a form of a man-in-the-middle attack. 1288 Deployment recommendations on the appropriate key lengths and cypher 1289 suites are beyond the scope of this document. Please refer to TLS 1290 Recommendations [RFC7525] for the best current practices. Keep in 1291 mind that best practices only exist for a snapshot in time and 1292 recommendations will continue to change. Updated versions or errata 1293 may exist for these recommendations. 1295 7.2. TLS Name Authentication 1297 As described in Section 6.1, the client discovers the DNS Push 1298 Notification server using an SRV lookup for the record name 1299 "_dns-push._tcp.". The server connection endpoint SHOULD then 1300 be authenticated using DANE TLSA records for the associated SRV 1301 record. This associates the target's name and port number with a 1302 trusted TLS certificate [RFC7673]. This procedure uses the TLS 1303 Server Name Indication (SNI) extension [RFC6066] to inform the server 1304 of the name the client has authenticated through the use of TLSA 1305 records. Therefore, if the SRV record passes DNSSEC validation and a 1306 TLSA record matching the target name is useable, an SNI extension 1307 must be used for the target name to ensure the client is connecting 1308 to the server it has authenticated. If the target name does not have 1309 a usable TLSA record, then the use of the SNI extension is optional. 1310 See Usage Profiles for DNS over TLS and DNS over DTLS [RFC8310] for 1311 more information on authenticating domain names. 1313 7.3. TLS Session Resumption 1315 TLS Session Resumption is permissible on DNS Push Notification 1316 servers. The server may keep TLS state with Session IDs [RFC8446] or 1317 operate in stateless mode by sending a Session Ticket [RFC5077] to 1318 the client for it to store. However, closing the TLS connection 1319 terminates the DSO session. When the TLS session is resumed, the DNS 1320 Push Notification server will not have any subscription state and 1321 will proceed as with any other new DSO session. Use of TLS Session 1322 Resumption may allow a TLS connection to be set up more quickly, but 1323 the client will still have to recreate any desired subscriptions. 1325 8. IANA Considerations 1327 This document defines a new service name to be published in the IANA 1328 Registry Service Types [RFC6335][ST] that is only applicable for the 1329 TCP protocol. 1331 +---------------------------+------+------------------+-------------+ 1332 | Name | Port | Value | Definition | 1333 +---------------------------+------+------------------+-------------+ 1334 | DNS Push Notification | None | "_dns-push._tcp" | Section 6.1 | 1335 | Service Type | | | | 1336 +---------------------------+------+------------------+-------------+ 1338 Table 4: IANA Service Type Assignments 1340 This document also defines four new DNS Stateful Operation TLV types 1341 to be recorded in the IANA DSO Type Code Registry. 1343 +-------------+------------+---------+-----------------+------------+ 1344 | Name | Value | Early | Status | Definition | 1345 | | | Data | | | 1346 +-------------+------------+---------+-----------------+------------+ 1347 | SUBSCRIBE | TBA (0x40) | NO | Standards Track | Section | 1348 | | | | | 6.2 | 1349 | PUSH | TBA (0x41) | NA | Standards Track | Section | 1350 | | | | | 6.3 | 1351 | UNSUBSCRIBE | TBA (0x42) | NA | Standards Track | Section | 1352 | | | | | 6.4 | 1353 | RECONFIRM | TBA (0x43) | NA | Standards Track | Section | 1354 | | | | | 6.5 | 1355 +-------------+------------+---------+-----------------+------------+ 1357 Table 5: IANA DSO TLV Type Code Assignments 1359 9. Acknowledgements 1361 The authors would like to thank Kiren Sekar and Marc Krochmal for 1362 previous work completed in this field. 1364 This draft has been improved due to comments from Ran Atkinson, Tim 1365 Chown, Mark Delany, Ralph Droms, Bernie Volz, Jan Komissar, Manju 1366 Shankar Rao, Markus Stenberg, Dave Thaler, Soraia Zlatkovic, Sara 1367 Dickinson, and Andrew Sullivan. Ted Lemon provided clarifying text 1368 that was greatly appreciated. 1370 10. References 1372 10.1. Normative References 1374 [RFC0768] Postel, J., "User Datagram Protocol", STD 6, RFC 768, 1375 DOI 10.17487/RFC0768, August 1980, 1376 . 1378 [RFC0793] Postel, J., "Transmission Control Protocol", STD 7, 1379 RFC 793, DOI 10.17487/RFC0793, September 1981, 1380 . 1382 [RFC1034] Mockapetris, P., "Domain names - concepts and facilities", 1383 STD 13, RFC 1034, DOI 10.17487/RFC1034, November 1987, 1384 . 1386 [RFC1035] Mockapetris, P., "Domain names - implementation and 1387 specification", STD 13, RFC 1035, DOI 10.17487/RFC1035, 1388 November 1987, . 1390 [RFC1123] Braden, R., Ed., "Requirements for Internet Hosts - 1391 Application and Support", STD 3, RFC 1123, 1392 DOI 10.17487/RFC1123, October 1989, 1393 . 1395 [RFC2119] Bradner, S., "Key words for use in RFCs to Indicate 1396 Requirement Levels", BCP 14, RFC 2119, 1397 DOI 10.17487/RFC2119, March 1997, 1398 . 1400 [RFC2136] Vixie, P., Ed., Thomson, S., Rekhter, Y., and J. Bound, 1401 "Dynamic Updates in the Domain Name System (DNS UPDATE)", 1402 RFC 2136, DOI 10.17487/RFC2136, April 1997, 1403 . 1405 [RFC2181] Elz, R. and R. Bush, "Clarifications to the DNS 1406 Specification", RFC 2181, DOI 10.17487/RFC2181, July 1997, 1407 . 1409 [RFC2782] Gulbrandsen, A., Vixie, P., and L. Esibov, "A DNS RR for 1410 specifying the location of services (DNS SRV)", RFC 2782, 1411 DOI 10.17487/RFC2782, February 2000, 1412 . 1414 [RFC6066] Eastlake 3rd, D., "Transport Layer Security (TLS) 1415 Extensions: Extension Definitions", RFC 6066, 1416 DOI 10.17487/RFC6066, January 2011, 1417 . 1419 [RFC6335] Cotton, M., Eggert, L., Touch, J., Westerlund, M., and S. 1420 Cheshire, "Internet Assigned Numbers Authority (IANA) 1421 Procedures for the Management of the Service Name and 1422 Transport Protocol Port Number Registry", BCP 165, 1423 RFC 6335, DOI 10.17487/RFC6335, August 2011, 1424 . 1426 [RFC6895] Eastlake 3rd, D., "Domain Name System (DNS) IANA 1427 Considerations", BCP 42, RFC 6895, DOI 10.17487/RFC6895, 1428 April 2013, . 1430 [RFC7673] Finch, T., Miller, M., and P. Saint-Andre, "Using DNS- 1431 Based Authentication of Named Entities (DANE) TLSA Records 1432 with SRV Records", RFC 7673, DOI 10.17487/RFC7673, October 1433 2015, . 1435 [RFC7766] Dickinson, J., Dickinson, S., Bellis, R., Mankin, A., and 1436 D. Wessels, "DNS Transport over TCP - Implementation 1437 Requirements", RFC 7766, DOI 10.17487/RFC7766, March 2016, 1438 . 1440 [RFC8174] Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC 1441 2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174, 1442 May 2017, . 1444 [RFC8446] Rescorla, E., "The Transport Layer Security (TLS) Protocol 1445 Version 1.3", RFC 8446, DOI 10.17487/RFC8446, August 2018, 1446 . 1448 [RFC8490] Bellis, R., Cheshire, S., Dickinson, J., Dickinson, S., 1449 Lemon, T., and T. Pusateri, "DNS Stateful Operations", 1450 RFC 8490, DOI 10.17487/RFC8490, March 2019, 1451 . 1453 [ST] "Service Name and Transport Protocol Port Number 1454 Registry", . 1457 10.2. Informative References 1459 [DisProx] Cheshire, S., "Discovery Proxy for Multicast DNS-Based 1460 Service Discovery", draft-ietf-dnssd-hybrid-10 (work in 1461 progress), March 2019. 1463 [I-D.dukkipati-tcpm-tcp-loss-probe] 1464 Dukkipati, N., Cardwell, N., Cheng, Y., and M. Mathis, 1465 "Tail Loss Probe (TLP): An Algorithm for Fast Recovery of 1466 Tail Losses", draft-dukkipati-tcpm-tcp-loss-probe-01 (work 1467 in progress), February 2013. 1469 [LLQ] Cheshire, S. and M. Krochmal, "DNS Long-Lived Queries", 1470 draft-sekar-dns-llq-03 (work in progress), March 2019. 1472 [obs] "Observer Pattern", 1473 . 1475 [RFC2308] Andrews, M., "Negative Caching of DNS Queries (DNS 1476 NCACHE)", RFC 2308, DOI 10.17487/RFC2308, March 1998, 1477 . 1479 [RFC3123] Koch, P., "A DNS RR Type for Lists of Address Prefixes 1480 (APL RR)", RFC 3123, DOI 10.17487/RFC3123, June 2001, 1481 . 1483 [RFC4287] Nottingham, M., Ed. and R. Sayre, Ed., "The Atom 1484 Syndication Format", RFC 4287, DOI 10.17487/RFC4287, 1485 December 2005, . 1487 [RFC4953] Touch, J., "Defending TCP Against Spoofing Attacks", 1488 RFC 4953, DOI 10.17487/RFC4953, July 2007, 1489 . 1491 [RFC5077] Salowey, J., Zhou, H., Eronen, P., and H. Tschofenig, 1492 "Transport Layer Security (TLS) Session Resumption without 1493 Server-Side State", RFC 5077, DOI 10.17487/RFC5077, 1494 January 2008, . 1496 [RFC6281] Cheshire, S., Zhu, Z., Wakikawa, R., and L. Zhang, 1497 "Understanding Apple's Back to My Mac (BTMM) Service", 1498 RFC 6281, DOI 10.17487/RFC6281, June 2011, 1499 . 1501 [RFC6762] Cheshire, S. and M. Krochmal, "Multicast DNS", RFC 6762, 1502 DOI 10.17487/RFC6762, February 2013, 1503 . 1505 [RFC6763] Cheshire, S. and M. Krochmal, "DNS-Based Service 1506 Discovery", RFC 6763, DOI 10.17487/RFC6763, February 2013, 1507 . 1509 [RFC6824] Ford, A., Raiciu, C., Handley, M., and O. Bonaventure, 1510 "TCP Extensions for Multipath Operation with Multiple 1511 Addresses", RFC 6824, DOI 10.17487/RFC6824, January 2013, 1512 . 1514 [RFC6886] Cheshire, S. and M. Krochmal, "NAT Port Mapping Protocol 1515 (NAT-PMP)", RFC 6886, DOI 10.17487/RFC6886, April 2013, 1516 . 1518 [RFC6887] Wing, D., Ed., Cheshire, S., Boucadair, M., Penno, R., and 1519 P. Selkirk, "Port Control Protocol (PCP)", RFC 6887, 1520 DOI 10.17487/RFC6887, April 2013, 1521 . 1523 [RFC7413] Cheng, Y., Chu, J., Radhakrishnan, S., and A. Jain, "TCP 1524 Fast Open", RFC 7413, DOI 10.17487/RFC7413, December 2014, 1525 . 1527 [RFC7525] Sheffer, Y., Holz, R., and P. Saint-Andre, 1528 "Recommendations for Secure Use of Transport Layer 1529 Security (TLS) and Datagram Transport Layer Security 1530 (DTLS)", BCP 195, RFC 7525, DOI 10.17487/RFC7525, May 1531 2015, . 1533 [RFC7719] Hoffman, P., Sullivan, A., and K. Fujiwara, "DNS 1534 Terminology", RFC 7719, DOI 10.17487/RFC7719, December 1535 2015, . 1537 [RFC7858] Hu, Z., Zhu, L., Heidemann, J., Mankin, A., Wessels, D., 1538 and P. Hoffman, "Specification for DNS over Transport 1539 Layer Security (TLS)", RFC 7858, DOI 10.17487/RFC7858, May 1540 2016, . 1542 [RFC8010] Sweet, M. and I. McDonald, "Internet Printing 1543 Protocol/1.1: Encoding and Transport", STD 92, RFC 8010, 1544 DOI 10.17487/RFC8010, January 2017, 1545 . 1547 [RFC8011] Sweet, M. and I. McDonald, "Internet Printing 1548 Protocol/1.1: Model and Semantics", STD 92, RFC 8011, 1549 DOI 10.17487/RFC8011, January 2017, 1550 . 1552 [RFC8310] Dickinson, S., Gillmor, D., and T. Reddy, "Usage Profiles 1553 for DNS over TLS and DNS over DTLS", RFC 8310, 1554 DOI 10.17487/RFC8310, March 2018, 1555 . 1557 [SYN] Eddy, W., "Defenses Against TCP SYN Flooding Attacks", The 1558 Internet Protocol Journal, Cisco Systems, Volume 9, 1559 Number 4, December 2006. 1561 [XEP0060] Millard, P., Saint-Andre, P., and R. Meijer, "Publish- 1562 Subscribe", XSF XEP 0060, July 2010. 1564 Authors' Addresses 1566 Tom Pusateri 1567 Unaffiliated 1568 Raleigh, NC 27608 1569 USA 1571 Phone: +1 919 867 1330 1572 Email: pusateri@bangj.com 1574 Stuart Cheshire 1575 Apple Inc. 1576 One Apple Park Way 1577 Cupertino, CA 95014 1578 USA 1580 Phone: +1 (408) 996-1010 1581 Email: cheshire@apple.com