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If it is intended as a requirements expression, it should be rewritten using one of the combinations defined in RFC 2119; otherwise it should not be all-uppercase. -- The document date (March 24, 2019) is 1857 days in the past. Is this intentional? Checking references for intended status: Proposed Standard ---------------------------------------------------------------------------- (See RFCs 3967 and 4897 for information about using normative references to lower-maturity documents in RFCs) ** Obsolete normative reference: RFC 793 (Obsoleted by RFC 9293) -- Possible downref: Non-RFC (?) normative reference: ref. 'ST' == Outdated reference: A later version (-06) exists of draft-sekar-dns-llq-03 -- Obsolete informational reference (is this intentional?): RFC 5077 (Obsoleted by RFC 8446) -- Obsolete informational reference (is this intentional?): RFC 6824 (Obsoleted by RFC 8684) -- Obsolete informational reference (is this intentional?): RFC 7525 (Obsoleted by RFC 9325) -- Obsolete informational reference (is this intentional?): RFC 7719 (Obsoleted by RFC 8499) Summary: 1 error (**), 0 flaws (~~), 2 warnings (==), 7 comments (--). 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: September 25, 2019 Apple Inc. 6 March 24, 2019 8 DNS Push Notifications 9 draft-ietf-dnssd-push-19 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 September 25, 2019. 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 . . . . . . . . . . . . . . . . . . . . . . . . . . 7 61 5. State Considerations . . . . . . . . . . . . . . . . . . . . 8 62 6. Protocol Operation . . . . . . . . . . . . . . . . . . . . . 9 63 6.1. Discovery . . . . . . . . . . . . . . . . . . . . . . . . 10 64 6.2. DNS Push Notification SUBSCRIBE . . . . . . . . . . . . . 14 65 6.2.1. SUBSCRIBE Request . . . . . . . . . . . . . . . . . . 14 66 6.2.2. SUBSCRIBE Response . . . . . . . . . . . . . . . . . 17 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 . . . . . . . . . . . . . . . . . . . 32 76 7.1. Security Services . . . . . . . . . . . . . . . . . . . . 32 77 7.2. TLS Name Authentication . . . . . . . . . . . . . . . . . 32 78 7.3. TLS Session Resumption . . . . . . . . . . . . . . . . . 33 79 8. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 33 80 9. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . 34 81 10. References . . . . . . . . . . . . . . . . . . . . . . . . . 34 82 10.1. Normative References . . . . . . . . . . . . . . . . . . 34 83 10.2. Informative References . . . . . . . . . . . . . . . . . 36 84 Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 38 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-tls._tcp." SRV record for a zone 164 MAY reference the same target host and port as that zone's 165 "_dns-update-tls._tcp." SRV record. When the same target host 166 and port is offered for both DNS Updates and DNS Push Notifications, 167 a 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 also 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-tls._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 This DNS Push Notification specification includes support for DNS 190 classes, for completeness. However, in practice, it is anticipated 191 that for the foreseeable future the only DNS class in use will be DNS 192 class "IN", as is the reality today with existing DNS servers and 193 clients. A DNS Push Notification server MAY choose to implement only 194 DNS class "IN". If messages are received for a class other than 195 "IN", and that class is not supported, an error with RCODE NOTIMPL 196 (Not Implemented) should be returned. 198 DNS Push Notifications impose less load on the responding server than 199 rapid polling would, but Push Notifications do still have a cost, so 200 DNS Push Notification clients MUST NOT recklessly create an excessive 201 number of Push Notification subscriptions. Specifically: 203 (a) A subscription should only be active when there is a valid reason 204 to need live data (for example, an on-screen display is currently 205 showing the results to the user) and the subscription SHOULD be 206 cancelled as soon as the need for that data ends (for example, when 207 the user dismisses that display). In the case of a device like a 208 smartphone which, after some period of inactivity, goes to sleep or 209 otherwise darkens its screen, it should cancel its subscriptions when 210 darkening the screen (since the user cannot see any changes in the 211 display anyway) and reinstate its subscriptions when re-awakening 212 from display sleep. 214 (b) A DNS Push Notification client SHOULD NOT routinely keep a DNS 215 Push Notification subscription active 24 hours a day, 7 days a week, 216 just to keep a list in memory up to date so that if the user does 217 choose to bring up an on-screen display of that data, it can be 218 displayed really fast. DNS Push Notifications are designed to be 219 fast enough that there is no need to pre-load a "warm" list in memory 220 just in case it might be needed later. 222 Generally, as described in the DNS Stateful Operations specification 223 [RFC8490], a client must not keep a session to a server open 224 indefinitely if it has no subscriptions (or other operations) active 225 on that session. A client MAY close a session as soon as it becomes 226 idle, and then if needed in the future, open a new session when 227 required. Alternatively, a client MAY speculatively keep an idle 228 session open for some time, subject to the constraint that it MUST 229 NOT keep a session open that has been idle for more than the 230 session's idle timeout (15 seconds by default) [RFC8490]. 232 4. Transport 234 Other DNS operations like DNS Update [RFC2136] MAY use either User 235 Datagram Protocol (UDP) [RFC0768] or Transmission Control Protocol 236 (TCP) [RFC0793] as the transport protocol, in keeping with the 237 historical precedent that DNS queries must first be sent over UDP 238 [RFC1123]. This requirement to use UDP has subsequently been relaxed 239 [RFC7766]. 241 In keeping with the more recent precedent, DNS Push Notification is 242 defined only for TCP. DNS Push Notification clients MUST use DNS 243 Stateful Operations [RFC8490] running over TLS over TCP [RFC7858]. 245 Connection setup over TCP ensures return reachability and alleviates 246 concerns of state overload at the server through anonymous 247 subscriptions. All subscribers are guaranteed to be reachable by the 248 server by virtue of the TCP three-way handshake. Flooding attacks 249 are possible with any protocol, and a benefit of TCP is that there 250 are already established industry best practices to guard against SYN 251 flooding and similar attacks [SYN] [RFC4953]. 253 Use of TCP also allows DNS Push Notifications to take advantage of 254 current and future developments in TCP, such as Multipath TCP (MPTCP) 255 [RFC6824], TCP Fast Open (TFO) [RFC7413], Tail Loss Probe (TLP) 256 [I-D.dukkipati-tcpm-tcp-loss-probe], and so on. 258 Transport Layer Security (TLS) [RFC8446] is well understood and 259 deployed across many protocols running over TCP. It is designed to 260 prevent eavesdropping, tampering, and message forgery. TLS is 261 REQUIRED for every connection between a client subscriber and server 262 in this protocol specification. Additional security measures such as 263 client authentication during TLS negotiation MAY also be employed to 264 increase the trust relationship between client and server. 266 5. State Considerations 268 Each DNS Push Notification server is capable of handling some finite 269 number of Push Notification subscriptions. This number will vary 270 from server to server and is based on physical machine 271 characteristics, network bandwidth, and operating system resource 272 allocation. After a client establishes a session to a DNS server, 273 each subscription is individually accepted or rejected. Servers may 274 employ various techniques to limit subscriptions to a manageable 275 level. Correspondingly, the client is free to establish simultaneous 276 sessions to alternate DNS servers that support DNS Push Notifications 277 for the zone and distribute subscriptions at the client's discretion. 278 In this way, both clients and servers can react to resource 279 constraints. 281 6. Protocol Operation 283 The DNS Push Notification protocol is a session-oriented protocol, 284 and makes use of DNS Stateful Operations (DSO) [RFC8490]. 286 For details of the DSO message format refer to the DNS Stateful Oper- 287 ations specification [RFC8490]. Those details are not repeated here. 289 DNS Push Notification clients and servers MUST support DSO. A single 290 server can support DNS Queries, DNS Updates, and DNS Push 291 Notifications (using DSO) on the same TCP port. 293 A DNS Push Notification exchange begins with the client discovering 294 the appropriate server, using the procedure described in Section 6.1, 295 and then making a TLS/TCP connection to it. 297 A typical DNS Push Notification client will immediately issue a DSO 298 Keepalive operation to request a session timeout and/or keepalive 299 interval longer than the the 15-second default values, but this is 300 not required. A DNS Push Notification client MAY issue other 301 requests on the session first, and only issue a DSO Keepalive 302 operation later if it determines that to be necessary. Sending 303 either a DSO Keepalive operation or a Push Notification subscription 304 over the TLS/TCP connection to the server signals the client's 305 support of DSO and serves to establish a DSO session. 307 In accordance with the current set of active subscriptions, the 308 server sends relevant asynchronous Push Notifications to the client. 309 Note that a client MUST be prepared to receive (and silently ignore) 310 Push Notifications for subscriptions it has previously removed, since 311 there is no way to prevent the situation where a Push Notification is 312 in flight from server to client while the client's UNSUBSCRIBE 313 message cancelling that subscription is simultaneously in flight from 314 client to server. 316 6.1. Discovery 318 The first step in DNS Push Notification subscription is to discover 319 an appropriate DNS server that supports DNS Push Notifications for 320 the desired zone. 322 The client begins by opening a DSO Session to its normal configured 323 DNS recursive resolver and requesting a Push Notification 324 subscription. This connection is made to TCP port 853, the default 325 port for DNS-over-TLS DNS over TLS [RFC7858]. If the request for a 326 Push Notification subscription is successful, then the recursive 327 resolver will make a corresponding Push Notification subscription on 328 the client's behalf (if the recursive resolver doesn't already have 329 an active subscription for that name, type, and class), and pass on 330 any results it receives back to the client. This is closely 331 analogous to how a client sends normal DNS queries to its configured 332 DNS recursive resolver, which issues queries on the client's behalf 333 (if the recursive resolver doesn't already have appropriate answer(s) 334 in its cache), and passes on any results it receives back to the 335 client. 337 In many contexts, the recursive resolver will be able to handle Push 338 Notifications for all names that the client may need to follow. Use 339 of VPN tunnels and split-view DNS can create some additional 340 complexity in the client software here; the techniques to handle VPN 341 tunnels and split-view DNS for DNS Push Notifications are the same as 342 those already used to handle this for normal DNS queries. 344 If the recursive resolver does not support DNS over TLS, or does 345 support DNS over TLS but is not listening on TCP port 853, or does 346 support DNS over TLS on TCP port 853 but does not support DSO on that 347 port, then the DSO Session session establishment will fail [RFC8490]. 349 If the recursive resolver does support DSO but not Push Notification 350 subscriptions, then it will return the DSO error code, DSOTYPENI 351 (11). 353 In some cases, the recursive resolver may support DSO and Push 354 Notification subscriptions, but may not be able to subscribe for Push 355 Notifications for a particular name. In this case, the recursive 356 resolver should return an informative error code to the client so 357 that the client can make an informed decision how to handle the 358 error. If the recursive resolver is unable to establish a connection 359 to the zone's DNS Push Notification server (perhaps because the 360 required SRV record does not exist) the recursive resolver should 361 return SERVFAIL. If the recursive resolver is able to establish a 362 connection to the zone's DNS Push Notification server and some other 363 error code is then received, the recursive resolver should pass on 364 this received error code back to the client. In some cases, where 365 the client has a pre-established trust relationship with the owner of 366 the zone (that is not handled via the usual mechanisms for VPN 367 software) the client may handle these failures by contacting the 368 zone's DNS Push server directly. 370 In any of the cases described above where the client fails to 371 establish a DNS Push Notification subscription via its configured 372 recursive resolver, the client should proceed to discover the 373 appropriate server for direct communication. The client MUST also 374 determine which TCP port on the server is listening for connections, 375 which need not be (and often is not) the typical TCP port 53 used for 376 conventional DNS, or TCP port 853 used for DNS over TLS. 378 The discovery algorithm described here is an iterative algorithm, 379 which starts with the full name of the record to which the client 380 wishes to subscribe. Successive SOA queries are then issued, 381 trimming one label each time, until the closest enclosing 382 authoritative server is discovered. There is also an optimization to 383 enable the client to take a "short cut" directly to the SOA record of 384 the closest enclosing authoritative server in many cases. 386 1. The client begins the discovery by sending a DNS query to its 387 local resolver, with record type SOA [RFC1035] for the record 388 name to which it wishes to subscribe. As an example, suppose the 389 client wishes to subscribe to PTR records with the name 390 _ipp._tcp.foo.example.com (to discover Internet Printing Protocol 391 (IPP) printers [RFC8010] [RFC8011] being advertised at 392 "foo.example.com"). The client begins by sending an SOA query 393 for _ipp._tcp.foo.example.com to the local recursive resolver. 394 The goal is to determine the server authoritative for the name 395 _ipp._tcp.foo.example.com. The closest enclosing DNS zone 396 containing the name _ipp._tcp.foo.example.com could be 397 example.com, or foo.example.com, or _tcp.foo.example.com, or even 398 _ipp._tcp.foo.example.com. The client does not know in advance 399 where the closest enclosing zone cut occurs, which is why it uses 400 the iterative procedure described here to discover this 401 information. 403 2. If the requested SOA record exists, it will be returned in the 404 Answer section with a NOERROR response code, and the client has 405 succeeded in discovering the information it needs. 406 (This language is not placing any new requirements on DNS 407 recursive resolvers. This text merely describes the existing 408 operation of the DNS protocol [RFC1034] [RFC1035].) 410 3. If the requested SOA record does not exist, the client will get 411 back a NOERROR/NODATA response or an NXDOMAIN/Name Error 412 response. In either case, the local resolver would normally 413 include the SOA record for the closest enclosing zone of the 414 requested name in the Authority Section. If the SOA record is 415 received in the Authority Section, then the client has succeeded 416 in discovering the information it needs. 417 (This language is not placing any new requirements on DNS 418 recursive resolvers. This text merely describes the existing 419 operation of the DNS protocol regarding negative responses 420 [RFC2308].) 422 4. If the client receives a response containing no SOA record, then 423 it proceeds with the iterative approach. The client strips the 424 leading label from the current query name and if the resulting 425 name has at least one label in it, the client sends an SOA query 426 for that new name, and processing continues at step 2 above, 427 repeating the iterative search until either an SOA is received, 428 or the query name consists of a single label, i.e., a Top Level 429 Domain (TLD). In the case of a single-label TLD, this is a 430 network configuration error which should not happen and the 431 client gives up. The client may retry the operation at a later 432 time, of the client's choosing, such after a change in network 433 attachment. 435 5. Once the SOA is known (either by virtue of being seen in the 436 Answer Section, or in the Authority Section), the client sends a 437 DNS query with type SRV [RFC2782] for the record name 438 "_dns-push-tls._tcp.", where is the owner name of 439 the discovered SOA record. 441 6. If the zone in question is set up to offer DNS Push Notifications 442 then this SRV record MUST exist. (If this SRV record does not 443 exist then the zone is not correctly configured for DNS Push 444 Notifications as specified in this document.) The SRV "target" 445 contains the name of the server providing DNS Push Notifications 446 for the zone. The port number on which to contact the server is 447 in the SRV record "port" field. The address(es) of the target 448 host MAY be included in the Additional Section, however, the 449 address records SHOULD be authenticated before use as described 450 below in Section 7.2 and in the specification for using DANE TLSA 451 Records with SRV Records [RFC7673], if applicable. 453 7. More than one SRV record may be returned. In this case, the 454 "priority" and "weight" values in the returned SRV records are 455 used to determine the order in which to contact the servers for 456 subscription requests. As described in the SRV specification 457 [RFC2782], the server with the lowest "priority" is first 458 contacted. If more than one server has the same "priority", the 459 "weight" indicates the weighted probability that the client 460 should contact that server. Higher weights have higher 461 probabilities of being selected. If a server is not willing to 462 accept a subscription request, or is not reachable within a 463 reasonable time, as determined by the client, then a subsequent 464 server is to be contacted. 466 Each time a client makes a new DNS Push Notification subscription 467 session, it SHOULD repeat the discovery process in order to determine 468 the preferred DNS server for subscriptions at that time. However, 469 the client device MUST respect the DNS TTL values on records it 470 receives, and store them in its local cache with this lifetime. This 471 means that, as long as the DNS TTL values on the authoritative 472 records were set to reasonable values, repeated application of this 473 discovery process can be completed nearly instantaneously by the 474 client, using only locally-stored cached data. 476 6.2. DNS Push Notification SUBSCRIBE 478 After connecting, and requesting a longer idle timeout and/or 479 keepalive interval if necessary, a DNS Push Notification client 480 then indicates its desire to receive DNS Push Notifications for 481 a given domain name by sending a SUBSCRIBE request to the server. 482 A SUBSCRIBE request is encoded in a DSO message [RFC8490]. 483 This specification defines a primary DSO TLV for DNS Push 484 Notification SUBSCRIBE Requests (tentatively DSO Type Code 0x40). 486 The entity that initiates a SUBSCRIBE request is by definition the 487 client. A server MUST NOT send a SUBSCRIBE request over an existing 488 session from a client. If a server does send a SUBSCRIBE request 489 over a DSO session initiated by a client, this is a fatal error and 490 the client should immediately abort the connection with a TCP RST (or 491 equivalent for other protocols). 493 6.2.1. SUBSCRIBE Request 495 A SUBSCRIBE request begins with the standard DSO 12-byte header 496 [RFC8490], followed by the SUBSCRIBE primary TLV. A SUBSCRIBE 497 request message is illustrated in Figure 1. 499 The MESSAGE ID field MUST be set to a unique value, that the client 500 is not using for any other active operation on this DSO session. For 501 the purposes here, a MESSAGE ID is in use on this session if the 502 client has used it in a request for which it has not yet received a 503 response, or if the client has used it for a subscription which it 504 has not yet cancelled using UNSUBSCRIBE. In the SUBSCRIBE response 505 the server MUST echo back the MESSAGE ID value unchanged. 507 The other header fields MUST be set as described in the DSO spec- 508 ification [RFC8490]. The DNS OPCODE field contains the OPCODE value 509 for DNS Stateful Operations (6). The four count fields MUST be zero, 510 and the corresponding four sections MUST be empty (i.e., absent). 512 The DSO-TYPE is SUBSCRIBE (tentatively 0x40). 514 The DSO-LENGTH is the length of the DSO-DATA that follows, which 515 specifies the name, type, and class of the record(s) being sought. 517 1 1 1 1 1 1 518 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 519 +--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+ \ 520 | MESSAGE ID | \ 521 +--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+ | 522 |QR| OPCODE(6) | Z | RCODE | | 523 +--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+ | 524 | QDCOUNT (MUST BE ZERO) | | 525 +--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+ > HEADER 526 | ANCOUNT (MUST BE ZERO) | | 527 +--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+ | 528 | NSCOUNT (MUST BE ZERO) | | 529 +--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+ | 530 | ARCOUNT (MUST BE ZERO) | / 531 +--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+ / 532 | DSO-TYPE = SUBSCRIBE (tentatively 0x40) | 533 +--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+ 534 | DSO-LENGTH (number of octets in DSO-DATA) | 535 +--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+ \ 536 | | \ 537 \ NAME \ | 538 \ \ | 539 +--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+ > DSO-DATA 540 | TYPE | | 541 +--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+ | 542 | CLASS | / 543 +--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+ / 545 Figure 1: SUBSCRIBE Request 547 The DSO-DATA for a SUBSCRIBE request MUST contain exactly one NAME, 548 TYPE, and CLASS. Since SUBSCRIBE requests are sent over TCP, 549 multiple SUBSCRIBE DSO request messages can be concatenated in a 550 single TCP stream and packed efficiently into TCP segments. 552 If accepted, the subscription will stay in effect until the client 553 cancels the subscription using UNSUBSCRIBE or until the DSO session 554 between the client and the server is closed. 556 SUBSCRIBE requests on a given session MUST be unique. A client MUST 557 NOT send a SUBSCRIBE message that duplicates the NAME, TYPE and CLASS 558 of an existing active subscription on that DSO session. For the 559 purpose of this matching, the established DNS case-insensitivity for 560 US-ASCII letters applies (e.g., "example.com" and "Example.com" are 561 the same). If a server receives such a duplicate SUBSCRIBE message 562 this is an error and the server MUST immediately terminate the 563 connection with a TCP RST (or equivalent for other protocols). 565 DNS wildcarding is not supported. That is, a wildcard ("*") in a 566 SUBSCRIBE message matches only a literal wildcard character ("*") in 567 the zone, and nothing else. 569 Aliasing is not supported. That is, a CNAME in a SUBSCRIBE message 570 matches only a literal CNAME record in the zone, and nothing else. 572 A client may SUBSCRIBE to records that are unknown to the server at 573 the time of the request (providing that the name falls within one of 574 the zone(s) the server is responsible for) and this is not an error. 575 The server MUST NOT return NXDOMAIN in this case. The server MUST 576 accept these requests and send Push Notifications if and when 577 matching records are found in the future. 579 If neither TYPE nor CLASS are ANY (255) then this is a specific 580 subscription to changes for the given NAME, TYPE and CLASS. If one 581 or both of TYPE or CLASS are ANY (255) then this subscription matches 582 any type and/or any class, as appropriate. 584 NOTE: A little-known quirk of DNS is that in DNS QUERY requests, 585 QTYPE and QCLASS 255 mean "ANY" not "ALL". They indicate that the 586 server should respond with ANY matching records of its choosing, not 587 necessarily ALL matching records. This can lead to some surprising 588 and unexpected results, where a query returns some valid answers but 589 not all of them, and makes QTYPE=ANY queries less useful than people 590 sometimes imagine. 592 When used in conjunction with SUBSCRIBE, TYPE and CLASS 255 should be 593 interpreted to mean "ALL", not "ANY". After accepting a subscription 594 where one or both of TYPE or CLASS are 255, the server MUST send Push 595 Notification Updates for ALL record changes that match the 596 subscription, not just some of them. 598 6.2.2. SUBSCRIBE Response 600 Each SUBSCRIBE request generates exactly one SUBSCRIBE response from 601 the server. 603 A SUBSCRIBE response begins with the standard DSO 12-byte header 604 [RFC8490], possibly followed by one or more optional TLVs, such as a 605 Retry Delay TLV. 607 The MESSAGE ID field MUST echo the value given in the ID field of the 608 SUBSCRIBE request. This is how the client knows which request is 609 being responded to. 611 A SUBSCRIBE response message MUST NOT include a SUBSCRIBE TLV. If a 612 client receives a SUBSCRIBE response message containing a SUBSCRIBE 613 TLV then the response message is processed but the SUBSCRIBE TLV MUST 614 be silently ignored. 616 In the SUBSCRIBE response the RCODE indicates whether or not the 617 subscription was accepted. Supported RCODEs are as follows: 619 +-----------+-------+-----------------------------------------------+ 620 | Mnemonic | Value | Description | 621 +-----------+-------+-----------------------------------------------+ 622 | NOERROR | 0 | SUBSCRIBE successful. | 623 | FORMERR | 1 | Server failed to process request due to a | 624 | | | malformed request. | 625 | SERVFAIL | 2 | Server failed to process request due to a | 626 | | | problem with the server. | 627 | NOTIMP | 4 | Server does not implement DSO. | 628 | REFUSED | 5 | Server refuses to process request for policy | 629 | | | or security reasons. | 630 | NOTAUTH | 9 | Server is not authoritative for the requested | 631 | | | name. | 632 | DSOTYPENI | 11 | SUBSCRIBE operation not supported. | 633 +-----------+-------+-----------------------------------------------+ 635 Table 1: SUBSCRIBE Response codes 637 This document specifies only these RCODE values for SUBSCRIBE 638 Responses. Servers sending SUBSCRIBE Responses SHOULD use one of 639 these values. Note that NXDOMAIN is not a valid RCODE in response to 640 a SUBSCRIBE Request. However, future circumstances may create 641 situations where other RCODE values are appropriate in SUBSCRIBE 642 Responses, so clients MUST be prepared to accept SUBSCRIBE Responses 643 with any other RCODE value. 645 If the server sends a nonzero RCODE in the SUBSCRIBE response, that 646 means: 648 a. the client is (at least partially) misconfigured, 649 b. the server resources are exhausted, or 650 c. there is some other unknown failure on the server. 652 In any case, the client shouldn't retry the subscription to this 653 server right away. If multiple SRV records were returned as 654 described in Section 6.1, Paragraph 7, a subsequent server can be 655 tried immediately. 657 If the client has other successful subscriptions to this server, 658 these subscriptions remain even though additional subscriptions may 659 be refused. Neither the client nor the server are required to close 660 the connection, although, either end may choose to do so. 662 If the server sends a nonzero RCODE then it SHOULD append a Retry 663 Delay TLV [RFC8490] to the response specifying a delay before the 664 client attempts this operation again. Recommended values for the 665 delay for different RCODE values are given below. These recommended 666 values apply both to the default values a server should place in the 667 Retry Delay TLV, and the default values a client should assume if the 668 server provides no Retry Delay TLV. 670 For RCODE = 1 (FORMERR) the delay may be any value selected by the 671 implementer. A value of five minutes is RECOMMENDED, to reduce 672 the risk of high load from defective clients. 674 For RCODE = 2 (SERVFAIL) the delay should be chosen according to 675 the level of server overload and the anticipated duration of that 676 overload. By default, a value of one minute is RECOMMENDED. If a 677 more serious server failure occurs, the delay may be longer in 678 accordance with the specific problem encountered. 680 For RCODE = 4 (NOTIMP), which occurs on a server that doesn't 681 implement DNS Stateful Operations [RFC8490], it is unlikely that 682 the server will begin supporting DSO in the next few minutes, so 683 the retry delay SHOULD be one hour. Note that in such a case, a 684 server that doesn't implement DSO is unlikely to place a Retry 685 Delay TLV in its response, so this recommended value in particular 686 applies to what a client should assume by default. 688 For RCODE = 5 (REFUSED), which occurs on a server that implements 689 DNS Push Notifications, but is currently configured to disallow 690 DNS Push Notifications, the retry delay may be any value selected 691 by the implementer and/or configured by the operator. 693 If the server being queried is listed in a 694 "_dns-push-tls._tcp." SRV record for the zone, then this is 695 a misconfiguration, since this server is being advertised as 696 supporting DNS Push Notifications for this zone, but the server 697 itself is not currently configured to perform that task. Since it 698 is possible that the misconfiguration may be repaired at any time, 699 the retry delay should not be set too high. By default, a value 700 of 5 minutes is RECOMMENDED. 702 For RCODE = 9 (NOTAUTH), which occurs on a server that implements 703 DNS Push Notifications, but is not configured to be authoritative 704 for the requested name, the retry delay may be any value selected 705 by the implementer and/or configured by the operator. 707 If the server being queried is listed in a 708 "_dns-push-tls._tcp." SRV record for the zone, then this is 709 a misconfiguration, since this server is being advertised as 710 supporting DNS Push Notifications for this zone, but the server 711 itself is not currently configured to perform that task. Since it 712 is possible that the misconfiguration may be repaired at any time, 713 the retry delay should not be set too high. By default, a value 714 of 5 minutes is RECOMMENDED. 716 For RCODE = 11 (DSOTYPENI), which occurs on a server that 717 implements DSO but doesn't implement DNS Push Notifications, it is 718 unlikely that the server will begin supporting DNS Push 719 Notifications in the next few minutes, so the retry delay SHOULD 720 be one hour. 722 For other RCODE values, the retry delay should be set by the 723 server as appropriate for that error condition. By default, a 724 value of 5 minutes is RECOMMENDED. 726 For RCODE = 9 (NOTAUTH), the time delay applies to requests for other 727 names falling within the same zone. Requests for names falling 728 within other zones are not subject to the delay. For all other 729 RCODEs the time delay applies to all subsequent requests to this 730 server. 732 After sending an error response the server MAY allow the session to 733 remain open, or MAY send a DNS Push Notification Retry Delay 734 Operation TLV instructing the client to close the session, as 735 described in the DSO specification [RFC8490]. Clients MUST correctly 736 handle both cases. 738 6.3. DNS Push Notification Updates 740 Once a subscription has been successfully established, the server 741 generates PUSH messages to send to the client as appropriate. In the 742 case that the answer set was already non-empty at the moment the 743 subscription was established, an initial PUSH message will be sent 744 immediately following the SUBSCRIBE Response. Subsequent changes to 745 the answer set are then communicated to the client in subsequent PUSH 746 messages. 748 6.3.1. PUSH Message 750 A PUSH unidirectional message begins with the standard DSO 12-byte 751 header [RFC8490], followed by the PUSH primary TLV. A PUSH message 752 is illustrated in Figure 2. 754 In accordance with the definition of DSO unidirectional messages, the 755 MESSAGE ID field MUST be zero. There is no client response to a PUSH 756 message. 758 The other header fields MUST be set as described in the DSO spec- 759 ification [RFC8490]. The DNS OPCODE field contains the OPCODE value 760 for DNS Stateful Operations (6). The four count fields MUST be zero, 761 and the corresponding four sections MUST be empty (i.e., absent). 763 The DSO-TYPE is PUSH (tentatively 0x41). 765 The DSO-LENGTH is the length of the DSO-DATA that follows, which 766 specifies the changes being communicated. 768 The DSO-DATA contains one or more change notifications. A PUSH 769 Message MUST contain at least one change notification. If a PUSH 770 Message is received that contains no change notifications, this is a 771 fatal error, and the receiver MUST immediately terminate the 772 connection with a TCP RST (or equivalent for other protocols). 774 The change notification records are formatted similarly to how DNS 775 Resource Records are conventionally expressed in DNS messages, as 776 illustrated in Figure 2, and are interpreted as described below. 778 The TTL field holds an unsigned 32-bit integer [RFC2181]. If the TTL 779 is in the range 0 to 2,147,483,647 seconds (2^31 - 1, or 0x7FFFFFFF), 780 then a new DNS Resource Record with the given name, type, class and 781 RDATA is added. A TTL of 0 means that this record should be retained 782 for as long as the subscription is active, and should be discarded 783 immediately the moment the subscription is cancelled. 785 If the TTL has the value 0xFFFFFFFF, then the DNS Resource Record 786 with the given name, type, class and RDATA is removed. 788 If the TTL has the value 0xFFFFFFFE, then this is a 'collective' 789 remove notification. For collective remove notifications RDLEN MUST 790 be zero and consequently the RDATA MUST be empty. If a change 791 notification is received where TTL = 0xFFFFFFFE and RDLEN is not 792 zero, this is a fatal error, and the receiver MUST immediately 793 terminate the connection with a TCP RST (or equivalent for other 794 protocols). There are three types of collective remove notification: 796 For collective remove notifications, if CLASS is 255 (ANY), then for 797 the given name this deletes all records of all types in all classes. 798 In this case TYPE MUST be set to zero on transmission, and MUST be 799 silently ignored on reception. 801 For collective remove notifications, if CLASS is not 255 (ANY) and 802 TYPE is 255 (ANY) then for the given name this deletes all records of 803 all types in the specified class. 805 For collective remove notifications, if CLASS is not 255 (ANY) and 806 TYPE is not 255 (ANY) then for the given name this deletes all 807 records of the specified type in the specified class. 809 Summary of change notification types: 811 Delete all RRsets from a name, in all classes 812 TTL=0xFFFFFFFE, RDLENGTH=0, CLASS=255 (ANY) 814 Delete all RRsets from a name, in given class: 815 TTL=0xFFFFFFFE, RDLENGTH=0, CLASS specifies class, TYPE=255 (ANY) 817 Delete specified RRset from a name, in given class: 818 TTL=0xFFFFFFFE, RDLENGTH=0 819 CLASS and TYPE specify the RRset being deleted 821 Delete an individual RR from a name: 822 TTL=0xFFFFFFFF 823 CLASS, TYPE, RDLENGTH and RDATA specify the RR being deleted. 825 Add individual RR to a name 826 TTL>=0 827 CLASS, TYPE, RDLENGTH, RDATA and TTL specify the RR being added. 829 Note that it is valid for the RDATA of an added or removed DNS 830 Resource Record to be empty (zero length). For example, an Address 831 Prefix List Resource Record [RFC3123] may have empty RDATA. 832 Therefore, a change notification with RDLEN=0 does not automatically 833 indicate a remove notification. If RDLEN=0 and TTL is the in the 834 range 0 - 0x7FFFFFFF, this change notification signals the addition 835 of a record with the given name, type, class, and empty RDATA. If 836 RDLEN=0 and TTL = 0xFFFFFFFF, this change notification signals the 837 removal specifically of that single record with the given name, type, 838 class, and empty RDATA. 840 If the TTL is any value other than 0xFFFFFFFF, 0xFFFFFFFE, or a value 841 in the range 0 - 0x7FFFFFFF, then the receiver SHOULD silently ignore 842 this particular change notification record. The connection is not 843 terminated and other valid change notification records within this 844 PUSH message are processed as usual. 846 For efficiency, when generating a PUSH message, a server SHOULD 847 include as many change notifications as it has immediately available 848 to send, rather than sending each change notification as a separate 849 DSO message. Once it has exhausted the list of change notifications 850 immediately available to send, a server SHOULD then send the PUSH 851 message immediately, rather than waiting to see if additional change 852 notifications become available. 854 For efficiency, when generating a PUSH message, a server SHOULD use 855 standard DNS name compression, with offsets relative to the beginning 856 of the DNS message [RFC1035]. When multiple change notifications in 857 a single PUSH message have the same owner name, this name compression 858 can yield significant savings. Name compression should be performed 859 as specified in Section 18.14 of the Multicast DNS specification 860 [RFC6762], namely, owner names should always be compressed, and names 861 appearing within RDATA should be compressed for only the RR types 862 listed below: 864 NS, CNAME, PTR, DNAME, SOA, MX, AFSDB, RT, KX, RP, PX, SRV, NSEC 866 Servers may generate PUSH messages up to a maximum DNS message length 867 of 16,382 bytes, counting from the start of the DSO 12-byte header. 868 Including the two-byte length prefix that is used to frame DNS over a 869 byte stream like TLS, this makes a total of 16,384 bytes. Servers 870 MUST NOT generate PUSH messages larger than this. Where the 871 immediately available change notifications are sufficient to exceed a 872 DNS message length of 16,382 bytes, the change notifications MUST be 873 communicated in separate PUSH messages of up to 16,382 bytes each. 874 DNS name compression becomes less effective for messages larger than 875 16,384 bytes, so little efficiency benefit is gained by sending 876 messages larger than this. 878 If a client receives a PUSH message with a DNS message length larger 879 than 16,382 bytes, the this is a fatal error, and the receiver MUST 880 immediately terminate the connection with a TCP RST (or equivalent 881 for other protocols). 883 1 1 1 1 1 1 884 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 885 +--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+ \ 886 | MESSAGE ID (MUST BE ZERO) | \ 887 +--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+ | 888 |QR| OPCODE(6) | Z | RCODE | | 889 +--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+ | 890 | QDCOUNT (MUST BE ZERO) | | 891 +--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+ > HEADER 892 | ANCOUNT (MUST BE ZERO) | | 893 +--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+ | 894 | NSCOUNT (MUST BE ZERO) | | 895 +--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+ | 896 | ARCOUNT (MUST BE ZERO) | / 897 +--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+ / 898 | DSO-TYPE = PUSH (tentatively 0x41) | 899 +--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+ 900 | DSO-LENGTH (number of octets in DSO-DATA) | 901 +--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+ \ 902 \ NAME \ \ 903 +--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+ | 904 | TYPE | | 905 +--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+ | 906 | CLASS | | 907 +--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+ | 908 | TTL | | 909 | (32-bit unsigned big-endian integer) | > DSO-DATA 910 +--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+ | 911 | RDLEN | | 912 +--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+ | 913 \ RDATA (sized as necessary) \ | 914 +--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+ | 915 : NAME, TYPE, CLASS, TTL, RDLEN, RDATA : | 916 : Repeated As Necessary : / 917 +--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+ / 919 Figure 2: PUSH Message 921 When processing the records received in a PUSH Message, the receiving 922 client MUST validate that the records being added or deleted 923 correspond with at least one currently active subscription on that 924 session. Specifically, the record name MUST match the name given in 925 a SUBSCRIBE request, subject to the usual established DNS case- 926 insensitivity for US-ASCII letters. If the TYPE in the SUBSCRIBE 927 request was not ANY (255) then the TYPE of the record must match the 928 TYPE given in the SUBSCRIBE request. If the CLASS in the SUBSCRIBE 929 request was not ANY (255) then the CLASS of the record must match the 930 CLASS given in the SUBSCRIBE request. If a matching active 931 subscription on that session is not found, then that individual 932 record addition/deletion is silently ignored. Processing of other 933 additions and deletions in this message is not affected. The DSO 934 session is not closed. This is to allow for the unavoidable race 935 condition where a client sends an outbound UNSUBSCRIBE while inbound 936 PUSH messages for that subscription from the server are still in 937 flight. 939 In the case where a single change affects more than one active 940 subscription, only one PUSH message is sent. For example, a PUSH 941 message adding a given record may match both a SUBSCRIBE request with 942 the same TYPE and a different SUBSCRIBE request with TYPE=ANY (255). 943 It is not the case that two PUSH messages are sent because the new 944 record matches two active subscriptions. 946 The server SHOULD encode change notifications in the most efficient 947 manner possible. For example, when three AAAA records are deleted 948 from a given name, and no other AAAA records exist for that name, the 949 server SHOULD send a "delete an RRset from a name" PUSH message, not 950 three separate "delete an individual RR from a name" PUSH messages. 951 Similarly, when both an SRV and a TXT record are deleted from a given 952 name, and no other records of any kind exist for that name, the 953 server SHOULD send a "delete all RRsets from a name" PUSH message, 954 not two separate "delete an RRset from a name" PUSH messages. 956 A server SHOULD combine multiple change notifications in a single 957 PUSH message when possible, even if those change notifications apply 958 to different subscriptions. Conceptually, a PUSH message is a 959 session-level mechanism, not a subscription-level mechanism. 961 The TTL of an added record is stored by the client. While the 962 subscription is active, the TTL is not decremented, because a change 963 to the TTL would produce a new update. For as long as a relevant 964 subscription remains active, the client SHOULD assume that when a 965 record goes away the server will notify it of that fact. 966 Consequently, a client does not have to poll to verify that the 967 record is still there. Once a subscription is cancelled 968 (individually, or as a result of the DSO session being closed) record 969 aging for records covered by the subscription resumes and records are 970 removed from the local cache when their TTL reaches zero. 972 6.4. DNS Push Notification UNSUBSCRIBE 974 To cancel an individual subscription without closing the entire DSO 975 session, the client sends an UNSUBSCRIBE message over the established 976 DSO session to the server. The UNSUBSCRIBE message is encoded as a 977 DSO unidirectional message [RFC8490]. This specification defines a 978 primary unidirectional DSO TLV for DNS Push Notification UNSUBSCRIBE 979 Messages (tentatively DSO Type Code 0x42). 981 A server MUST NOT initiate an UNSUBSCRIBE message. If a server does 982 send an UNSUBSCRIBE message over a DSO session initiated by a client, 983 this is a fatal error and the client should immediately abort the 984 connection with a TCP RST (or equivalent for other protocols). 986 6.4.1. UNSUBSCRIBE Message 988 An UNSUBSCRIBE unidirectional message begins with the standard DSO 989 12-byte header [RFC8490], followed by the UNSUBSCRIBE primary TLV. 990 An UNSUBSCRIBE message is illustrated in Figure 3. 992 In accordance with the definition of DSO unidirectional messages, the 993 MESSAGE ID field MUST be zero. There is no server response to an 994 UNSUBSCRIBE message. 996 The other header fields MUST be set as described in the DSO spec- 997 ification [RFC8490]. The DNS OPCODE field contains the OPCODE value 998 for DNS Stateful Operations (6). The four count fields MUST be zero, 999 and the corresponding four sections MUST be empty (i.e., absent). 1001 The DSO-TYPE is UNSUBSCRIBE (tentatively 0x42). 1003 The DSO-LENGTH field contains the value 2, the length of the 2-octet 1004 MESSAGE ID contained in the DSO-DATA. 1006 The DSO-DATA contains the value given in the MESSAGE ID field of an 1007 active SUBSCRIBE request. This is how the server knows which 1008 SUBSCRIBE request is being cancelled. After receipt of the 1009 UNSUBSCRIBE message, the SUBSCRIBE request is no longer active. 1011 It is allowable for the client to issue an UNSUBSCRIBE message for a 1012 previous SUBSCRIBE request for which the client has not yet received 1013 a SUBSCRIBE response. This is to allow for the case where a client 1014 starts and stops a subscription in less than the round-trip time to 1015 the server. The client is NOT required to wait for the SUBSCRIBE 1016 response before issuing the UNSUBSCRIBE message. 1018 Consequently, it is possible for a server to receive an UNSUBSCRIBE 1019 message that does not match any currently active subscription. This 1020 can occur when a client sends a SUBSCRIBE request, which subsequently 1021 fails and returns an error code, but the client sent an UNSUBSCRIBE 1022 message before it became aware that the SUBSCRIBE request had failed. 1023 Because of this, servers MUST silently ignore UNSUBSCRIBE messages 1024 that do not match any currently active subscription. 1026 1 1 1 1 1 1 1027 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 1028 +--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+ \ 1029 | MESSAGE ID (MUST BE ZERO) | \ 1030 +--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+ | 1031 |QR| OPCODE(6) | Z | RCODE | | 1032 +--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+ | 1033 | QDCOUNT (MUST BE ZERO) | | 1034 +--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+ > HEADER 1035 | ANCOUNT (MUST BE ZERO) | | 1036 +--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+ | 1037 | NSCOUNT (MUST BE ZERO) | | 1038 +--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+ | 1039 | ARCOUNT (MUST BE ZERO) | / 1040 +--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+ / 1041 | DSO-TYPE = UNSUBSCRIBE (tentatively 0x42) | 1042 +--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+ 1043 | DSO-LENGTH (2) | 1044 +--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+ \ 1045 | SUBSCRIBE MESSAGE ID | > DSO-DATA 1046 +--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+ / 1048 Figure 3: UNSUBSCRIBE Message 1050 6.5. DNS Push Notification RECONFIRM 1052 Sometimes, particularly when used with a Discovery Proxy [DisProx], a 1053 DNS Zone may contain stale data. When a client encounters data that 1054 it believes may be stale (e.g., an SRV record referencing a target 1055 host+port that is not responding to connection requests) the client 1056 can send a RECONFIRM message to ask the server to re-verify that the 1057 data is still valid. For a Discovery Proxy, this causes it to issue 1058 new Multicast DNS queries to ascertain whether the target device is 1059 still present. How the Discovery Proxy causes these new Multicast 1060 DNS queries to be issued depends on the details of the underlying 1061 Multicast DNS implementation being used. For example, a Discovery 1062 Proxy built on Apple's dns_sd.h API responds to a DNS Push 1063 Notification RECONFIRM message by calling the underlying API's 1064 DNSServiceReconfirmRecord() routine. 1066 For other types of DNS server, the RECONFIRM operation is currently 1067 undefined, and SHOULD result in a NOERROR response, but otherwise 1068 need not cause any action to occur. 1070 Frequent use of RECONFIRM operations may be a sign of network 1071 unreliability, or some kind of misconfiguration, so RECONFIRM 1072 operations MAY be logged or otherwise communicated to a human 1073 administrator to assist in detecting, and remedying, such network 1074 problems. 1076 If, after receiving a valid RECONFIRM message, the server determines 1077 that the disputed records are in fact no longer valid, then 1078 subsequent DNS PUSH Messages will be generated to inform interested 1079 clients. Thus, one client discovering that a previously-advertised 1080 device (like a network printer) is no longer present has the side 1081 effect of informing all other interested clients that the device in 1082 question is now gone. 1084 6.5.1. RECONFIRM Message 1086 A RECONFIRM unidirectional message begins with the standard DSO 1087 12-byte header [RFC8490], followed by the RECONFIRM primary TLV. 1088 A RECONFIRM message is illustrated in Figure 4. 1090 In accordance with the definition of DSO unidirectional messages, the 1091 MESSAGE ID field MUST be zero. There is no server response to a 1092 RECONFIRM message. 1094 The other header fields MUST be set as described in the DSO spec- 1095 ification [RFC8490]. The DNS OPCODE field contains the OPCODE value 1096 for DNS Stateful Operations (6). The four count fields MUST be zero, 1097 and the corresponding four sections MUST be empty (i.e., absent). 1099 The DSO-TYPE is RECONFIRM (tentatively 0x43). 1101 The DSO-LENGTH is the length of the data that follows, which 1102 specifies the name, type, class, and content of the record being 1103 disputed. 1105 1 1 1 1 1 1 1106 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 1107 +--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+ \ 1108 | MESSAGE ID | \ 1109 +--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+ | 1110 |QR| OPCODE(6) | Z | RCODE | | 1111 +--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+ | 1112 | QDCOUNT (MUST BE ZERO) | | 1113 +--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+ > HEADER 1114 | ANCOUNT (MUST BE ZERO) | | 1115 +--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+ | 1116 | NSCOUNT (MUST BE ZERO) | | 1117 +--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+ | 1118 | ARCOUNT (MUST BE ZERO) | / 1119 +--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+ / 1120 | DSO-TYPE = RECONFIRM (tentatively 0x43) | 1121 +--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+ 1122 | DSO-LENGTH (number of octets in DSO-DATA) | 1123 +--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+ \ 1124 \ NAME \ \ 1125 +--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+ | 1126 | TYPE | | 1127 +--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+ > DSO-DATA 1128 | CLASS | | 1129 +--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+ | 1130 \ RDATA \ / 1131 +--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+ / 1133 Figure 4: RECONFIRM Message 1135 The DSO-DATA for a RECONFIRM message MUST contain exactly one record. 1136 The DSO-DATA for a RECONFIRM message has no count field to specify 1137 more than one record. Since RECONFIRM messages are sent over TCP, 1138 multiple RECONFIRM messages can be concatenated in a single TCP 1139 stream and packed efficiently into TCP segments. 1141 TYPE MUST NOT be the value ANY (255) and CLASS MUST NOT be the value 1142 ANY (255). 1144 DNS wildcarding is not supported. That is, a wildcard ("*") in a 1145 RECONFIRM message matches only a literal wildcard character ("*") in 1146 the zone, and nothing else. 1148 Aliasing is not supported. That is, a CNAME in a RECONFIRM message 1149 matches only a literal CNAME record in the zone, and nothing else. 1151 6.6. DNS Stateful Operations TLV Context Summary 1153 This document defines four new DSO TLVs. As suggested in Section 8.2 1154 of the DNS Stateful Operations specification [RFC8490], the valid 1155 contexts of these new TLV types are summarized below. 1157 The client TLV contexts are: 1159 C-P: Client request message, primary TLV 1160 C-U: Client unidirectional message, primary TLV 1161 C-A: Client request or unidirectional message, additional TLV 1162 CRP: Response back to client, primary TLV 1163 CRA: Response back to client, additional TLV 1165 +-------------+-----+-----+-----+-----+-----+ 1166 | TLV Type | C-P | C-U | C-A | CRP | CRA | 1167 +-------------+-----+-----+-----+-----+-----+ 1168 | SUBSCRIBE | X | | | | | 1169 | PUSH | | | | | | 1170 | UNSUBSCRIBE | | X | | | | 1171 | RECONFIRM | X | | | | | 1172 +-------------+-----+-----+-----+-----+-----+ 1174 Table 2: DSO TLV Client Context Summary 1176 The server TLV contexts are: 1178 S-P: Server request message, primary TLV 1179 S-U: Server unidirectional message, primary TLV 1180 S-A: Server request or unidirectional message, additional TLV 1181 SRP: Response back to server, primary TLV 1182 SRA: Response back to server, additional TLV 1184 +-------------+-----+-----+-----+-----+-----+ 1185 | TLV Type | S-P | S-U | S-A | SRP | SRA | 1186 +-------------+-----+-----+-----+-----+-----+ 1187 | SUBSCRIBE | | | | | | 1188 | PUSH | | X | | | | 1189 | UNSUBSCRIBE | | | | | | 1190 | RECONFIRM | | | | | | 1191 +-------------+-----+-----+-----+-----+-----+ 1193 Table 3: DSO TLV Server Context Summary 1195 6.7. Client-Initiated Termination 1197 An individual subscription is terminated by sending an UNSUBSCRIBE 1198 TLV for that specific subscription, or all subscriptions can be 1199 cancelled at once by the client closing the DSO session. When a 1200 client terminates an individual subscription (via UNSUBSCRIBE) or all 1201 subscriptions on that DSO session (by ending the session) it is 1202 signaling to the server that it is longer interested in receiving 1203 those particular updates. It is informing the server that the server 1204 may release any state information it has been keeping with regards to 1205 these particular subscriptions. 1207 After terminating its last subscription on a session via UNSUBSCRIBE, 1208 a client MAY close the session immediately, or it may keep it open if 1209 it anticipates performing further operations on that session in the 1210 future. If a client wishes to keep an idle session open, it MUST 1211 respect the maximum idle time required by the server [RFC8490]. 1213 If a client plans to terminate one or more subscriptions on a session 1214 and doesn't intend to keep that session open, then as an efficiency 1215 optimization it MAY instead choose to simply close the session, which 1216 implicitly terminates all subscriptions on that session. This may 1217 occur because the client computer is being shut down, is going to 1218 sleep, the application requiring the subscriptions has terminated, or 1219 simply because the last active subscription on that session has been 1220 cancelled. 1222 When closing a session, a client will generally do an abortive 1223 disconnect, sending a TCP RST. This immediately discards all 1224 remaining inbound and outbound data, which is appropriate if the 1225 client no longer has any interest in this data. In the BSD Sockets 1226 API, sending a TCP RST is achieved by setting the SO_LINGER option 1227 with a time of 0 seconds and then closing the socket. 1229 If a client has performed operations on this session that it would 1230 not want lost (like DNS updates) then the client SHOULD do an orderly 1231 disconnect, sending a TLS close_notify followed by a TCP FIN. (In 1232 the BSD Sockets API, sending a TCP FIN is achieved by calling 1233 "shutdown(s,SHUT_WR)" and keeping the socket open until all remaining 1234 data has been read from it.) 1236 7. Security Considerations 1238 The Strict Privacy Usage Profile for DNS over TLS is REQUIRED for DNS 1239 Push Notifications [RFC8310]. Cleartext connections for DNS Push 1240 Notifications are not permissible. Since this is a new protocol, 1241 transition mechanisms from the Opportunistic Privacy profile are 1242 unnecessary. 1244 Also, see Section 9 of the DNS over (D)TLS Usage Profiles document 1245 [RFC8310] for additional recommendations for various versions of TLS 1246 usage. 1248 DNSSEC is RECOMMENDED for the authentication of DNS Push Notification 1249 servers. TLS alone does not provide complete security. TLS 1250 certificate verification can provide reasonable assurance that the 1251 client is really talking to the server associated with the desired 1252 host name, but since the desired host name is learned via a DNS SRV 1253 query, if the SRV query is subverted then the client may have a 1254 secure connection to a rogue server. DNSSEC can provided added 1255 confidence that the SRV query has not been subverted. 1257 7.1. Security Services 1259 It is the goal of using TLS to provide the following security 1260 services: 1262 Confidentiality: All application-layer communication is encrypted 1263 with the goal that no party should be able to decrypt it except 1264 the intended receiver. 1266 Data integrity protection: Any changes made to the communication in 1267 transit are detectable by the receiver. 1269 Authentication: An end-point of the TLS communication is 1270 authenticated as the intended entity to communicate with. 1272 Deployment recommendations on the appropriate key lengths and cypher 1273 suites are beyond the scope of this document. Please refer to TLS 1274 Recommendations [RFC7525] for the best current practices. Keep in 1275 mind that best practices only exist for a snapshot in time and 1276 recommendations will continue to change. Updated versions or errata 1277 may exist for these recommendations. 1279 7.2. TLS Name Authentication 1281 As described in Section 6.1, the client discovers the DNS Push 1282 Notification server using an SRV lookup for the record name 1283 "_dns-push-tls._tcp.". The server connection endpoint SHOULD 1284 then be authenticated using DANE TLSA records for the associated SRV 1285 record. This associates the target's name and port number with a 1286 trusted TLS certificate [RFC7673]. This procedure uses the TLS 1287 Server Name Indication (SNI) extension [RFC6066] to inform the server 1288 of the name the client has authenticated through the use of TLSA 1289 records. Therefore, if the SRV record passes DNSSEC validation and a 1290 TLSA record matching the target name is useable, an SNI extension 1291 must be used for the target name to ensure the client is connecting 1292 to the server it has authenticated. If the target name does not have 1293 a usable TLSA record, then the use of the SNI extension is optional. 1294 See Usage Profiles for DNS over TLS and DNS over DTLS [RFC8310] for 1295 more information on authenticating domain names. 1297 7.3. TLS Session Resumption 1299 TLS Session Resumption is permissible on DNS Push Notification 1300 servers. The server may keep TLS state with Session IDs [RFC8446] or 1301 operate in stateless mode by sending a Session Ticket [RFC5077] to 1302 the client for it to store. However, closing the TLS connection 1303 terminates the DSO session. When the TLS session is resumed, the DNS 1304 Push Notification server will not have any subscription state and 1305 will proceed as with any other new DSO session. Use of TLS Session 1306 Resumption may allow a TLS connection to be set up more quickly, but 1307 the client will still have to recreate any desired subscriptions. 1309 8. IANA Considerations 1311 This document defines a new service name to be published in the IANA 1312 Registry Service Types [RFC6335][ST] that is only applicable for the 1313 TCP protocol. 1315 +-----------------------+------+----------------------+-------------+ 1316 | Name | Port | Value | Definition | 1317 +-----------------------+------+----------------------+-------------+ 1318 | DNS Push Notification | None | "_dns-push-tls._tcp" | Section 6.1 | 1319 | Service Type | | | | 1320 +-----------------------+------+----------------------+-------------+ 1322 Table 4: IANA Service Type Assignments 1324 This document also defines four new DNS Stateful Operation TLV types 1325 to be recorded in the IANA DSO Type Code Registry. 1327 +-------------+------------------------+-------------+ 1328 | Name | Value | Definition | 1329 +-------------+------------------------+-------------+ 1330 | SUBSCRIBE | TBA (tentatively 0x40) | Section 6.2 | 1331 | PUSH | TBA (tentatively 0x41) | Section 6.3 | 1332 | UNSUBSCRIBE | TBA (tentatively 0x42) | Section 6.4 | 1333 | RECONFIRM | TBA (tentatively 0x43) | Section 6.5 | 1334 +-------------+------------------------+-------------+ 1336 Table 5: IANA DSO TLV Type Code Assignments 1338 9. Acknowledgements 1340 The authors would like to thank Kiren Sekar and Marc Krochmal for 1341 previous work completed in this field. 1343 This draft has been improved due to comments from Ran Atkinson, Tim 1344 Chown, Mark Delany, Ralph Droms, Bernie Volz, Jan Komissar, Manju 1345 Shankar Rao, Markus Stenberg, Dave Thaler, Soraia Zlatkovic, Sara 1346 Dickinson, and Andrew Sullivan. Ted Lemon provided clarifying text 1347 that was greatly appreciated. 1349 10. References 1351 10.1. Normative References 1353 [RFC0768] Postel, J., "User Datagram Protocol", STD 6, RFC 768, 1354 DOI 10.17487/RFC0768, August 1980, 1355 . 1357 [RFC0793] Postel, J., "Transmission Control Protocol", STD 7, 1358 RFC 793, DOI 10.17487/RFC0793, September 1981, 1359 . 1361 [RFC1034] Mockapetris, P., "Domain names - concepts and facilities", 1362 STD 13, RFC 1034, DOI 10.17487/RFC1034, November 1987, 1363 . 1365 [RFC1035] Mockapetris, P., "Domain names - implementation and 1366 specification", STD 13, RFC 1035, DOI 10.17487/RFC1035, 1367 November 1987, . 1369 [RFC1123] Braden, R., Ed., "Requirements for Internet Hosts - 1370 Application and Support", STD 3, RFC 1123, 1371 DOI 10.17487/RFC1123, October 1989, 1372 . 1374 [RFC2119] Bradner, S., "Key words for use in RFCs to Indicate 1375 Requirement Levels", BCP 14, RFC 2119, 1376 DOI 10.17487/RFC2119, March 1997, 1377 . 1379 [RFC2136] Vixie, P., Ed., Thomson, S., Rekhter, Y., and J. Bound, 1380 "Dynamic Updates in the Domain Name System (DNS UPDATE)", 1381 RFC 2136, DOI 10.17487/RFC2136, April 1997, 1382 . 1384 [RFC2181] Elz, R. and R. Bush, "Clarifications to the DNS 1385 Specification", RFC 2181, DOI 10.17487/RFC2181, July 1997, 1386 . 1388 [RFC2782] Gulbrandsen, A., Vixie, P., and L. Esibov, "A DNS RR for 1389 specifying the location of services (DNS SRV)", RFC 2782, 1390 DOI 10.17487/RFC2782, February 2000, 1391 . 1393 [RFC6066] Eastlake 3rd, D., "Transport Layer Security (TLS) 1394 Extensions: Extension Definitions", RFC 6066, 1395 DOI 10.17487/RFC6066, January 2011, 1396 . 1398 [RFC6335] Cotton, M., Eggert, L., Touch, J., Westerlund, M., and S. 1399 Cheshire, "Internet Assigned Numbers Authority (IANA) 1400 Procedures for the Management of the Service Name and 1401 Transport Protocol Port Number Registry", BCP 165, 1402 RFC 6335, DOI 10.17487/RFC6335, August 2011, 1403 . 1405 [RFC6895] Eastlake 3rd, D., "Domain Name System (DNS) IANA 1406 Considerations", BCP 42, RFC 6895, DOI 10.17487/RFC6895, 1407 April 2013, . 1409 [RFC7673] Finch, T., Miller, M., and P. Saint-Andre, "Using DNS- 1410 Based Authentication of Named Entities (DANE) TLSA Records 1411 with SRV Records", RFC 7673, DOI 10.17487/RFC7673, October 1412 2015, . 1414 [RFC7766] Dickinson, J., Dickinson, S., Bellis, R., Mankin, A., and 1415 D. Wessels, "DNS Transport over TCP - Implementation 1416 Requirements", RFC 7766, DOI 10.17487/RFC7766, March 2016, 1417 . 1419 [RFC8174] Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC 1420 2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174, 1421 May 2017, . 1423 [RFC8446] Rescorla, E., "The Transport Layer Security (TLS) Protocol 1424 Version 1.3", RFC 8446, DOI 10.17487/RFC8446, August 2018, 1425 . 1427 [RFC8490] Bellis, R., Cheshire, S., Dickinson, J., Dickinson, S., 1428 Lemon, T., and T. Pusateri, "DNS Stateful Operations", 1429 RFC 8490, DOI 10.17487/RFC8490, March 2019, 1430 . 1432 [ST] "Service Name and Transport Protocol Port Number 1433 Registry", . 1436 10.2. Informative References 1438 [DisProx] Cheshire, S., "Discovery Proxy for Multicast DNS-Based 1439 Service Discovery", draft-ietf-dnssd-hybrid-10 (work in 1440 progress), March 2019. 1442 [I-D.dukkipati-tcpm-tcp-loss-probe] 1443 Dukkipati, N., Cardwell, N., Cheng, Y., and M. Mathis, 1444 "Tail Loss Probe (TLP): An Algorithm for Fast Recovery of 1445 Tail Losses", draft-dukkipati-tcpm-tcp-loss-probe-01 (work 1446 in progress), February 2013. 1448 [LLQ] Cheshire, S. and M. Krochmal, "DNS Long-Lived Queries", 1449 draft-sekar-dns-llq-03 (work in progress), March 2019. 1451 [obs] "Observer Pattern", 1452 . 1454 [RFC2308] Andrews, M., "Negative Caching of DNS Queries (DNS 1455 NCACHE)", RFC 2308, DOI 10.17487/RFC2308, March 1998, 1456 . 1458 [RFC3123] Koch, P., "A DNS RR Type for Lists of Address Prefixes 1459 (APL RR)", RFC 3123, DOI 10.17487/RFC3123, June 2001, 1460 . 1462 [RFC4287] Nottingham, M., Ed. and R. Sayre, Ed., "The Atom 1463 Syndication Format", RFC 4287, DOI 10.17487/RFC4287, 1464 December 2005, . 1466 [RFC4953] Touch, J., "Defending TCP Against Spoofing Attacks", 1467 RFC 4953, DOI 10.17487/RFC4953, July 2007, 1468 . 1470 [RFC5077] Salowey, J., Zhou, H., Eronen, P., and H. Tschofenig, 1471 "Transport Layer Security (TLS) Session Resumption without 1472 Server-Side State", RFC 5077, DOI 10.17487/RFC5077, 1473 January 2008, . 1475 [RFC6281] Cheshire, S., Zhu, Z., Wakikawa, R., and L. Zhang, 1476 "Understanding Apple's Back to My Mac (BTMM) Service", 1477 RFC 6281, DOI 10.17487/RFC6281, June 2011, 1478 . 1480 [RFC6762] Cheshire, S. and M. Krochmal, "Multicast DNS", RFC 6762, 1481 DOI 10.17487/RFC6762, February 2013, 1482 . 1484 [RFC6763] Cheshire, S. and M. Krochmal, "DNS-Based Service 1485 Discovery", RFC 6763, DOI 10.17487/RFC6763, February 2013, 1486 . 1488 [RFC6824] Ford, A., Raiciu, C., Handley, M., and O. Bonaventure, 1489 "TCP Extensions for Multipath Operation with Multiple 1490 Addresses", RFC 6824, DOI 10.17487/RFC6824, January 2013, 1491 . 1493 [RFC6886] Cheshire, S. and M. Krochmal, "NAT Port Mapping Protocol 1494 (NAT-PMP)", RFC 6886, DOI 10.17487/RFC6886, April 2013, 1495 . 1497 [RFC6887] Wing, D., Ed., Cheshire, S., Boucadair, M., Penno, R., and 1498 P. Selkirk, "Port Control Protocol (PCP)", RFC 6887, 1499 DOI 10.17487/RFC6887, April 2013, 1500 . 1502 [RFC7413] Cheng, Y., Chu, J., Radhakrishnan, S., and A. Jain, "TCP 1503 Fast Open", RFC 7413, DOI 10.17487/RFC7413, December 2014, 1504 . 1506 [RFC7525] Sheffer, Y., Holz, R., and P. Saint-Andre, 1507 "Recommendations for Secure Use of Transport Layer 1508 Security (TLS) and Datagram Transport Layer Security 1509 (DTLS)", BCP 195, RFC 7525, DOI 10.17487/RFC7525, May 1510 2015, . 1512 [RFC7719] Hoffman, P., Sullivan, A., and K. Fujiwara, "DNS 1513 Terminology", RFC 7719, DOI 10.17487/RFC7719, December 1514 2015, . 1516 [RFC7858] Hu, Z., Zhu, L., Heidemann, J., Mankin, A., Wessels, D., 1517 and P. Hoffman, "Specification for DNS over Transport 1518 Layer Security (TLS)", RFC 7858, DOI 10.17487/RFC7858, May 1519 2016, . 1521 [RFC8010] Sweet, M. and I. McDonald, "Internet Printing 1522 Protocol/1.1: Encoding and Transport", STD 92, RFC 8010, 1523 DOI 10.17487/RFC8010, January 2017, 1524 . 1526 [RFC8011] Sweet, M. and I. McDonald, "Internet Printing 1527 Protocol/1.1: Model and Semantics", STD 92, RFC 8011, 1528 DOI 10.17487/RFC8011, January 2017, 1529 . 1531 [RFC8310] Dickinson, S., Gillmor, D., and T. Reddy, "Usage Profiles 1532 for DNS over TLS and DNS over DTLS", RFC 8310, 1533 DOI 10.17487/RFC8310, March 2018, 1534 . 1536 [SYN] Eddy, W., "Defenses Against TCP SYN Flooding Attacks", The 1537 Internet Protocol Journal, Cisco Systems, Volume 9, 1538 Number 4, December 2006. 1540 [XEP0060] Millard, P., Saint-Andre, P., and R. Meijer, "Publish- 1541 Subscribe", XSF XEP 0060, July 2010. 1543 Authors' Addresses 1545 Tom Pusateri 1546 Unaffiliated 1547 Raleigh, NC 27608 1548 USA 1550 Phone: +1 919 867 1330 1551 Email: pusateri@bangj.com 1553 Stuart Cheshire 1554 Apple Inc. 1555 One Apple Park Way 1556 Cupertino, CA 95014 1557 USA 1559 Phone: +1 (408) 996-1010 1560 Email: cheshire@apple.com