<?xml version="1.0" encoding="UTF-8"?><!-- This template is for creating an Internet Draft using xml2rfc, which is available here: http://xml.resource.org. --><!DOCTYPE rfc SYSTEM"rfc2629.dtd" [ <!-- One method to get references from the online citation libraries. There has to be one entity for each item to be referenced. An alternate method (rfc include) is described in the references. --> <!ENTITY RFC0020 SYSTEM "https://xml2rfc.tools.ietf.org/public/rfc/bibxml/reference.RFC.0020.xml"> <!ENTITY RFC0768 SYSTEM "https://xml2rfc.tools.ietf.org/public/rfc/bibxml/reference.RFC.0768.xml"> <!ENTITY RFC0793 SYSTEM "https://xml2rfc.tools.ietf.org/public/rfc/bibxml/reference.RFC.0793.xml"> <!ENTITY RFC1034 SYSTEM "https://xml2rfc.tools.ietf.org/public/rfc/bibxml/reference.RFC.1034.xml"> <!ENTITY RFC1035 SYSTEM "https://xml2rfc.tools.ietf.org/public/rfc/bibxml/reference.RFC.1035.xml"> <!ENTITY RFC1123 SYSTEM "https://xml2rfc.tools.ietf.org/public/rfc/bibxml/reference.RFC.1123.xml"> <!ENTITY RFC2119 SYSTEM "https://xml2rfc.tools.ietf.org/public/rfc/bibxml/reference.RFC.2119.xml"> <!ENTITY RFC2136 SYSTEM "https://xml2rfc.tools.ietf.org/public/rfc/bibxml/reference.RFC.2136.xml"> <!ENTITY RFC2181 SYSTEM "https://xml2rfc.tools.ietf.org/public/rfc/bibxml/reference.RFC.2181.xml"> <!ENTITY RFC2308 SYSTEM "https://xml2rfc.tools.ietf.org/public/rfc/bibxml/reference.RFC.2308.xml"> <!ENTITY RFC3123 SYSTEM "https://xml2rfc.tools.ietf.org/public/rfc/bibxml/reference.RFC.3123.xml"> <!ENTITY RFC2782 SYSTEM "https://xml2rfc.tools.ietf.org/public/rfc/bibxml/reference.RFC.2782.xml"> <!ENTITY RFC4287 SYSTEM "https://xml2rfc.tools.ietf.org/public/rfc/bibxml/reference.RFC.4287.xml"> <!ENTITY RFC4953 SYSTEM "https://xml2rfc.tools.ietf.org/public/rfc/bibxml/reference.RFC.4953.xml"> <!ENTITY RFC6066 SYSTEM "https://xml2rfc.tools.ietf.org/public/rfc/bibxml/reference.RFC.6066.xml"> <!ENTITY RFC6281 SYSTEM "https://xml2rfc.tools.ietf.org/public/rfc/bibxml/reference.RFC.6281.xml"> <!ENTITY RFC6762 SYSTEM "https://xml2rfc.tools.ietf.org/public/rfc/bibxml/reference.RFC.6762.xml"> <!ENTITY RFC6763 SYSTEM "https://xml2rfc.tools.ietf.org/public/rfc/bibxml/reference.RFC.6763.xml"> <!ENTITY RFC6824 SYSTEM "https://xml2rfc.tools.ietf.org/public/rfc/bibxml/reference.RFC.6824.xml"> <!ENTITY RFC6886 SYSTEM "https://xml2rfc.tools.ietf.org/public/rfc/bibxml/reference.RFC.6886.xml"> <!ENTITY RFC6887 SYSTEM "https://xml2rfc.tools.ietf.org/public/rfc/bibxml/reference.RFC.6887.xml"> <!ENTITY RFC6895 SYSTEM "https://xml2rfc.tools.ietf.org/public/rfc/bibxml/reference.RFC.6895.xml"> <!ENTITY RFC7413 SYSTEM "https://xml2rfc.tools.ietf.org/public/rfc/bibxml/reference.RFC.7413.xml"> <!ENTITY RFC7673 SYSTEM "https://xml2rfc.tools.ietf.org/public/rfc/bibxml/reference.RFC.7673.xml"> <!ENTITY RFC7719 SYSTEM "https://xml2rfc.tools.ietf.org/public/rfc/bibxml/reference.RFC.7719.xml"> <!ENTITY RFC7766 SYSTEM "https://xml2rfc.tools.ietf.org/public/rfc/bibxml/reference.RFC.7766.xml"> <!ENTITY RFC7858 SYSTEM "https://xml2rfc.tools.ietf.org/public/rfc/bibxml/reference.RFC.7858.xml"> <!ENTITY RFC8010 SYSTEM "https://xml2rfc.tools.ietf.org/public/rfc/bibxml/reference.RFC.8010.xml"> <!ENTITY RFC8011 SYSTEM "https://xml2rfc.tools.ietf.org/public/rfc/bibxml/reference.RFC.8011.xml"> <!ENTITY RFC8174 SYSTEM "https://xml2rfc.tools.ietf.org/public/rfc/bibxml/reference.RFC.8174.xml"> <!ENTITY RFC8310 SYSTEM "https://xml2rfc.tools.ietf.org/public/rfc/bibxml/reference.RFC.8310.xml"> <!ENTITY RFC8446 SYSTEM "https://xml2rfc.tools.ietf.org/public/rfc/bibxml/reference.RFC.8446.xml"> <!ENTITY RFC8490 SYSTEM "https://xml2rfc.tools.ietf.org/public/rfc/bibxml/reference.RFC.8490.xml"> <!ENTITY RFC8499 SYSTEM "https://xml2rfc.tools.ietf.org/public/rfc/bibxml/reference.RFC.8499.xml"> <!ENTITY I-D.ietf-tcpm-rack SYSTEM "https://xml2rfc.tools.ietf.org/public/rfc/bibxml3/reference.I-D.ietf-tcpm-rack.xml"> ]> <?xml-stylesheet type='text/xsl' href='rfc2629.xslt' ?> <!-- used by XSLT processors --> <!-- For a complete list and description of processing instructions (PIs), please see http://xml.resource.org/authoring/README.html. --> <!-- Below are generally applicable Processing Instructions (PIs) that most I-Ds might want to use. (Here they are set differently than their defaults in xml2rfc v1.32) --> <?rfc strict="yes" ?> <!-- give errors regarding ID-nits and DTD validation --> <!-- control the table of contents (ToC) --> <?rfc toc="yes"?> <!-- generate a ToC --> <?rfc tocdepth="4"?> <!-- the number of levels of subsections in ToC. default: 3 --> <!-- control references --> <?rfc symrefs="yes"?> <!-- use symbolic references tags, i.e, [RFC2119] instead of [1] --> <?rfc sortrefs="yes" ?> <!-- sort the reference entries alphabetically --> <!-- control vertical white space (using these PIs as follows is recommended by the RFC Editor) --> <?rfc compact="yes" ?> <!-- do not start each main section on a new page --> <?rfc subcompact="no" ?> <!-- keep one blank line between list items --> <!-- end of list of popular I-D processing instructions -->"rfc2629-xhtml.ent"> <rfc xmlns:xi="http://www.w3.org/2001/XInclude" category="std" consensus="true" docName="draft-ietf-dnssd-push-25"ipr="trust200902"> <!-- category values: std, bcp, info, exp, and historic ipr values: trust200902, noModificationTrust200902, noDerivativesTrust200902, or pre5378Trust200902 you can add the attributes updates="NNNN" and obsoletes="NNNN" they will automatically be output with "(if approved)" --> <!-- ***** FRONT MATTER ***** -->number="8765" ipr="trust200902" obsoletes="" updates="" submissionType="IETF" xml:lang="en" tocInclude="true" tocDepth="4" symRefs="true" sortRefs="true" version="3"> <front><!-- The abbreviated title is used in the page header - it is only necessary if the full title is longer than 39 characters --><title abbrev="DNS Push Notifications">DNS Push Notifications</title><!-- add 'role="editor"' below for the editors if appropriate --> <!-- Another author who claims to be an editor --><seriesInfo name="RFC" value="8765"/> <author fullname="Tom Pusateri"initials="T.J."initials="T." surname="Pusateri"> <organization>Unaffiliated</organization> <address> <postal><street></street> <!-- Reorder these if your country does things differently --><street/> <city>Raleigh</city> <region>NC</region> <code>27608</code><country>USA</country><country>United States of America</country> </postal> <phone>+1 919 867 1330</phone> <email>pusateri@bangj.com</email><!-- uri and facsimile elements may also be added --></address> </author> <author fullname="Stuart Cheshire" initials="S." surname="Cheshire"> <organization>Apple Inc.</organization> <address> <postal> <street>One Apple Park Way</street><!-- Reorder these if your country does things differently --><city>Cupertino</city> <region>CA</region> <code>95014</code><country>USA</country><country>United States of America</country> </postal> <phone>+1 (408) 996-1010</phone> <email>cheshire@apple.com</email><!-- uri and facsimile elements may also be added --></address> </author> <dateyear='2019' month='October' day='13'/> <!-- If the month and year are both specified and are the current ones, xml2rfc will fill in the current day for you. If only the current year is specified, xml2rfc will fill in the current day and month for you. If the year is not the current one, it is necessary to specify at least a month (xml2rfc assumes day="1" if not specified for the purpose of calculating the expiry date). With drafts it is normally sufficient to specify just the year. --> <!-- Meta-data Declarations --> <area>DNSSD</area> <workgroup>Internet Engineering Task Force</workgroup> <!-- WG name at the upper left corner of the doc, IETF is fine for individual submissions. If this element is not present, the default is "Network Working Group", which is used by the RFC Editor as a nod to the history of the IETF. --> <keyword>dns update pushyear="2020" month="June"/> <area>INT</area> <workgroup>DNSSD</workgroup> <keyword>Push notification</keyword> <keyword>Asynchronous notification</keyword><!-- Keywords will be incorporated into HTML output files in a meta tag but they have no effect on text or nroff output. If you submit your draft to the RFC Editor, the keywords will be used for the search engine. --><abstract> <t>The Domain Name System (DNS) was designed to return matching records efficiently for queries for data that are relatively static. When those records change frequently, DNS is still efficient at returning the updated results when polled, as long as the polling rate is not too high.ButBut, there exists no mechanism for a client to be asynchronously notified when these changes occur. This document defines a mechanism for a client to be notified of such changes to DNS records, called DNS Push Notifications.</t> </abstract> </front> <middle><?rfc needLines="14" ?><sectiontitle="Introduction">numbered="true" toc="default"> <name>Introduction</name> <t>Domain Name System (DNS) records may be updated using DNS Update <xreftarget="RFC2136">DNS Update</xref>.target="RFC2136" format="default"></xref>. Other mechanisms such as a Discovery Proxy <xreftarget="DisProx">Discovery Proxy</xref>target="RFC8766" format="default"></xref> can also generate changes to a DNS zone. This document specifies a protocol for DNS clients to subscribe to receive asynchronous notifications of changes to RRsets of interest. It is immediately relevant in the case of<xref target="RFC6763">DNSDNS-based ServiceDiscovery</xref>Discovery <xref target="RFC6763" format="default"></xref> but is not limited to that usecase, andcase; it provides a general DNS mechanism for DNS record change notifications. Familiarity with the DNS protocol and DNS packet formats is assumed <xreftarget="RFC1034"/>target="RFC1034" format="default"/> <xreftarget="RFC1035"/>target="RFC1035" format="default"/> <xreftarget="RFC6895"/>.</t> <?rfc needLines="7" ?>target="RFC6895" format="default"/>.</t> <sectiontitle="Requirements Language"> <t>Thenumbered="true" toc="default"> <name>Requirements Language</name> <t> The key words"MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", "SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY","<bcp14>MUST</bcp14>", "<bcp14>MUST NOT</bcp14>", "<bcp14>REQUIRED</bcp14>", "<bcp14>SHALL</bcp14>", "<bcp14>SHALL NOT</bcp14>", "<bcp14>SHOULD</bcp14>", "<bcp14>SHOULD NOT</bcp14>", "<bcp14>RECOMMENDED</bcp14>", "<bcp14>NOT RECOMMENDED</bcp14>", "<bcp14>MAY</bcp14>", and"OPTIONAL""<bcp14>OPTIONAL</bcp14>" in this document are to be interpreted as described in BCP 14 <xref target="RFC2119"/> <xref target="RFC8174"/> when, and only when, they appear inall capitals, as shown here. These words may also appear in this document in lower caseall capitals, asplain English words, absent their normative meanings.</t>shown here. </t> </section> <sectiontitle="Fatal Errors">numbered="true" toc="default"> <name>Fatal Errors</name> <t>Certain invalid situations are described in this specification,likesuch as a server sending a Push Notification subscription request to a client, or a client sending a Push Notification response to a server. These should never occur with a correctly implemented client and server, and if they dooccuroccur, then they indicate a serious implementation error. In these extremecasescases, there is no reasonable expectation of a graceful recovery, and the recipient detecting the error should respond by unilaterally aborting the session without regard for data loss. Such cases are addressed by having an engineer investigate the cause of the failure and fixing the problem in the software.</t> <t>Where this specification says "forcibly abort", it means sending a TCP RST to terminate the TCPconnection,connection and the TLS session running over that TCP connection. In the BSD Sockets API, this is achieved by setting the SO_LINGER option to zero before closing the socket.</t><?rfc needLines="40" ?></section> </section> <sectiontitle="Motivation">numbered="true" toc="default"> <name>Motivation</name> <t>As the domain name system continues to adapt to new uses and changes in deployment, polling has the potential to burden DNS servers at many levels throughout the network. Other network protocols have successfully deployed a publish/subscribe model following the<xref target="obs">ObserverObserver designpattern</xref>.pattern <xref target="OBS" format="default"></xref>. Extensible Messaging and Presence Protocol (XMPP) Publish-Subscribe <xreftarget="XEP0060">XMPP Publish-Subscribe</xref>target="XEP0060" format="default"></xref> and Atom <xreftarget="RFC4287">Atom</xref>target="RFC4287" format="default"></xref> are examples. While DNS servers are generally highly tuned and capable of a high rate of query/response traffic, adding a publish/subscribe model for tracking changes to DNS records can deliver more timelynotificationnotifications of changes with reduced CPU usage and lower network traffic.</t><t><xref target="RFC6762">Multicast DNS</xref><t>The guiding design principle of DNS Push Notifications is that clients that choose to use DNS Push Notifications, instead of repeated polling with DNS queries, will receive the same results as they could via sufficiently rapid polling, except more efficiently. This means that the rules for which records match a given DNS Push Notification subscription are the same as the already established rules used to determine which records match a given DNS query <xref target="RFC1034" format="default"/>. For example, name comparisons are done in a case-insensitive manner, and a record of type CNAME in a zone matches any DNS TYPE in a query or subscription.</t> <t>Multicast DNS <xref target="RFC6762" format="default"></xref> implementations always listen on awell knownwell-known link-local IP multicast group address, and changes are sent to that multicast group address for all group members to receive. Therefore, Multicast DNS already has asynchronous change notification capability. When<xref target="RFC6763">DNSDNS-based ServiceDiscovery</xref>Discovery <xref target="RFC6763" format="default"></xref> is used across a wide area network using Unicast DNS (possibly facilitated via a Discovery Proxy <xreftarget="DisProx">Discovery Proxy</xref>)target="RFC8766" format="default"></xref>), it would be beneficial to have an equivalent capability for UnicastDNS,DNS in order to allow clients to learn about DNS record changes in a timely manner without polling.</t> <t>The<xref target="LLQ">DNSDNS Long-Lived Queries (LLQ)mechanism</xref>mechanism <xref target="RFC8764" format="default"></xref> is an existing deployed solution to provide asynchronous changenotifications,notifications; it was used by Apple's<xref target="RFC6281">BackBack to MyMac</xref>Mac <xref target="RFC6281" format="default"></xref> service introduced in Mac OS X 10.5 Leopard in 2007. Back to My Mac was designed in an era when the data center operations staff asserted that it was impossible for a server to handle large numbers ofmostly-idleTCP connections,soeven if those connections carried very little traffic and spent most of their time idle. Consequently, LLQ was defined as a UDP-based protocol, effectively replicating much of TCP's connection state management logic in userspace,space and creating its own imitation of existing TCP features likethe three-way handshake,flow control, reliability, andreliability.</t>the three-way handshake.</t> <t>This document builds on experience gained with the LLQ protocol, with an improved design. Instead of using UDP, this specification uses<xref target="RFC8490">DNSDNS Stateful Operations(DSO)</xref>(DSO) <xref target="RFC8490" format="default"></xref> running over TLS over TCP, and therefore doesn't need to reinvent existing TCP functionality. Using TCP also gives long-lived low-traffic connections better longevity through NAT gateways without depending on the gateway to support<xref target="RFC6886">NATNAT Port Mapping Protocol(NAT-PMP)</xref> or(NAT-PMP) <xreftarget="RFC6887">Porttarget="RFC6886" format="default"></xref> or Port Control Protocol(PCP)</xref>,(PCP) <xref target="RFC6887" format="default"></xref>, or resorting to excessive keepalive traffic.</t><?rfc needLines="9" ?></section> <sectiontitle="Overview">numbered="true" toc="default"> <name>Overview</name> <t>A DNS Push Notification client subscribes for Push Notifications for a particular RRset by connecting to the appropriate Push Notification server for thatRRset,RRset and sending DSO message(s) indicating the RRset(s) of interest. When the client loses interest in receiving further updates to these records, it unsubscribes.</t> <t>The DNS Push Notification server for a DNS zone is any server capable of generating the correct change notifications for a name. It may be a primary, secondary, or stealth name server <xreftarget="RFC7719"/>.</t>target="RFC8499" format="default"/>.</t> <t>The<spanx style="verb">_dns&nbhy;push&nbhy;tls._tcp.<zone></spanx><tt>_dns&nbhy;push&nbhy;tls._tcp.<zone></tt> SRV record for a zoneMAY<bcp14>MAY</bcp14> reference the same target host and port as that zone's<spanx style="verb">_dns&nbhy;update&nbhy;tls._tcp.<zone></spanx><tt>_dns&nbhy;update&nbhy;tls._tcp.<zone></tt> SRV record. When the same target host and port is offered for both DNS Updates and DNS Push Notifications, a clientMAY<bcp14>MAY</bcp14> use a single DSO session to that server for both DNS Updates and DNS Push NotificationSubscriptions.subscriptions. DNS Updates and DNS Push Notifications may be handled on different ports on the same target host, in which case they are not considered to be the "same server" for the purposes of this specification, and communications with these two ports are handled independently. Supporting DNS Updates and DNS Push Notifications on the same server isOPTIONAL.<bcp14>OPTIONAL</bcp14>. A DNS Push Notification server is not required to support DNS Update.</t> <t>Standard DNS QueriesMAY<bcp14>MAY</bcp14> be sent over a DNS Push Notification (i.e., DSO) session. For any zone for which the server is authoritative, itMUST<bcp14>MUST</bcp14> respond authoritatively for queries for names falling within that zone (e.g., the<spanx style="verb">_dns&nbhy;push&nbhy;tls._tcp.<zone></spanx><tt>_dns&nbhy;push&nbhy;tls._tcp.<zone></tt> SRV record) both for normal DNS queries and for DNS Push Notification subscriptions. For names for which the server is acting as a recursive resolver (e.g., when the server is the local recursive resolver) for any query for which it supports DNS Push Notification subscriptions, itMUST<bcp14>MUST</bcp14> also support standard queries.</t> <t>DNS Push Notifications impose less load on the responding server than rapid polling would, but Push Notifications do still have acost, socost. Therefore, DNS Push Notification clientsMUST NOT<bcp14>MUST NOT</bcp14> recklessly create an excessive number of Push Notification subscriptions. Specifically:</t><t>(a) A<ol type="(%c)" > <li>A subscription should only be active when there is a valid reason to need live data (for example, an on-screen display is currently showing the results to theuser)user), and the subscriptionSHOULD<bcp14>SHOULD</bcp14> becancelledcanceled as soon as the need for that data ends (for example, when the user dismisses that display). In the case of a device like a smartphonewhich,that, after some period of inactivity, goes to sleep or otherwise darkens its screen, it should cancel its subscriptions when darkening the screen (since the user cannot see any changes on the display anyway) and reinstate its subscriptions whenre-awakeningreawakening from displaysleep.</t> <t>(b) Asleep. </li> <li>A DNS Push Notification clientSHOULD NOT<bcp14>SHOULD NOT</bcp14> routinely keep a DNS Push Notification subscription active 24 hours a day, 7 days a week, just to keep a list in memory up to date so that if the user does choose to bring up an on-screen display of that data, it can be displayed really fast. DNS Push Notifications are designed to be fast enough that there is no need to pre-load a "warm" list in memory just in case it might be neededlater.</t>later. </li> </ol> <t>Generally, as described in the<xref target="RFC8490">DNSDNS Stateful Operationsspecification</xref>,specification <xref target="RFC8490" format="default"></xref>, a client must not keep a DSO session to a server open indefinitely if it has no subscriptions (or other operations) active on that session. A clientmay closeshould begin closing a DSO session immediately after it becomes idle, andthenthen, if needed in the future, open a new session when required. Alternatively, a client may speculatively keep an idle DSO session open for some time, subject to the constraint that it must not keep a session open that has been idle for more than the session's idle timeout (15 seconds by default) <xreftarget="RFC8490"/>.</t>target="RFC8490" format="default"/>.</t> <t>Note that a DSO session that has an active DNS Push Notification subscription is not considered idle, even if there is no traffic flowing for an extended period of time. In thiscasecase, the DSO inactivity timeout does not apply, because the session is not inactive, but the keepalive interval does still apply, to ensure the generation of sufficient messages to maintain state in middleboxes (such at NAT gateways or firewalls) and for the client and server to periodically verify that they still have connectivity to each other. This is described inSection 6.2 of the<xreftarget="RFC8490">DSOtarget="RFC8490" sectionFormat="of" section="6.2">the DSO specification</xref>.</t><?rfc needLines="14" ?></section> <sectiontitle="State Considerations">numbered="true" toc="default"> <name>State Considerations</name> <t>Each DNS Push Notification server is capable of handling some finite number of Push Notification subscriptions. This number will vary from server to server and is based on physical machine characteristics, networkbandwidth,capacity, and operating system resource allocation. After a client establishes a session to a DNS server, each subscription is individually accepted or rejected. Servers may employ various techniques to limit subscriptions to a manageable level. Correspondingly, the client is free to establish simultaneous sessions to alternate DNS servers that support DNS Push Notifications for the zone and distribute subscriptions at the client's discretion. In this way, both clients and servers can react to resource constraints.</t><?rfc needLines="35" ?></section> <sectiontitle="Transport">numbered="true" toc="default"> <name>Transport</name> <t>Other DNS operations like DNS Update <xreftarget="RFC2136">DNS Update</xref> MAYtarget="RFC2136" format="default"></xref> <bcp14>MAY</bcp14> use either DNS over User Datagram Protocol (UDP) <xreftarget="RFC0768">(UDP)</xref>target="RFC0768" format="default"></xref> or DNS over Transmission Control Protocol (TCP) <xreftarget="RFC0793">(TCP)</xref>target="RFC0793" format="default"></xref> as the transport protocol,in keeping withprovided they follow the historical precedent that DNS queries must first be sent using DNS over UDP and only switch to DNS over TCP if needed <xreftarget="RFC1123"/>.target="RFC1123" format="default"/>. This requirement touseprefer UDP has subsequently been relaxed <xreftarget="RFC7766"/>.</t>target="RFC7766" format="default"/>.</t> <t>In keeping with the more recent precedent, DNS Push Notification is defined only for TCP. DNS Push Notification clientsMUST<bcp14>MUST</bcp14> use<xref target="RFC8490">DNSDNS StatefulOperations</xref>Operations <xref target="RFC8490" format="default"></xref> running over TLS over TCP <xreftarget="RFC7858"/>.</t> <t>Connectiontarget="RFC7858" format="default"/>.</t> <t> Connection setup over TCP ensures return reachability and alleviates concerns of state overload at the server,which isa potential problem with connectionless protocols, which can be more vulnerable to being exploited by attackers using spoofed source addresses. All subscribers are guaranteed to be reachable by the server by virtue of the TCP three-way handshake. Flooding attacks are possible with any protocol, and a benefit of TCP is that there are already established industry best practices to guard against SYN flooding and similar attacks <xreftarget="SYN"/>target="SYN" format="default"/> <xreftarget="RFC4953"/>.</t>target="RFC4953" format="default"/>.</t> <t>Use of TCP also allows DNS Push Notifications to take advantage of current and future developments inTCP,TCP such as<xref target="RFC6824">MultipathMultipath TCP(MPTCP)</xref>,(MPTCP) <xreftarget="RFC7413">TCPtarget="RFC8684" format="default"></xref>, TCP Fast Open(TFO)</xref>,(TFO) <xreftarget="I-D.ietf-tcpm-rack">thetarget="RFC7413" format="default"></xref>, the TCP RACK fast loss detectionalgorithm</xref>,algorithm <xref target="I-D.ietf-tcpm-rack" format="default"></xref>, and so on.</t> <t>Transport Layer Security (TLS) <xreftarget="RFC8446">(TLS)</xref>target="RFC8446" format="default"></xref> is wellunderstood,understood and is used by many application-layer protocols running over TCP. TLS is designed to prevent eavesdropping, tampering, and message forgery. TLS isREQUIRED<bcp14>REQUIRED</bcp14> for every connection between a client subscriber and server in this protocol specification. Additional security measures such as client authentication during TLS negotiation may also be employed to increase the trust relationship between client and server.</t><?rfc needLines="25" ?></section> <sectiontitle="Protocol Operation">numbered="true" toc="default"> <name>Protocol Operation</name> <t>The DNS Push Notification protocol is a session-orientedprotocol,protocol and makes use of<xref target="RFC8490">DNSDNS Stateful Operations(DSO)</xref>.</t>(DSO) <xref target="RFC8490" format="default"></xref>.</t> <t>For details of the DSO messageformatformat, refer to the<xref target="RFC8490">DNSDNS StatefulOper-ations specification</xref>.Operations specification <xref target="RFC8490" format="default"></xref>. Those details are not repeated here.</t> <t>DNS Push Notification clients and serversMUST<bcp14>MUST</bcp14> support DSO. A single server can support DNS Queries, DNS Updates, and DNS Push Notifications (using DSO) on the same TCP port.</t> <t>A DNS Push Notification exchange begins with the client discovering the appropriate server, using the procedure described in <xreftarget="discovery"/>,target="discovery" format="default"/>, and then making a TLS/TCP connection to it.</t><t>A<t>After making the TLS/TCP connection to the server, a typical DNS Push Notification client will then immediately issue a DSO Keepalive operation to establish the DSO session and request a session timeout and/or keepalive interval longer than the 15-second default values, but this is not required. A DNS Push Notification clientMAY<bcp14>MAY</bcp14> issue other requests on the session first, and only issue a DSO Keepalive operation later if it determines that to be necessary. Sending either a DSO Keepalive operation or a Push Notification subscription request over the TLS/TCP connection to the server signals the client's support of DSO and serves to establish a DSO session.</t> <t>In accordance with the current set of active subscriptions, the server sends relevant asynchronous Push Notifications to the client. Note that a clientMUST<bcp14>MUST</bcp14> be prepared to receive (and silently ignore) Push Notifications for subscriptions it has previously removed, since there is no way to prevent the situation where a Push Notification is in flight from server to client while the client's UNSUBSCRIBE messagecancellingcanceling that subscription is simultaneously in flight from client to server.</t><?rfc needLines="30" ?><sectiontitle="Discovery" anchor="discovery">anchor="discovery" numbered="true" toc="default"> <name>Discovery</name> <t>The first step in establishing a DNS Push Notification subscription is to discover an appropriate DNS server that supports DNS Push Notifications for the desired zone.</t> <t>The client begins by opening a DSOSessionsession to its normal configured DNS recursive resolver and requesting a Push Notification subscription. This connection is made to TCP port 853, the default port for DNS over TLS <xreftarget="RFC7858">DNS-over-TLS</xref>.target="RFC7858" format="default"></xref>. If the request for a Push Notification subscription is successful, and the recursive resolver doesn't already have an active subscription for that name, type, and class, then the recursive resolver will make a corresponding Push Notification subscription on the client's behalf. Results received are relayed to the client. This is closely analogous to how a client sends a normal DNS query to its configured DNS recursiveresolverresolver, which, if it doesn't already have appropriate answer(s) in its cache, issues an upstream query to satisfy the request.</t> <t>In many contexts, the recursive resolver will be able to handle Push Notifications for all names that the client may need to follow. Use of VPN tunnels and Private DNS <xreftarget="RFC8499"/>target="RFC8499" format="default"/> can create some additional complexity in the client software here; the techniques to handle VPN tunnels and Private DNS for DNS Push Notifications are the same as those already used to handle this for normal DNS queries.</t> <t>If the recursive resolver does not support DNS over TLS, or supports DNS over TLS but is not listening on TCP port 853, or supports DNS over TLS on TCP port 853 but does not support DSO on that port, then the DSOSessionsession establishment will fail <xreftarget="RFC8490"/>.</t>target="RFC8490" format="default"/>.</t> <t>If the recursive resolver does support DSO on TCP port 853 but does not support Push Notification subscriptions, thenitwhen the client attempts to create a subscription, the server will return the DSO error code DSOTYPENI (11).</t> <t>In some cases, the recursive resolver may support DSO and Push Notificationsubscriptions,subscriptions but may not be able to subscribe for Push Notifications for a particular name. In this case, the recursive resolver should return SERVFAIL to the client. This includes being unable to establish a connection to the zone's DNS Push Notification server or establishing a connection but receiving anon successnon-success response code. In some cases, where the client has a pre-established trust relationship with the owner of the zone (that is not handled via the usual mechanisms for VPNsoftware)software), the client may handle these failures by contacting the zone's DNS Push Notification server directly.</t> <t>In any of the cases described above where the client fails to establish a DNS Push Notification subscription via its configured recursive resolver, the client should proceed to discover the appropriate server for direct communication. The clientMUST<bcp14>MUST</bcp14> also determine on which TCP portonthe server is listening for connections, which need notbe (andbe, and often isnot) the typicalnot, TCP port 53 (traditionally used for conventionalDNS,DNS) or TCP port 853 (traditionally used for DNS overTLS.</t>TLS).</t> <t>The discovery algorithm described here is an iterative algorithm, which starts with the full name of the record to which the client wishes to subscribe. Successive SOA queries are then issued, trimming one label each time, until the closest enclosing authoritative server is discovered. There is also an optimization to enable the client to take a "short cut" directly to the SOA record of the closest enclosing authoritative server in many cases.</t><t> <list style="numbers"> <t>The<ol spacing="normal" type="1"> <li>The client begins the discovery by sending a DNS query to its local resolver, with record type SOA <xreftarget="RFC1035">SOA</xref>target="RFC1035" format="default"></xref> for the record name to which it wishes to subscribe. As an example, suppose the client wishes to subscribe to PTR records with the name_ipp._tcp.headoffice.example.com<tt>_ipp._tcp.headoffice.example.com</tt> (to discover Internet Printing Protocol (IPP) printers <xreftarget="RFC8010"/>target="RFC8010" format="default"/> <xreftarget="RFC8011"/>target="RFC8011" format="default"/> being advertised in the head office of ExampleCompany.).Company). The client begins by sending an SOA query for_ipp._tcp.headoffice.example.com<tt>_ipp._tcp.headoffice.example.com</tt> to the local recursive resolver. The goal is to determine the server that is authoritative for the name_ipp._tcp.headoffice.example.com.<tt>_ipp._tcp.headoffice.example.com</tt>. The closest enclosing DNS zone containing the name_ipp._tcp.headoffice.example.com<tt>_ipp._tcp.headoffice.example.com</tt> could beexample.com,<tt>example.com</tt>, orheadoffice.example.com,<tt>headoffice.example.com</tt>, or_tcp.headoffice.example.com,<tt>_tcp.headoffice.example.com</tt>, or even_ipp._tcp.headoffice.example.com.<tt>_ipp._tcp.headoffice.example.com</tt>. The client does not know in advance where the closest enclosing zone cut occurs, which is why it uses the iterative procedure described here to discover thisinformation.</t>information.</li> <li> <t>If the requested SOA record exists, it will be returned in the AnswersectionSection with a NOERROR response code, and the client has succeeded in discovering the information it needs.<vspace /></t> <t> (This language is not placing any new requirements on DNS recursive resolvers. This text merely describes the existing operation of the DNS protocol <xreftarget="RFC1034"/>target="RFC1034" format="default"/> <xreftarget="RFC1035"/>.)</t>target="RFC1035" format="default"/>.)</t> </li> <li> <t>If the requested SOA record does not exist, the client will get back a NOERROR/NODATA response or an NXDOMAIN/Name Error response. In either case, the local resolver would normally include the SOA record for the closest enclosing zone of the requested name in the Authority Section. If the SOA record is received in the Authority Section, then the client has succeeded in discovering the information it needs.<vspace /></t> <t> (This language is not placing any new requirements on DNS recursive resolvers. This text merely describes the existing operation of the DNS protocol regarding negative responses <xreftarget="RFC2308"/>.)</t> <t>Iftarget="RFC2308" format="default"/>.)</t> </li> <li>If the client receives a response containing no SOA record, then it proceeds with the iterative approach. The client strips the leading label from the current query name, and if the resulting name has at least two labels in it, then the client sends an SOA query for that newname,name and processing continues at step 2 above, repeating the iterative search until either an SOA isreceived,received or the query name consists of a single label, i.e., aTop LevelTop-Level Domain (TLD). In the case of a single-label name (TLD), this is a network configuration error, which should not happen, and the client gives up. The client may retry the operation at a latertime,time of the client's choosing, such as after a change in networkattachment.</t> <t>Onceattachment.</li> <li>Once the SOA is known(either by(by virtue of being seen either in the AnswerSection,Section or in the Authority Section), the client sends a DNS query with type SRV <xreftarget="RFC2782">SRV</xref>target="RFC2782" format="default"></xref> for the record name<spanx style="verb">_dns&nbhy;push&nbhy;tls._tcp.<zone></spanx>,<tt>_dns&nbhy;push&nbhy;tls._tcp.<zone></tt>, where <zone> is the owner name of the discovered SOArecord.</t> <t>Ifrecord.</li> <li>If the zone in question is set up to offer DNS PushNotificationsNotifications, then this SRV recordMUST<bcp14>MUST</bcp14> exist. (If this SRV record does notexistexist, then the zone is not correctly configured for DNS Push Notifications as specified in this document.) The SRV<spanx style="verb">target</spanx><tt>target</tt> contains the name of the server providing DNS Push Notifications for the zone. The port number on which to contact the server is in the SRV record<spanx style="verb">port</spanx><tt>port</tt> field. The address(es) of the target hostMAY<bcp14>MAY</bcp14> be included in the Additional Section, however, the address recordsSHOULD<bcp14>SHOULD</bcp14> be authenticated before use as describedbelowin <xreftarget="tls_name_auth"/>target="tls_name_auth" format="default"/> and in<xref target="RFC7673">thethe specification for usingDANEDNS-Based Authentication of Named Entities (DANE) TLSA Records with SRVRecords</xref>,Records <xref target="RFC7673" format="default"></xref>, ifapplicable.</t> <tapplicable.</li> <li anchor="SRV">More than one SRV record may be returned. In this case, the<spanx style="verb">priority</spanx><tt>priority</tt> and<spanx style="verb">weight</spanx><tt>weight</tt> values in the returned SRV records are used to determine the order in which to contact the servers for subscription requests. As described in<xref target="RFC2782">thethe SRVspecification</xref>,specification <xref target="RFC2782" format="default"></xref>, the server with the lowest<spanx style="verb">priority</spanx><tt>priority</tt> is first contacted. If more than one server has the same<spanx style="verb">priority</spanx>,<tt>priority</tt>, the<spanx style="verb">weight</spanx><tt>weight</tt> indicates the weighted probability that the client should contact that server. Higher weights have higher probabilities of being selected. If a server is not willing to accept a subscription request, or is not reachable within a reasonable time, as determined by the client, then a subsequent server is to becontacted.</t> </list> </t>contacted.</li> </ol> <t>Each time a client makes a new DNS Push Notification subscription, itSHOULD<bcp14>SHOULD</bcp14> repeat the discovery process in order to determine the preferred DNS server for that subscription at that time. If a client already has a DSO session with that DNSserverserver, the clientSHOULD<bcp14>SHOULD</bcp14> reuse that existing DSO session for the newsubscription,subscription; otherwise, a new DSO session is established. The clientMUST<bcp14>MUST</bcp14> respect the DNS TTL values on records it receives while performing the discovery process and store them in its local cache with this lifetime (as it will generallybedo anyway for all DNS queries it performs). This means that, as long as the DNS TTL values on the authoritative records are set to reasonable values, repeated application of the discovery process can be completednearlypractically instantaneously by the client, using onlylocally-storedlocally stored cached data.</t><?rfc needLines="48" ?></section> <sectiontitle="DNSanchor="subscribe" numbered="true" toc="default"> <name>DNS Push NotificationSUBSCRIBE" anchor="subscribe">SUBSCRIBE</name> <t>After connecting, and requesting a longer idle timeout and/or keepalive interval if necessary, a DNS Push Notificationclient<vspace />client then indicates its desire to receive DNS Push Notificationsfor<vspace />for a given domain name by sending a SUBSCRIBE request to theserver.<vspace />server. A SUBSCRIBE request is encoded in a DSO message <xreftarget="RFC8490"/>.<vspace />target="RFC8490" format="default"/>. This specification defines aprimaryDSO Primary TLV for DNS Push Notification SUBSCRIBE Requests(tentatively DSO(DSO Type Code0x40).</t>0x0040).</t> <t>DSO messages with the SUBSCRIBE TLV as the Primary TLV are permitted in TLS early data, provided that the precautions described in <xreftarget="early_data"/>target="early_data" format="default"/> are followed.</t> <t>The entity that initiates a SUBSCRIBE request is by definition the client. A serverMUST NOT<bcp14>MUST NOT</bcp14> send a SUBSCRIBE request over an existing session from a client. If a server does send a SUBSCRIBE request over a DSO session initiated by a client, this is a fatal error and the clientMUST<bcp14>MUST</bcp14> forcibly abort the connection immediately.</t> <t>Each SUBSCRIBE request generates exactly one SUBSCRIBE response from the server. The entity that initiates a SUBSCRIBE response is by definition the server. A clientMUST NOT<bcp14>MUST NOT</bcp14> send a SUBSCRIBE response. If a client does send a SUBSCRIBE response, this is a fatal error and the serverMUST<bcp14>MUST</bcp14> forcibly abort the connection immediately.</t> <sectiontitle="SUBSCRIBE Request">numbered="true" toc="default"> <name>SUBSCRIBE Request</name> <t>A SUBSCRIBE request begins with the standard<xref target="RFC8490">DSODSO 12-byteheader</xref>,header <xref target="RFC8490" format="default"></xref>, followed by the SUBSCRIBEprimaryPrimary TLV. A SUBSCRIBE request is illustrated in <xreftarget="subscribe_req"/>.</t>target="subscribe_req" format="default"/>.</t> <t>The MESSAGE ID fieldMUST<bcp14>MUST</bcp14> be set to a uniquevalue,value that the client is not using for any other active operation on this DSO session. For the purposes here, a MESSAGE ID is in use on this session if either the client has used it in a request for which it has not yet received a response, or if the client has used it for a subscriptionwhichthat it has not yetcancelledcanceled using UNSUBSCRIBE. In the SUBSCRIBEresponseresponse, the serverMUST<bcp14>MUST</bcp14> echo back the MESSAGE ID value unchanged.</t> <t>The other header fieldsMUST<bcp14>MUST</bcp14> be set as described in the <xreftarget="RFC8490">DSO spec-ification</xref>.target="RFC8490" format="default">DSO specification</xref>. The DNS OPCODE field contains the OPCODE value for DNS Stateful Operations (6). The four count fields must be zero, and the corresponding four sections must be empty (i.e., absent).</t> <t>The DSO-TYPE is SUBSCRIBE(tentatively 0x40).</t>(0x0040).</t> <t>The DSO-LENGTH is the length of the DSO-DATA that follows, which specifies the name, type, and class of the record(s) being sought.</t> <figure anchor="subscribe_req"> <name>SUBSCRIBE Request</name> <artwork align="left"anchor="subscribe_req" title="SUBSCRIBE Request"><artwork align="left"><![CDATA[name="" type="" alt=""><![CDATA[ 1 1 1 1 1 1 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 +--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+ \ | MESSAGE ID | \ +--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+ | |QR| OPCODE(6) | Z | RCODE | | +--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+ | | QDCOUNT (MUST BE ZERO) | | +--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+ > HEADER | ANCOUNT (MUST BE ZERO) | | +--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+ | | NSCOUNT (MUST BE ZERO) | | +--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+ | | ARCOUNT (MUST BE ZERO) | / +--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+ / | DSO-TYPE = SUBSCRIBE(tentatively 0x40)(0x0040) | +--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+ | DSO-LENGTH (number of octets in DSO-DATA) | +--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+ \ \ NAME \ \ +--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+ | | TYPE | > DSO-DATA +--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+ | | CLASS | / +--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+/]]></artwork></figure>/]]></artwork> </figure> <t>The DSO-DATA for a SUBSCRIBE requestMUST<bcp14>MUST</bcp14> contain exactly one NAME, TYPE, and CLASS. Since SUBSCRIBE requests are sent over TCP, multiple SUBSCRIBE DSO request messages can be concatenated in a single TCP stream and packed efficiently into TCP segments.</t> <t>If accepted, the subscription will stay in effect until the client cancels the subscription using UNSUBSCRIBE or until the DSO session between the client and the server is closed.</t> <t>SUBSCRIBE requests on a given sessionMUST<bcp14>MUST</bcp14> be unique. A clientMUST NOT<bcp14>MUST NOT</bcp14> send a SUBSCRIBE message that duplicates theNAME, TYPEname, type andCLASSclass of an existing active subscription on that DSO session. For the purpose of this matching, the established DNScase-insensitivitycase insensitivity for US-ASCII letters <xref target="RFC0020"/>format="default"/> applies (e.g., "example.com" and "Example.com" are the same). If a server receives such a duplicate SUBSCRIBE message, this is a fatal error and the serverMUST<bcp14>MUST</bcp14> forcibly abort the connection immediately.</t> <t>DNS wildcarding is not supported. That is,a wildcardan asterisk character ("*") in a SUBSCRIBE message matches only a literalwildcardasterisk character ("*") inthe zone,a name and nothingelse.</t> <t>Aliasing is not supported. That is,else. Similarly, a CNAME in a SUBSCRIBE message matches only aliteralCNAME record with that name in thezone,zone and no other records withthe same ownerthat name.</t> <t>A client may SUBSCRIBE to records that are unknown to the server at the time of the request (providing that the name falls within one of the zone(s) the server is responsiblefor)for), and this is not an error. The serverMUST NOT<bcp14>MUST NOT</bcp14> return NXDOMAIN in this case. The serverMUST<bcp14>MUST</bcp14> accept these requests and send Push Notifications if and when matching records are found in the future.</t> <t>If neither TYPE nor CLASS are ANY(255)(255), then this is a specific subscription to changes for the givenNAME, TYPEname, type, andCLASS.class. If one or both of TYPE or CLASS are ANY(255)(255), then this subscription matchesany typeall types and/orany class,all classes as appropriate.</t><?rfc needLines="14" ?><t>NOTE: A little-known quirk of DNS is that in DNS QUERY requests, QTYPE and QCLASS 255 mean"ANY""ANY", not "ALL". They indicate that the server should respond with ANY matching records of its choosing, not necessarily ALL matching records. This can lead to some surprising and unexpected results, where a query returns some validanswersanswers, but not all of them, and makes QTYPE = 255 (ANY) queries less useful than people sometimes imagine.</t> <t>When used in conjunction with SUBSCRIBE, TYPE 255 and CLASS 255 should be interpreted to mean "ALL", not "ANY". After accepting a subscription where one or both of TYPE or CLASS are 255, the serverMUST<bcp14>MUST</bcp14> send Push Notification Updates for ALL record changes that match the subscription, not just some of them.</t><?rfc needLines="48" ?></section> <sectiontitle="SUBSCRIBE Response" anchor="subresp">anchor="subresp" numbered="true" toc="default"> <name>SUBSCRIBE Response</name> <t>A SUBSCRIBE response begins with the standard<xref target="RFC8490">DSODSO 12-byteheader</xref>.header <xref target="RFC8490" format="default"></xref>. The QR bit in the header is set indicating it is a response. The headerMAY<bcp14>MAY</bcp14> be followed by one or more optionalTLVs,Additional TLVs such as a Retry Delay Additional TLV. A SUBSCRIBE response is illustrated in <xreftarget="subscribe_resp"/>.</t>target="subscribe_resp" format="default"/>.</t> <t>The MESSAGE ID fieldMUST<bcp14>MUST</bcp14> echo the value given in the MESSAGE ID field of the SUBSCRIBE request. This is how the client knows which request is being responded to.</t> <t>The other header fieldsMUST<bcp14>MUST</bcp14> be set as described in the <xreftarget="RFC8490">DSO spec-ification</xref>.target="RFC8490" format="default">DSO specification</xref>. The DNS OPCODE field contains the OPCODE value for DNS Stateful Operations (6). The four count fields must be zero, and the corresponding four sections must be empty (i.e., absent).</t> <t>A SUBSCRIBE response messageMUST NOT<bcp14>MUST NOT</bcp14> include a SUBSCRIBE TLV. If a client receives a SUBSCRIBE response message containing a SUBSCRIBETLVTLV, then the response message is processed but the SUBSCRIBE TLVMUST<bcp14>MUST</bcp14> be silently ignored.</t> <figure anchor="subscribe_resp"> <name>SUBSCRIBE Response</name> <artwork align="left"anchor="subscribe_resp" title="SUBSCRIBE Response"><artwork align="left"><![CDATA[name="" type="" alt=""><![CDATA[ 1 1 1 1 1 1 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 +--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+ \ | MESSAGE ID | \ +--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+ | |QR| OPCODE(6) | Z | RCODE | | +--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+ | | QDCOUNT (MUST BE ZERO) | | +--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+ > HEADER | ANCOUNT (MUST BE ZERO) | | +--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+ | | NSCOUNT (MUST BE ZERO) | | +--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+ | | ARCOUNT (MUST BE ZERO) | / +--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+ /]]> </artwork></figure> <?rfc needLines="20" ?>]]></artwork> </figure> <t>In the SUBSCRIBEresponseresponse, the RCODE indicates whether or not the subscription was accepted. Supported RCODEs are as follows:</t><texttable title="SUBSCRIBE<table anchor="subscribe_rcodes" align="center"> <name>SUBSCRIBE Responsecodes" anchor="subscribe_rcodes"> <ttcol align="left">Mnemonic</ttcol> <ttcol align="center">Value</ttcol> <ttcol align="left">Description</ttcol> <c>NOERROR</c><c>0</c><c>SUBSCRIBE successful.</c> <c>FORMERR</c><c>1</c><c>ServerCodes</name> <thead> <tr> <th align="left">Mnemonic</th> <th align="center">Value</th> <th align="left">Description</th> </tr> </thead> <tbody> <tr> <td align="left">NOERROR</td> <td align="center">0</td> <td align="left">SUBSCRIBE successful.</td> </tr> <tr> <td align="left">FORMERR</td> <td align="center">1</td> <td align="left">Server failed to process request due to a malformedrequest.</c> <c>SERVFAIL</c><c>2</c><c>Serverrequest.</td> </tr> <tr> <td align="left">SERVFAIL</td> <td align="center">2</td> <td align="left">Server failed to process request due to a problem with theserver.</c> <c>NOTIMP</c><c>4</c><c>Serverserver.</td> </tr> <tr> <td align="left">NOTIMP</td> <td align="center">4</td> <td align="left">Server does not implementDSO.</c> <c>REFUSED</c><c>5</c><c>ServerDSO.</td> </tr> <tr> <td align="left">REFUSED</td> <td align="center">5</td> <td align="left">Server refuses to process request for policy or securityreasons.</c> <c>NOTAUTH</c><c>9</c><c>Serverreasons.</td> </tr> <tr> <td align="left">NOTAUTH</td> <td align="center">9</td> <td align="left">Server is not authoritative for the requestedname.</c> <c>DSOTYPENI</c><c>11</c><c>SUBSCRIBEname.</td> </tr> <tr> <td align="left">DSOTYPENI</td> <td align="center">11</td> <td align="left">SUBSCRIBE operation notsupported.</c> </texttable>supported.</td> </tr> </tbody> </table> <t>This document specifies only these RCODE values for SUBSCRIBE Responses. Servers sending SUBSCRIBE ResponsesSHOULD<bcp14>SHOULD</bcp14> use one of these values. Note that NXDOMAIN is not a valid RCODE in response to a SUBSCRIBE Request. However, future circumstances may create situations where other RCODE values are appropriate in SUBSCRIBE Responses, so clientsMUST<bcp14>MUST</bcp14> be prepared to accept and handle SUBSCRIBE Responses with any other nonzero RCODEvalue.</t>error values.</t> <t>If the server sends a nonzero RCODE in the SUBSCRIBE response, that means:<?rfc subcompact="yes" ?> <list style="letters"> <t>the</t> <ol spacing="compact" type="a"> <li>the client is (at least partially) misconfigured,or</t> <t>theor</li> <li>the server resources are exhausted,or</t> <t>thereor</li> <li>there is some other unknown failure on theserver.</t> </list> <?rfc subcompact="no" ?>server.</li> </ol> <t> In any case, the client shouldn't retry the subscription to this server right away. If multiple SRV records were returned as described in <xreftarget="discovery"/>, <xref target="SRV"/>,target="SRV" format="default"/>, a subsequent serverMAY<bcp14>MAY</bcp14> be tried immediately.</t> <t>If the client has other successful subscriptions to this server, these subscriptions remain even though additional subscriptions may be refused. Neither the client nor the serverareis required to close the connection,although,although either end may choose to do so.</t> <t>If the server sends a nonzeroRCODERCODE, then itSHOULD<bcp14>SHOULD</bcp14> append a Retry Delay Additional TLV <xreftarget="RFC8490"/>target="RFC8490" format="default"/> to the response specifying a delay before the client attempts this operation again. Recommended values for the delay for different RCODE values are given below. These recommended values apply both to the default values a server should place in the Retry DelayTLV,Additional TLV and the default values a client should assume if the server provides no Retry Delay Additional TLV.<list style="bullets"> <t>For</t> <ul spacing="normal" empty="true"> <li>For RCODE = 1(FORMERR)(FORMERR), the delay may be any value selected by the implementer. A value of five minutes isRECOMMENDED,<bcp14>RECOMMENDED</bcp14> to reduce the risk of high load from defectiveclients.</t> <t>Forclients.</li> <li>For RCODE = 2(SERVFAIL)(SERVFAIL), the delay should be chosen according to the level of server overload and the anticipated duration of that overload. By default, a value of one minute isRECOMMENDED.<bcp14>RECOMMENDED</bcp14>. If a more serious server failure occurs, the delay may be longer in accordance with the specific problemencountered.</t> <t>Forencountered.</li> <li>For RCODE = 4 (NOTIMP), which occurs on a server that doesn't implement<xref target="RFC8490">DNSDNS StatefulOperations</xref>,Operations <xref target="RFC8490" format="default"></xref>, it is unlikely that the server will begin supporting DSO in the next few minutes, so the retry delaySHOULD<bcp14>SHOULD</bcp14> be one hour. Note that in such a case, a server that doesn't implement DSO is unlikely to place a Retry Delay Additional TLV in its response, so this recommended value in particular applies to what a client should assume bydefault.</t> <t>Fordefault.</li> <li>For RCODE = 5 (REFUSED), which occurs on a server that implements DNS PushNotifications,Notifications but is currently configured to disallow DNS Push Notifications, the retry delay may be any value selected by the implementer and/or configured by theoperator.</t> <t>Ifoperator.</li> <li>If the server being queried is listed in a<spanx style="verb">_dns&nbhy;push&nbhy;tls._tcp.<zone></spanx><tt>_dns&nbhy;push&nbhy;tls._tcp.<zone></tt> SRV record for the zone, then this is a misconfiguration, since this server is being advertised as supporting DNS Push Notifications for this zone, but the server itself is not currently configured to perform that task. Since it is possible that the misconfiguration may be repaired at any time, the retry delay should not be set too high. By default, a value of 5 minutes isRECOMMENDED.</t> <t>For<bcp14>RECOMMENDED</bcp14>.</li> <li>For RCODE = 9 (NOTAUTH), which occurs on a server that implements DNS PushNotifications,Notifications but is not configured to be authoritative for the requested name, the retry delay may be any value selected by the implementer and/or configured by theoperator.</t> <t>Ifoperator.</li> <li>If the server being queried is listed in a<spanx style="verb">_dns&nbhy;push&nbhy;tls._tcp.<zone></spanx><tt>_dns&nbhy;push&nbhy;tls._tcp.<zone></tt> SRV record for the zone, then this is a misconfiguration, since this server is being advertised as supporting DNS Push Notifications for this zone, but the server itself is not currently configured to perform that task. Since it is possible that the misconfiguration may be repaired at any time, the retry delay should not be set too high. By default, a value of 5 minutes isRECOMMENDED.</t> <t>For<bcp14>RECOMMENDED</bcp14>.</li> <li>For RCODE = 11 (DSOTYPENI), which occurs on a server that implements DSO but doesn't implement DNS Push Notifications, it is unlikely that the server will begin supporting DNS Push Notifications in the next few minutes, so the retry delaySHOULD<bcp14>SHOULD</bcp14> be onehour.</t> <t>Forhour.</li> <li>For other RCODE values, the retry delay should be set by the server as appropriate for that error condition. By default, a value of 5 minutes isRECOMMENDED.</t> </list> </t><bcp14>RECOMMENDED</bcp14>.</li> </ul> <t>For RCODE = 9 (NOTAUTH), the time delay applies to requests for other names falling within the same zone. Requests for names falling within other zones are not subject to the delay. For all otherRCODEsRCODEs, the time delay applies to all subsequent requests to this server.</t> <t>After sending an errorresponseresponse, the serverMAY<bcp14>MAY</bcp14> allow the session to remain open, orMAY send<bcp14>MAY</bcp14> follow it with aDNS Push NotificationDSO Retry DelayOperation TLVoperation (using the Retry Delay Primary TLV) instructing the client to close thesession,session as described in the <xreftarget="RFC8490">DSOtarget="RFC8490" format="default">DSO specification</xref>. ClientsMUST<bcp14>MUST</bcp14> correctly handle bothcases.</t> <?rfc needLines="48" ?>cases. Note that the DSO Retry Delay operation (using the Retry Delay Primary TLV) is different to the Retry Delay Additional TLV mentioned above. </t> </section> </section> <sectiontitle="DNSanchor="push" numbered="true" toc="default"> <name>DNS Push NotificationUpdates" anchor="push">Updates</name> <t>Once a subscription has been successfully established, the server generates PUSH messages to send to the client as appropriate. In the case that the answer set was already non-empty at the moment the subscription was established, an initial PUSH message will be sent immediately following the SUBSCRIBE Response. Subsequent changes to the answer set are then communicated to the client in subsequent PUSH messages.</t> <t>A clientMUST NOT<bcp14>MUST NOT</bcp14> send a PUSH message. If a client does send a PUSH message, or a PUSH message is sent with the QR bit set indicating that it is a response, this is a fatal error and the receiverMUST<bcp14>MUST</bcp14> forcibly abort the connection immediately.</t> <sectiontitle="PUSH Message">numbered="true" toc="default"> <name>PUSH Message</name> <t>A PUSH unidirectional message begins with the standard<xref target="RFC8490">DSODSO 12-byteheader</xref>,header <xref target="RFC8490" format="default"></xref>, followed by the PUSHprimaryPrimary TLV. A PUSH message is illustrated in <xreftarget="push_msg"/>.</t>target="push_msg" format="default"/>.</t> <t>In accordance with the definition of DSO unidirectional messages, the MESSAGE ID fieldMUST<bcp14>MUST</bcp14> be zero. There is no client response to a PUSH message.</t> <t>The other header fieldsMUST<bcp14>MUST</bcp14> be set as described in the DSO specification <xreftarget="RFC8490">DSO spec-ification</xref>.target="RFC8490" format="default"></xref>. The DNS OPCODE field contains the OPCODE value for DNS Stateful Operations (6). The four count fields must be zero, and the corresponding four sections must be empty (i.e., absent).</t> <t>The DSO-TYPE is PUSH(tentatively 0x41).</t>(0x0041).</t> <t>The DSO-LENGTH is the length of the DSO-DATA that follows, which specifies the changes being communicated.</t> <t>The DSO-DATA contains one or more change notifications. A PUSH MessageMUST<bcp14>MUST</bcp14> contain at least one change notification. If a PUSH Message is received that contains no change notifications, this is a fatalerror,error and the clientMUST<bcp14>MUST</bcp14> forcibly abort the connection immediately.</t> <t>The change notification records are formatted similarly to how DNS Resource Records are conventionally expressed in DNS messages, as illustrated in <xreftarget="push_msg"/>,target="push_msg" format="default"/>, and are interpreted as described below.</t><?rfc needLines="6" ?><t>The TTL field holds an unsigned 32-bit integer <xreftarget="RFC2181"/>.target="RFC2181" format="default"/>. If the TTL is in the range 0 to 2,147,483,647 seconds (0 to2^312<sup>31</sup> - 1, or 0x7FFFFFFF), then a new DNS Resource Record with the given name, type,classclass, and RDATA is added. Type and classMUST NOT<bcp14>MUST NOT</bcp14> be 255 (ANY). If either type or class are 255(ANY)(ANY), this is a fatalerror,error and the clientMUST<bcp14>MUST</bcp14> forcibly abort the connection immediately. A TTL of 0 means that this record should be retained for as long as the subscription isactive,active and should be discarded immediately the moment the subscription iscancelled.</t>canceled.</t> <t>If the TTL has the value 0xFFFFFFFF, then the DNS Resource Record with the given name, type,classclass, and RDATA is removed. Type and classMUST NOT<bcp14>MUST NOT</bcp14> be 255 (ANY). If either type or class are 255(ANY)(ANY), this is a fatalerror,error and the clientMUST<bcp14>MUST</bcp14> forcibly abort the connection immediately.</t> <t>If the TTL has the value 0xFFFFFFFE, then this is a 'collective' remove notification. For collective removenotificationsnotifications, RDLENMUST<bcp14>MUST</bcp14> bezerozero, andconsequentlyconsequently, the RDATAMUST<bcp14>MUST</bcp14> be empty. If a change notification is received where TTL = 0xFFFFFFFE and RDLEN is not zero, this is a fatalerror,error and the clientMUST<bcp14>MUST</bcp14> forcibly abort the connection immediately.</t> <t>There are three types of collective removenotification:</t> <t>Fornotification. For collective removenotifications, ifnotifications:</t> <ul> <li>If CLASS is not 255 (ANY) and TYPE is not 255(ANY)(ANY), then for the givennamename, this removes all records of the specified type in the specifiedclass.</t> <t>For collective remove notifications, ifclass. </li> <li>If CLASS is not 255 (ANY) and TYPE is 255(ANY)(ANY), then for the givennamename, this removes all records of all types in the specifiedclass.</t> <t>For collective remove notifications, ifclass. </li> <li>If CLASS is 255 (ANY), then for the givennamename, this removes all records of all types in all classes. In thiscasecase, TYPEMUST<bcp14>MUST</bcp14> be set to zero ontransmission,transmission andMUST<bcp14>MUST</bcp14> be silently ignored onreception.</t> <?rfc needLines="19" ?>reception. </li> </ul> <t>Summary of change notification types:<list style="bullets"></t> <ul spacing="normal"> <li> <t>Remove all RRsets from aname,name in allclasses<vspace />classes:<br/> TTL = 0xFFFFFFFE, RDLEN = 0, CLASS = 255(ANY)</t>(ANY).</t> </li> <li> <t>Remove all RRsets from aname,name in givenclass:<vspace />class:<br/> TTL = 0xFFFFFFFE, RDLEN = 0, CLASS gives class, TYPE = 255(ANY)</t>(ANY).</t> </li> <li> <t>Remove specified RRset from aname,name in givenclass:<vspace />class:<br/> TTL = 0xFFFFFFFE, RDLEN =0<vspace />0,<br/> CLASS and TYPE specify the RRset beingremoved</t>removed.</t> </li> <li> <t>Remove an individual RR from aname:<vspace />name:<br/> TTL =0xFFFFFFFF<vspace />0xFFFFFFFF,<br/> CLASS, TYPE,RDLENRDLEN, and RDATA specify the RR beingremoved</t>removed.</t> </li> <li> <t>Add individual RR to aname<vspace />name:<br/> TTL >= 0 and TTL <=0x7FFFFFFF<vspace />0x7FFFFFFF,<br/> CLASS, TYPE, RDLEN,RDATARDATA, and TTL specify the RR beingadded</t> </list> </t>added.</t> </li> </ul> <t>Note that it is valid for the RDATA of an added or removed DNS Resource Record to be empty (zero length). For example, an<xref target="RFC3123">AddressAddress Prefix List ResourceRecord</xref>Record <xref target="RFC3123" format="default"></xref> may have empty RDATA. Therefore, a change notification with RDLEN = 0 does not automatically indicate a remove notification. If RDLEN = 0 and TTL isthein the range 0-to 0x7FFFFFFF, this change notification signals the addition of a record with the given name, type, class, and empty RDATA. If RDLEN = 0 and TTL = 0xFFFFFFFF, this change notification signals the removal specifically of that single record with the given name, type, class, and empty RDATA.</t> <t>If the TTL is any value other than 0xFFFFFFFF, 0xFFFFFFFE, or a value in the range 0-to 0x7FFFFFFF, then the receiverSHOULD<bcp14>SHOULD</bcp14> silently ignore this particular change notification record. The connection is not terminated and other valid change notification records within this PUSHmessage are processed as usual.</t>message are processed as usual.</t> <t>In the case where a single change affects more than one active subscription, only one PUSH message is sent. For example, a PUSH message adding a given record may match both a SUBSCRIBE request with the same TYPE and a different SUBSCRIBE request with TYPE = 255 (ANY). It is not the case that two PUSH messages are sent because the new record matches two active subscriptions.</t> <t>The server <bcp14>SHOULD</bcp14> encode change notifications in the most efficient manner possible. For example, when three AAAA records are removed from a given name, and no other AAAA records exist for that name, the server <bcp14>SHOULD</bcp14> send a "Remove specified RRset from a name in given class" PUSH message, not three separate "Remove an individual RR from a name" PUSH messages. Similarly, when both an SRV and a TXT record are removed from a given name, and no other records of any kind exist for that name in that class, the server <bcp14>SHOULD</bcp14> send a "Remove all RRsets from a name in given class" PUSH message, not two separate "Remove specified RRset from a name in given class" PUSH messages.</t> <t>For efficiency, when generating a PUSH message, rather than sending each change notification as a separate DSO message, a serverSHOULD<bcp14>SHOULD</bcp14> include as many change notifications as it has immediately available tosend, rather than sending eachsend to that client, even if those changenotification asnotifications apply to different subscriptions from that client. Conceptually, aseparate DSO message.PUSH message is a session-level mechanism, not a subscription-level mechanism. Once it has exhausted the list of change notifications immediately available tosend,send to that client, a serverSHOULD<bcp14>SHOULD</bcp14> then send the PUSH messageimmediately,immediately rather than waiting speculatively to see if additional change notifications become available.</t><?rfc needLines="6" ?><t>For efficiency, when generating a PUSHmessage,message a serverSHOULD<bcp14>SHOULD</bcp14> use standard DNS name compression, with offsets relative to the beginning of the DNS message <xreftarget="RFC1035"/>.target="RFC1035" format="default"/>. When multiple change notifications in a single PUSH message have the same owner name, this name compression can yield significant savings. Name compression should be performed as specified inSection 18.14 of the<xreftarget="RFC6762">Multicasttarget="RFC6762" sectionFormat="of" section="18.14">the Multicast DNSspecification</xref>,specification</xref>; namely, owner names should always be compressed, and names appearing within RDATA should be compressed for only the RR types listed below:<list style="hanging"> <t>NS,</t> <dl newline="false" spacing="normal"> <dt/> <dd>NS, CNAME, PTR, DNAME, SOA, MX, AFSDB, RT, KX, RP, PX, SRV,NSEC</t> </list></t>NSEC</dd> </dl> <t>Servers may generate PUSH messages up to a maximum DNS message length of 16,382 bytes, counting from the start of the DSO 12-byte header. Including the two-byte length prefix that is used to frame DNS over a byte stream like TLS, this makes a total of 16,384 bytes. ServersMUST NOT<bcp14>MUST NOT</bcp14> generate PUSH messages larger than this. Where the immediately available change notifications are sufficient to exceed a DNS message length of 16,382 bytes, the change notificationsMUST<bcp14>MUST</bcp14> be communicated in separate PUSH messages of up to 16,382 bytes each. DNS name compression becomes less effective for messages larger than 16,384 bytes, so little efficiency benefit is gained by sending messages larger than this.</t> <t>If a client receives a PUSH message with a DNS message length larger than 16,382 bytes, this is a fatalerror,error and the clientMUST<bcp14>MUST</bcp14> forcibly abort the connection immediately.</t> <figure anchor="push_msg"> <name>PUSH Message</name> <artwork align="left"anchor="push_msg" title="PUSH Message"><artwork align="left"><![CDATA[name="" type="" alt=""><![CDATA[ 1 1 1 1 1 1 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 +--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+ \ | MESSAGE ID (MUST BE ZERO) | \ +--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+ | |QR| OPCODE(6) | Z | RCODE | | +--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+ | | QDCOUNT (MUST BE ZERO) | | +--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+ > HEADER | ANCOUNT (MUST BE ZERO) | | +--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+ | | NSCOUNT (MUST BE ZERO) | | +--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+ | | ARCOUNT (MUST BE ZERO) | / +--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+ / | DSO-TYPE = PUSH(tentatively 0x41)(0x0041) | +--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+ | DSO-LENGTH (number of octets in DSO-DATA) | +--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+ \ \ NAME \ \ +--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+ | | TYPE | | +--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+ | | CLASS | | +--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+ | | TTL | | | (32-bit unsigned big-endian integer) | > DSO-DATA +--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+ | | RDLEN (16-bit unsigned big-endian integer) | | +--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+ | \ RDATA (sized as necessary) \ | +--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+ | : NAME, TYPE, CLASS, TTL, RDLEN, RDATA : | : Repeated As Necessary : / +--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+/]]></artwork></figure>/]]></artwork> </figure> <t>When processing the records received in a PUSH Message, the receiving clientMUST<bcp14>MUST</bcp14> validate that the records being added or removed correspond with at least one currently active subscription on that session. Specifically, the record nameMUST<bcp14>MUST</bcp14> match the name given in the SUBSCRIBE request, subject to the usual established DNS case-insensitivity for US-ASCII letters. For individual additions and removals, if the TYPE in the SUBSCRIBE request was not ANY(255)(255), then the TYPE of the record must either be CNAME or match the TYPE given in the SUBSCRIBE request, and if the CLASS in the SUBSCRIBE request was not ANY(255)(255), then the CLASS of the record must match the CLASS given in the SUBSCRIBE request. Forcollective removals, at least one of the records being removed must match an active subscription. If a matching active subscription on that session is not found, then that particular addition/removal record is silently ignored. Processing of other additions and removal records in this message is not affected. The DSO session is not closed. This is to allow for the unavoidable race condition where a client sends an outbound UNSUBSCRIBE while inbound PUSH messages for that subscription from the server are still in flight.</t> <t>In the case where a single change affects more than one active subscription, only one PUSH message is sent. For example, a PUSH message adding a given record may match both a SUBSCRIBE request with the same TYPE and a different SUBSCRIBE request with TYPE = 255 (ANY). It is not the case that two PUSH messages are sent because the new record matches two active subscriptions.</t> <t>The server SHOULD encode change notifications in the most efficient manner possible. For example, when three AAAAcollective removals, at least one of the recordsarebeing removedfrom a given name, and no other AAAA records exist for that name, the server SHOULD send a "removemust match anRRset fromactive subscription. If aname" PUSH message,matching active subscription on that session is notthree separate "remove an individual RR from a name" PUSH messages. Similarly, when both an SRV and a TXTfound, then that particular addition/removal recordare removed from a given name, and nois silently ignored. The processing of other additions and removal recordsof any kind existin this message is not affected. The DSO session is not closed. This is to allow forthat name,theserver SHOULD send a "remove all RRsets fromunavoidable race condition where aname" PUSH message, not two separate "removeclient sends anRRset from a name"outbound UNSUBSCRIBE while inbound PUSHmessages.</t> <t>Amessages for that subscription from the serverSHOULD combine multiple change notificationsare still ina single PUSH message when possible, even if those change notifications apply to different subscriptions. Conceptually, a PUSH message is a session-level mechanism, not a subscription-level mechanism.</t>flight.</t> <t>The TTL of an added record is stored by the client. While the subscription isactive,active the TTL is not decremented, because a change to the TTL would produce a new update. For as long as a relevant subscription remains active, the clientSHOULD<bcp14>SHOULD</bcp14> assume that when a record goesawayaway, the server will notify it of that fact. Consequently, a client does not have to poll to verify that the record is still there. Once a subscription iscancelledcanceled (individually, or as a result of the DSO session beingclosed)closed), record aging for records covered by the subscription resumes and records are removed from the local cache when their TTL reaches zero.</t><?rfc needLines="48" ?></section> </section> <sectiontitle="DNSanchor="unsubscribe" numbered="true" toc="default"> <name>DNS Push NotificationUNSUBSCRIBE" anchor="unsubscribe">UNSUBSCRIBE</name> <t>To cancel an individual subscription without closing the entire DSO session, the client sends an UNSUBSCRIBE message over the established DSO session to the server.</t> <t>The entity that initiates an UNSUBSCRIBE message is by definition the client. A serverMUST NOT<bcp14>MUST NOT</bcp14> send an UNSUBSCRIBE message over an existing session from a client. If a server does send an UNSUBSCRIBE message over a DSO session initiated by a client, or an UNSUBSCRIBE message is sent with the QR bit set indicating that it is a response, this is a fatal error and the receiverMUST<bcp14>MUST</bcp14> forcibly abort the connection immediately.</t> <sectiontitle="UNSUBSCRIBE Message">numbered="true" toc="default"> <name>UNSUBSCRIBE Message</name> <t>An UNSUBSCRIBE unidirectional message begins with the standard<xref target="RFC8490">DSODSO 12-byteheader</xref>,header <xref target="RFC8490" format="default"></xref>, followed by the UNSUBSCRIBEprimaryPrimary TLV. An UNSUBSCRIBE message is illustrated in <xreftarget="unsubscribe_msg"/>.</t>target="unsubscribe_msg" format="default"/>.</t> <t>In accordance with the definition of DSO unidirectional messages, the MESSAGE ID fieldMUST<bcp14>MUST</bcp14> be zero. There is no server response to an UNSUBSCRIBE message.</t> <t>The other header fieldsMUST<bcp14>MUST</bcp14> be set as described in the <xreftarget="RFC8490">DSO spec-ification</xref>.target="RFC8490" format="default">DSO specification</xref>. The DNS OPCODE field contains the OPCODE value for DNS Stateful Operations (6). The four count fields must be zero, and the corresponding four sections must be empty (i.e., absent).</t> <t>The DSO-TYPE is UNSUBSCRIBE(tentatively 0x42).</t>(0x0042).</t> <t>The DSO-LENGTH field contains the value 2, the length of the 2-octet MESSAGE ID contained in the DSO-DATA.</t> <t>The DSO-DATA contains the value previously given in the MESSAGE ID field of an active SUBSCRIBE request. This is how the server knows which SUBSCRIBE request is beingcancelled.canceled. After receipt of the UNSUBSCRIBE message, the SUBSCRIBE request is no longer active.</t> <t>It is allowable for the client to issue an UNSUBSCRIBE message for a previous SUBSCRIBE request for which the client has not yet received a SUBSCRIBE response. This is to allow for the case where a client starts and stops a subscription in less than the round-trip time to the server. The client is NOT required to wait for the SUBSCRIBE response before issuing the UNSUBSCRIBE message.</t><?rfc needLines="6" ?><t>Consequently, it is possible for a server to receive an UNSUBSCRIBE message that does not match any currently active subscription. This can occur when a client sends a SUBSCRIBE request, which subsequently fails and returns an error code, but the client sent an UNSUBSCRIBE message before it became aware that the SUBSCRIBE request had failed. Because of this, serversMUST<bcp14>MUST</bcp14> silently ignore UNSUBSCRIBE messages that do not match any currently active subscription.</t> <figure anchor="unsubscribe_msg"> <name>UNSUBSCRIBE Message</name> <artwork align="left"anchor="unsubscribe_msg" title="UNSUBSCRIBE Message"><artwork align="left"><![CDATA[name="" type="" alt=""><![CDATA[ 1 1 1 1 1 1 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 +--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+ \ | MESSAGE ID (MUST BE ZERO) | \ +--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+ | |QR| OPCODE(6) | Z | RCODE | | +--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+ | | QDCOUNT (MUST BE ZERO) | | +--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+ > HEADER | ANCOUNT (MUST BE ZERO) | | +--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+ | | NSCOUNT (MUST BE ZERO) | | +--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+ | | ARCOUNT (MUST BE ZERO) | / +--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+ / | DSO-TYPE = UNSUBSCRIBE(tentatively 0x42)(0x0042) | +--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+ | DSO-LENGTH (2) | +--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+ \ | SUBSCRIBE MESSAGE ID | > DSO-DATA +--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+/]]></artwork></figure> <?rfc needLines="48" ?>/]]></artwork> </figure> </section> </section> <sectiontitle="DNSanchor="reconfirm" numbered="true" toc="default"> <name>DNS Push NotificationRECONFIRM" anchor="reconfirm">RECONFIRM</name> <t>Sometimes, particularly when used with a Discovery Proxy <xreftarget="DisProx">Discovery Proxy</xref>,target="RFC8766" format="default"></xref>, a DNS Zone may contain stale data. When a client encounters data that it believes may be stale (e.g., an SRV record referencing a target host+port that is not responding to connectionrequests)requests), the client can send a RECONFIRM message to ask the server to re-verify that the data is still valid. For a Discovery Proxy, this causes it to issue new Multicast DNS queries to ascertain whether the target device is still present. How the Discovery Proxy causes these new Multicast DNS queries to be issued depends on the details of the underlying Multicast DNS implementation being used. For example, a Discovery Proxy built on Apple's dns_sd.h API <xreftarget="SD-API"/>target="SD-API" format="default"/> responds to a DNS Push Notification RECONFIRM message by calling the underlying API's DNSServiceReconfirmRecord() routine.</t> <t>For other types of DNS server, the RECONFIRM operation is currentlyundefined,undefined andSHOULD<bcp14>SHOULD</bcp14> result in a NOERROR response, butotherwiseit need not cause any other action to occur.</t> <t>Frequent use of RECONFIRM operations may be a sign of network unreliability, or some kind of misconfiguration, so RECONFIRM operationsMAY<bcp14>MAY</bcp14> be logged or otherwise communicated to a human administrator to assist in detecting and remedying such network problems.</t> <t>If, after receiving a valid RECONFIRM message, the server determines that the disputed records are in fact no longer valid, then subsequent DNS PUSH Messages will be generated to inform interested clients. Thus, one client discovering that apreviously-advertisedpreviously advertised device (like a network printer) is no longer present has the side effect of informing all other interested clients that the device in question is now gone.</t> <t>The entity that initiates a RECONFIRM message is by definition the client. A serverMUST NOT<bcp14>MUST NOT</bcp14> send a RECONFIRM message over an existing session from a client. If a server does send a RECONFIRM message over a DSO session initiated by a client, or a RECONFIRM message is sent with the QR bit set indicating that it is a response, this is a fatal error and the receiverMUST<bcp14>MUST</bcp14> forcibly abort the connection immediately.</t><?rfc needLines="20" ?><sectiontitle="RECONFIRM Message">numbered="true" toc="default"> <name>RECONFIRM Message</name> <t>A RECONFIRM unidirectional message begins with the standard<xref target="RFC8490">DSODSO 12-byteheader</xref>,header <xref target="RFC8490" format="default"></xref>, followed by the RECONFIRMprimary TLV.<vspace />Primary TLV. A RECONFIRM message is illustrated in <xreftarget="reconfirm_msg"/>.</t>target="reconfirm_msg" format="default"/>.</t> <t>In accordance with the definition of DSO unidirectional messages, the MESSAGE ID fieldMUST<bcp14>MUST</bcp14> be zero. There is no server response to a RECONFIRM message.</t> <t>The other header fieldsMUST<bcp14>MUST</bcp14> be set as described in the <xreftarget="RFC8490">DSO spec-ification</xref>.target="RFC8490" format="default">DSO specification</xref>. The DNS OPCODE field contains the OPCODE value for DNS Stateful Operations (6). The four count fields must be zero, and the corresponding four sections must be empty (i.e., absent).</t> <t>The DSO-TYPE is RECONFIRM(tentatively 0x43).</t>(0x0043).</t> <t>The DSO-LENGTH is the length of the data that follows, which specifies the name, type, class, and content of the record being disputed.</t><t>The<t>A DNS Push Notifications RECONFIRM message contains exactly one RECONFIRM Primary TLV. The DSO-DATAforin a RECONFIRMmessage MUSTPrimary TLV <bcp14>MUST</bcp14> contain exactly one record. The DSO-DATAforin a RECONFIRMmessagePrimary TLV has no count field to specify more than one record. Since RECONFIRM messages are sent over TCP, multiple RECONFIRM messagescan be concatenated incan be concatenated in a single TCP stream and packed efficiently into TCP segments. Note that this means that DNS name compression cannot be used between different RECONFIRM messages. However, when a client is sending multiple RECONFIRM messages this indicates asingle TCP streamsituation with serious network problems, andpacked efficiently into TCP segments.</t>this is not expected to occur frequently enough that optimizing efficiency in this case is important. </t> <t>TYPEMUST NOT<bcp14>MUST NOT</bcp14> be the value ANY (255) and CLASSMUST NOT<bcp14>MUST NOT</bcp14> be the value ANY (255).</t> <t>DNS wildcarding is not supported. That is,a wildcardan asterisk character ("*") in a RECONFIRM message matches only a literalwildcardasterisk character ("*") inthe zone,a name and nothingelse.</t> <t>Aliasing is not supported. That is,else. Similarly, a CNAME in a RECONFIRM message matches only aliteralCNAME record with that name in thezone,zone and no other records withthe same ownerthat name.</t> <t>Note that there is no RDLEN field, since the length of the RDATA can be inferred from DSO-LENGTH, so an additional RDLEN field would be redundant.</t> <t>Following the same rules as for PUSH messages, DNS name compression SHOULD be used within the RDATA of the RECONFIRM message, with offsets relative to the beginning of the DNS message <xref target="RFC1035" format="default"/>.</t> <figure anchor="reconfirm_msg"> <name>RECONFIRM Message</name> <artwork align="left"anchor="reconfirm_msg" title="RECONFIRM Message"><artwork align="left"><![CDATA[name="" type="" alt=""><![CDATA[ 1 1 1 1 1 1 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 +--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+ \ | MESSAGE ID (MUST BE ZERO) | \ +--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+ | |QR| OPCODE(6) | Z | RCODE | | +--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+ | | QDCOUNT (MUST BE ZERO) | | +--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+ > HEADER | ANCOUNT (MUST BE ZERO) | | +--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+ | | NSCOUNT (MUST BE ZERO) | | +--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+ | | ARCOUNT (MUST BE ZERO) | / +--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+ / | DSO-TYPE = RECONFIRM(tentatively 0x43)(0x0043) | +--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+ | DSO-LENGTH (number of octets in DSO-DATA) | +--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+ \ \ NAME \ \ +--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+ | | TYPE | | +--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+ > DSO-DATA | CLASS | | +--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+ | \ RDATA \ / +--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+/]]></artwork></figure> <?rfc needLines="48" ?>/]]></artwork> </figure> </section> </section> <sectiontitle="DNSnumbered="true" toc="default"> <name>DNS Stateful Operations TLV ContextSummary">Summary</name> <t>This document defines four new DSO TLVs. As recommended inSection 8.2 of the<xreftarget="RFC8490">DNStarget="RFC8490" sectionFormat="of" section="8.2">the DNS Stateful Operations specification</xref>, the valid contexts of these new TLV types are summarized below.</t> <t>The client TLV contexts are:<?rfc subcompact="yes" ?> <list style="hanging"> <t hangText="C-P:">Client</t> <dl newline="false" spacing="compact"> <dt>C-P:</dt> <dd>Client request message,primary TLV</t> <t hangText="C-U:">Client unidirectionalPrimary TLV</dd> <dt>C-U:</dt> <dd>Client Unidirectional message, primaryTLV</t> <t hangText="C-A:">ClientTLV</dd> <dt>C-A:</dt> <dd>Client request or unidirectional message,additional TLV</t> <t hangText="CRP:">ResponseAdditional TLV</dd> <dt>CRP:</dt> <dd>Response back to client,primary TLV</t> <t hangText="CRA:">ResponsePrimary TLV</dd> <dt>CRA:</dt> <dd>Response back to client,additional TLV</t> </list> <?rfc subcompact="no" ?> </t> <texttable title="DSOAdditional TLV</dd> </dl> <table anchor="tlv_client_contexts" align="center"> <name>DSO TLV Client ContextSummary" anchor="tlv_client_contexts"> <ttcolSummary</name> <thead> <tr> <th align="right">TLVType</ttcol> <ttcol align="center">C-P</ttcol> <ttcol align="center">C-U</ttcol> <ttcol align="center">C-A</ttcol> <ttcol align="center">CRP</ttcol> <ttcol align="center">CRA</ttcol> <c>SUBSCRIBE</c><c>X</c><c></c><c></c><c></c><c></c> <c>PUSH</c><c></c><c></c><c></c><c></c><c></c> <c>UNSUBSCRIBE</c><c></c><c>X</c><c></c><c></c><c></c> <c>RECONFIRM</c><c></c><c>X</c><c></c><c></c><c></c> </texttable>Type</th> <th align="center">C-P</th> <th align="center">C-U</th> <th align="center">C-A</th> <th align="center">CRP</th> <th align="center">CRA</th> </tr> </thead> <tbody> <tr> <td align="right">SUBSCRIBE</td> <td align="center">X</td> <td align="center"/> <td align="center"/> <td align="center"/> <td align="center"/> </tr> <tr> <td align="right">PUSH</td> <td align="center"/> <td align="center"/> <td align="center"/> <td align="center"/> <td align="center"/> </tr> <tr> <td align="right">UNSUBSCRIBE</td> <td align="center"/> <td align="center">X</td> <td align="center"/> <td align="center"/> <td align="center"/> </tr> <tr> <td align="right">RECONFIRM</td> <td align="center"/> <td align="center">X</td> <td align="center"/> <td align="center"/> <td align="center"/> </tr> </tbody> </table> <t>The server TLV contexts are:<?rfc subcompact="yes" ?> <list style="hanging"> <t hangText="S-P:">Server</t> <dl newline="false" spacing="compact"> <dt>S-P:</dt> <dd>Server request message,primary TLV</t> <t hangText="S-U:">Server unidirectionalPrimary TLV</dd> <dt>S-U:</dt> <dd>Server Unidirectional message, primaryTLV</t> <t hangText="S-A:">ServerTLV</dd> <dt>S-A:</dt> <dd>Server request or unidirectional message,additional TLV</t> <t hangText="SRP:">ResponseAdditional TLV</dd> <dt>SRP:</dt> <dd>Response back to server,primary TLV</t> <t hangText="SRA:">ResponsePrimary TLV</dd> <dt>SRA:</dt> <dd>Response back to server,additional TLV</t> </list> <?rfc subcompact="no" ?> </t> <texttable title="DSOAdditional TLV</dd> </dl> <table anchor="tlv_server_contexts" align="center"> <name>DSO TLV Server ContextSummary" anchor="tlv_server_contexts"> <ttcolSummary</name> <thead> <tr> <th align="right">TLVType</ttcol> <ttcol align="center">S-P</ttcol> <ttcol align="center">S-U</ttcol> <ttcol align="center">S-A</ttcol> <ttcol align="center">SRP</ttcol> <ttcol align="center">SRA</ttcol> <c>SUBSCRIBE</c><c></c><c></c><c></c><c></c><c></c> <c>PUSH</c><c></c><c>X</c><c></c><c></c><c></c> <c>UNSUBSCRIBE</c><c></c><c></c><c></c><c></c><c></c> <c>RECONFIRM</c><c></c><c></c><c></c><c></c><c></c> </texttable>Type</th> <th align="center">S-P</th> <th align="center">S-U</th> <th align="center">S-A</th> <th align="center">SRP</th> <th align="center">SRA</th> </tr> </thead> <tbody> <tr> <td align="right">SUBSCRIBE</td> <td align="center"/> <td align="center"/> <td align="center"/> <td align="center"/> <td align="center"/> </tr> <tr> <td align="right">PUSH</td> <td align="center"/> <td align="center">X</td> <td align="center"/> <td align="center"/> <td align="center"/> </tr> <tr> <td align="right">UNSUBSCRIBE</td> <td align="center"/> <td align="center"/> <td align="center"/> <td align="center"/> <td align="center"/> </tr> <tr> <td align="right">RECONFIRM</td> <td align="center"/> <td align="center"/> <td align="center"/> <td align="center"/> <td align="center"/> </tr> </tbody> </table> </section> <sectiontitle="Client-Initiated Termination">numbered="true" toc="default"> <name>Client-Initiated Termination</name> <t>An individual subscription is terminated by sending an UNSUBSCRIBE TLV for that specific subscription, or all subscriptions can becancelledcanceled at once by the client closing the DSO session. When a client terminates an individual subscription (via UNSUBSCRIBE) or all subscriptions on that DSO session (by ending thesession)session), it is signaling to the server that it is no longer interested in receiving those particular updates. It is informing the server that the server may release any state information it has been keeping with regards to these particular subscriptions.</t> <t>After terminating its last subscription on a session via UNSUBSCRIBE, a clientMAY<bcp14>MAY</bcp14> close the sessionimmediately,immediately or it may keep it open if it anticipates performing further operations on that session in the future. If a client wishes to keep an idle session open, itMUST<bcp14>MUST</bcp14> respect the maximum idle time required by the server <xreftarget="RFC8490"/>.</t>target="RFC8490" format="default"/>.</t> <t>If a client plans to terminate one or more subscriptions on a session and doesn't intend to keep that session open, then as an efficiencyoptimizationoptimization, itMAY<bcp14>MAY</bcp14> instead choose to simply close the session, which implicitly terminates all subscriptions on that session. This may occur because the client computer is being shut down, is going to sleep, the application requiring the subscriptions has terminated, or simply because the last active subscription on that session has beencancelled.</t>canceled.</t> <t>When closing a session, a client should perform an orderly close of the TLS session. Typical APIs will provide a session close method that will send a TLS close_notify alert(see Section 6.1 of theas described in <xref target="RFC8446" sectionFormat="of" section="6.1">the TLS 1.3specification <xref target="RFC8446"/>).specification</xref>. This instructs the recipient that the sender will not send any more data over the session. After sending the TLS close_notifyalertalert, the clientMUST<bcp14>MUST</bcp14> gracefully close the underlying connection using a TCPFIN,FIN so that the TLS close_notify is reliably delivered. The mechanisms for gracefully closing a TCP connection with a TCP FIN vary depending on the networking API. For example, in the BSD Sockets API, sending a TCP FIN is achieved by calling "shutdown(s,SHUT_WR)" and keeping the socket open until all remaining data has been read from it.</t> <t>If the session is forcibly closed at the TCP level by sending a RST from either end of the connection, data may be lost.</t><?rfc needLines="10" ?></section> <sectiontitle="Clientanchor="polling" numbered="true" toc="default"> <name>Client Fallback toPolling" anchor="polling">Polling</name> <t>There are cases where a client may exhaust all avenues for establishing a DNS Push Notification subscription without success. This can happen if the client's configured recursive resolver does not support DNS over TLS, or supports DNS over TLS but is not listening on TCP port 853, or supports DNS over TLS on TCP port 853 but does not support DSO on that port, or for some other reason is unable to provide a DNS Push Notification subscription. In thiscasecase, the client will attempt to communicate directly with an appropriate server, and it may be that the zone apex discovery fails, or there is no<spanx style="verb">_dns&nbhy;push&nbhy;tls._tcp.<zone></spanx><tt>_dns&nbhy;push&nbhy;tls._tcp.<zone></tt> SRV record, or the server indicated in the SRV record is misconfigured, overloaded, or is unresponsive for some other reason.</t> <t>Regardless of the reason for the failure, after being unable to establish the desired DNS Push Notification subscription, it is likely that the client will still wish to know the answer it seeks, even if that answer cannot be obtained with the timely change notifications provided by DNS Push Notifications. In suchcasescases, it is likely that the client will obtain the answer it seeks via a conventional DNS query instead, repeated at some interval to detect when the answer RRset changes.</t> <t>In the case where a client responds to its failure to establish a DNS Push Notification subscription by falling back to polling with conventional DNS queries instead, the polling rate should be controlled to avoid placing excessive burden on the server. The interval between successive DNS queries for the same name,typetype, and classSHOULD<bcp14>SHOULD</bcp14> be at least the minimumof:of 900 seconds (15minutes),minutes) or two seconds more than the TTL of the answer RRset.</t> <t>The reason that for TTLsshorter thanup to 898 seconds the query should not be reissued until two seconds*after*<em>after</em> the answer RRset hasexpiredexpired, is to ensure that the answer RRset has also expired from the cache on the client's configured recursive resolver. Otherwise (particularly if the clocks on the client and the recursive resolver do not run at precisely the samerate)rate), there's a risk of a race condition where the client queries its configured recursive resolver just as the answer RRset has one second remaining in the recursive resolver's cache. The client wouldthenreceive a reply telling it that the answer RRset has one secondremaining, and thenremaining; the client would thenre-queryrequery the recursive resolver again one secondlater whenlater. If by this time the answer RRset has actuallyexpires, and only then wouldexpired from the recursive resolver's cache, the recursive resolver would then issue a new query to fetchnewfresh data from the authoritative server. Waiting until the answer RRset has definitely expired from thethecache on the client's configured recursive resolver avoids this race condition and any unnecessary additional queries it causes.</t> <t>Each time a client is about to reissue its query to discover changes to the answer RRset, it should first make a new attempt to establish a DNS Push Notificationsubscription,subscription using previously cached DNS answers as appropriate. After a temporary misconfiguration has been remedied, this allows a client that is polling to return to using DNS Push Notifications for asynchronous notification of changes.</t> </section> </section> <sectiontitle="Security Considerations" anchor="Security">anchor="Security" numbered="true" toc="default"> <name>Security Considerations</name> <t>The Strict PrivacyUsage Profileprofile for DNS over TLS isREQUIRED<bcp14>REQUIRED</bcp14> for DNS Push Notifications <xreftarget="RFC8310"/>.target="RFC8310" format="default"/>. Cleartext connections for DNS Push Notifications are not permissible. Since this is a new protocol, transition mechanisms from the Opportunistic Privacy profile are unnecessary.</t> <t>Also, seeSection 9 of the DNS over (D)TLS<xref target="RFC8310" sectionFormat="of" section="9">the document Usage Profilesdocument <xref target="RFC8310"/>for DNS over (D)TLS</xref> for additional recommendations for various versions of TLS usage.</t> <t>As a consequence of requiring TLS, client certificate authentication and verification may also be enforced by the server for stronger client-server security or end-to-end security. However, recommendations for security in particular deployment scenarios are outside the scope of this document.</t> <t>DNSSEC isRECOMMENDED<bcp14>RECOMMENDED</bcp14> for the authentication of DNS Push Notification servers. TLS alone does not provide complete security. TLS certificate verification can provide reasonable assurance that the client is really talking to the server associated with the desired host name, but since the desired host name is learned via a DNS SRV query, if the SRV query issubvertedsubverted, then the client may have a secure connection to a rogue server. DNSSEC can provide added confidence that the SRV query has not been subverted.</t><?rfc needLines="14" ?><sectiontitle="Security Services">numbered="true" toc="default"> <name>Security Services</name> <t>It is the goal of using TLS to provide the following security services:<list style="hanging"> <t hangText="Confidentiality:">All</t> <dl newline="false" spacing="normal"> <dt>Confidentiality:</dt> <dd>All application-layer communication is encrypted with the goal that no party should be able to decrypt it except the intendedreceiver.</t> <t hangText="Datareceiver.</dd> <dt>Data integrityprotection:">Anyprotection:</dt> <dd>Any changes made to the communication in transit are detectable by thereceiver.</t> <t hangText="Authentication:">An end-pointreceiver.</dd> <dt>Authentication:</dt> <dd>An endpoint of the TLS communication is authenticated as the intended entity to communicatewith.</t> <t hangText="Anti-replay protection:">TLSwith.</dd> <dt>Anti-replay protection:</dt> <dd>TLS provides for the detection of and prevention against messages sent previously over a TLS connection (such as DNS Push Notifications). If prior messages are re-sent at a later time as a form of a man-in-the-middleattackattack, then the receiver will detect this and reject the replayedmessages.</t> </list> </t>messages.</dd> </dl> <t>Deployment recommendations on the appropriate key lengths andcyphercipher suites are beyond the scope of this document. Please refer to the current TLS Recommendations <xreftarget="BCP195">TLS Recommendations</xref>target="BCP195" format="default"></xref> for the best current practices. Keep in mind that best practices only exist for a snapshot intimetime, and recommendations will continue to change. Updated versions or errata may exist for these recommendations.</t> </section> <sectiontitle="TLSanchor="tls_name_auth" numbered="true" toc="default"> <name>TLS NameAuthentication" anchor="tls_name_auth">Authentication</name> <t>As described in <xreftarget="discovery"/>,target="discovery" format="default"/>, the client discovers the DNS Push Notification server using an SRV lookup for the record name<spanx style="verb">_dns&nbhy;push&nbhy;tls._tcp.<zone></spanx>.<tt>_dns&nbhy;push&nbhy;tls._tcp.<zone></tt>. The server connection endpointSHOULD<bcp14>SHOULD</bcp14> then be authenticated using DANE TLSA records for the associated SRV record. This associates the target's name and port number with a trusted TLS certificate <xreftarget="RFC7673"/>.target="RFC7673" format="default"/>. This procedure uses the TLS Server Name Indication (SNI) extension <xreftarget="RFC6066"/>target="RFC6066" format="default"/> to inform the server of the name the client has authenticated through the use of TLSA records. Therefore, if the SRV record passes DNSSEC validation and a TLSA record matching the target name isuseable,usable, an SNI extension must be used for the target name to ensure the client is connecting to the server it has authenticated. If the target name does not have a usable TLSA record, then the use of the SNI extension is optional. See<xref target="RFC8310">UsageUsage Profiles for DNS over TLS and DNS overDTLS</xref>DTLS <xref target="RFC8310" format="default"></xref> for more information on authenticating domain names.</t> </section> <sectiontitle="TLSanchor="early_data" numbered="true" toc="default"> <name>TLS EarlyData" anchor="early_data">Data</name> <t>DSO messages with the SUBSCRIBE TLV as the Primary TLV are permitted in TLS early data. Using TLS early data can save one network roundtrip,trip and can result in the client obtaining results faster.</t> <t>However, there are some factors to consider before using TLS early data.</t> <t>TLSEarly Dataearly data is not forward secret. In cases where forward secrecy of DNS Push Notification subscriptions is required, the client should not use TLSEarly Data.</t>early data.</t> <t>With TLS earlydatadata, there are no guarantees of non-replay between connections. If packets are duplicated and delayed in the network, the later arrivals could be mistaken for new subscription requests.GenerallyGenerally, this is not a majorconcern,concern since the amount of state generated on the server for these spurious subscriptions is small andshort-lived,short lived since the TCP connection will not complete the three-way handshake. ServersMAY<bcp14>MAY</bcp14> choose to implement rate-limiting measures that are activated when the server detects an excessive number of spurious subscription requests.</t> <t>For further guidance on use of TLS early data, please see discussion of zero round-trip data(Section 2.3, Section 8,in Sections <xref target="RFC8446" sectionFormat="bare" section="2.3"/> and <xref target="RFC8446" sectionFormat="bare" section="8"/>, and AppendixE.5) in the<xref target="RFC8446" sectionFormat="bare" section="E.5"/>, of <xref target="RFC8446">the TLS 1.3specification, <xref target="RFC8446"/>.</t>specification</xref>.</t> </section> <sectiontitle="TLSanchor="resumption" numbered="true" toc="default"> <name>TLS SessionResumption" anchor="resumption">Resumption</name> <t>TLSSession Resumptionsession resumption <xreftarget="RFC8446"/>target="RFC8446" format="default"/> is permissible on DNS Push Notification servers. However, closing the TLS connection terminates the DSO session. When the TLS session is resumed, the DNS Push Notification server will not have any subscription state and will proceed as with any other new DSO session. Use of TLSSession Resumptionsession resumption may allow a TLS connection to be set up more quickly, but the client will still have to recreate any desired subscriptions.</t><?rfc needLines="30" ?></section> </section> <sectiontitle="IANA Considerations" anchor="IANA">anchor="IANA" numbered="true" toc="default"> <name>IANA Considerations</name> <t>This document defines a new service name, only applicable for the TCP protocol,to bewhich has been recorded in the IANAService Type Registry"Service Name and Transport Protocol Port Number Registry" <xref target="RFC6335" format="default"/> <xreftarget="RFC6335"/><xref target="SRVTYPE"/>.</t> <texttable title="IANAtarget="SRVTYPE" format="default"/>.</t> <table anchor="iana_service_table" align="center"> <name>IANA Service TypeAssignments" anchor="iana_service_table"> <ttcol width="25%" align="left">Name</ttcol> <ttcol align="center">Port</ttcol> <ttcol align="center">Value</ttcol> <ttcol align="left">Definition</ttcol> <c>DNSAssignments</name> <thead> <tr> <th align="left">Name</th> <th align="center">Port</th> <th align="center">Value</th> <th align="center">Section</th> </tr> </thead> <tbody> <tr> <td align="left">DNS Push Notification ServiceType</c> <c>None</c> <c><spanx style="verb">_dns&nbhy;push&nbhy;tls._tcp</spanx></c> <c><xref target="discovery"/></c> </texttable>Type</td> <td align="center">None</td> <td align="center"> <tt>_dns&nbhy;push&nbhy;tls._tcp</tt></td> <td align="center"> <xref target="discovery" format="counter"/></td> </tr> </tbody> </table> <t>This document defines four new DNS Stateful Operation TLVtypes to betypes, which have been recorded in the IANADSO"DSO TypeCode RegistryCodes" registry <xreftarget="RFC8490"/><xref target="DSOTYPE"/>.</t> <texttable title="IANAtarget="RFC8490" format="default"/> <xref target="DSOTYPE" format="default"/>.</t> <table anchor="iana_tlv_table" align="center"> <name>IANA DSO TLV Type CodeAssignments" anchor="iana_tlv_table"> <ttcol align="left" >Name</ttcol> <ttcol align="center" width="18%">Value</ttcol> <ttcol align="center" >Early Data</ttcol> <ttcol align="center" width="28%">Status</ttcol> <ttcol align="left" width="20%">Definition</ttcol> <c>SUBSCRIBE</c> <c>TBA (0x40)</c> <c>OK</c> <c>Standards Track</c> <c><xref target="subscribe"/></c> <c>PUSH</c> <c>TBA (0x41)</c> <c>NO</c> <c>Standards Track</c> <c><xref target="push"/></c> <c>UNSUBSCRIBE</c> <c>TBA (0x42)</c> <c>NO</c> <c>Standards Track</c> <c><xref target="unsubscribe"/></c> <c>RECONFIRM</c> <c>TBA (0x43)</c> <c>NO</c> <c>Standards Track</c> <c><xref target="reconfirm"/></c> </texttable>Assignments</name> <thead> <tr> <th align="left">Name</th> <th align="center">Value</th> <th align="center">Early Data</th> <th align="center">Status</th> <th align="center">Section</th> </tr> </thead> <tbody> <tr> <td align="left">SUBSCRIBE</td> <td align="center">0x0040</td> <td align="center">OK</td> <td align="center">Standards Track</td> <td align="center"> <xref target="subscribe" format="counter"/></td> </tr> <tr> <td align="left">PUSH</td> <td align="center">0x0041</td> <td align="center">NO</td> <td align="center">Standards Track</td> <td align="center"> <xref target="push" format="counter"/></td> </tr> <tr> <td align="left">UNSUBSCRIBE</td> <td align="center">0x0042</td> <td align="center">NO</td> <td align="center">Standards Track</td> <td align="center"> <xref target="unsubscribe" format="counter"/></td> </tr> <tr> <td align="left">RECONFIRM</td> <td align="center">0x0043</td> <td align="center">NO</td> <td align="center">Standards Track</td> <td align="center"> <xref target="reconfirm" format="counter"/></td> </tr> </tbody> </table> <t>This document defines no new DNS OPCODEs or RCODEs.</t><?rfc needLines="12" ?> </section> <section title="Acknowledgements" anchor="Acknowledgements"> <t>The authors would like to thank Kiren Sekar and Marc Krochmal for previous work completed in this field.</t> <t>This draft has been improved due to comments from Ran Atkinson, Tim Chown, Sara Dickinson, Mark Delany, Ralph Droms, Jan Komissar, Eric Rescorla, Michael Richardson, David Schinazi, Manju Shankar Rao, Robert Sparks, Markus Stenberg, Andrew Sullivan, Michael Sweet, Dave Thaler, Brian Trammell, Bernie Volz, Eric Vyncke, Christopher Wood, Liang Xia, and Soraia Zlatkovic. Ted Lemon provided clarifying text that was greatly appreciated.</t> <?rfc needLines="15" ?></section> </middle><!-- *****BACK MATTER ***** --><back><!-- References split into informative and normative --> <!-- There are 2 ways to insert reference entries from the citation libraries: 1. define an ENTITY at the top, and use "ampersand character"RFC2629; here (as shown) 2. simply use a PI "less than character"?rfc include="reference.RFC.2119.xml"?> here (for I-Ds: include="reference.I-D.narten-iana-considerations-rfc2434bis.xml") Both are cited textually in the same manner: by using xref elements. If you use the PI option, xml2rfc will, by default, try to find included files in the same directory as the including file. You can also define the XML_LIBRARY environment variable with a value containing a set of directories to search. These can be either in the local filing system or remote ones accessed by http (http://domain/dir/... ).--> <references title="Normative References"> &RFC0020; &RFC0768; &RFC0793; &RFC1034; &RFC1035; &RFC1123; &RFC2119; &RFC2136; &RFC2181; &RFC2782; &RFC6066; <?rfc include="reference.RFC.6335" ?> &RFC6895; &RFC7673; &RFC7766; &RFC7858; &RFC8174; &RFC8310; &RFC8446; &RFC8490;<displayreference target="I-D.ietf-tcpm-rack" to="TCPRACK"/> <references> <name>References</name> <references> <name>Normative References</name> <xi:include href="https://xml2rfc.ietf.org/public/rfc/bibxml/reference.RFC.0020.xml"/> <xi:include href="https://xml2rfc.ietf.org/public/rfc/bibxml/reference.RFC.0768.xml"/> <xi:include href="https://xml2rfc.ietf.org/public/rfc/bibxml/reference.RFC.0793.xml"/> <xi:include href="https://xml2rfc.ietf.org/public/rfc/bibxml/reference.RFC.1034.xml"/> <xi:include href="https://xml2rfc.ietf.org/public/rfc/bibxml/reference.RFC.1035.xml"/> <xi:include href="https://xml2rfc.ietf.org/public/rfc/bibxml/reference.RFC.1123.xml"/> <xi:include href="https://xml2rfc.ietf.org/public/rfc/bibxml/reference.RFC.2119.xml"/> <xi:include href="https://xml2rfc.ietf.org/public/rfc/bibxml/reference.RFC.2136.xml"/> <xi:include href="https://xml2rfc.ietf.org/public/rfc/bibxml/reference.RFC.2181.xml"/> <xi:include href="https://xml2rfc.ietf.org/public/rfc/bibxml/reference.RFC.2782.xml"/> <xi:include href="https://xml2rfc.ietf.org/public/rfc/bibxml/reference.RFC.6066.xml"/> <xi:include href="https://xml2rfc.ietf.org/public/rfc/bibxml/reference.RFC.6335.xml"/> <xi:include href="https://xml2rfc.ietf.org/public/rfc/bibxml/reference.RFC.6895.xml"/> <xi:include href="https://xml2rfc.ietf.org/public/rfc/bibxml/reference.RFC.7673.xml"/> <xi:include href="https://xml2rfc.ietf.org/public/rfc/bibxml/reference.RFC.7766.xml"/> <xi:include href="https://xml2rfc.ietf.org/public/rfc/bibxml/reference.RFC.7858.xml"/> <xi:include href="https://xml2rfc.ietf.org/public/rfc/bibxml/reference.RFC.8174.xml"/> <xi:include href="https://xml2rfc.ietf.org/public/rfc/bibxml/reference.RFC.8310.xml"/> <xi:include href="https://xml2rfc.ietf.org/public/rfc/bibxml/reference.RFC.8446.xml"/> <xi:include href="https://xml2rfc.ietf.org/public/rfc/bibxml/reference.RFC.8490.xml"/> <reference anchor="SRVTYPE"target="http://www.iana.org/assignments/service-names-port-numbers/">target="https://www.iana.org/assignments/service-names-port-numbers/"> <front> <title>Service Name and Transport Protocol Port Number Registry</title><author/> <date/><author><organization>IANA</organization></author> </front> </reference> <reference anchor="DSOTYPE" target="https://www.iana.org/assignments/dns-parameters/"> <front><title>DSO Type Code Registry</title> <author/> <date/><title>Domain Name System (DNS) Parameters</title> <author><organization>IANA</organization></author> </front> </reference> </references><!-- Use needLines to make sure "Authors' Addresses" line doesn't appear as the last line on the page --> <?rfc needLines="9" ?> <references title="Informative References"><references> <name>Informative References</name> <referenceanchor="BCP195" target="http://www.rfc-editor.org/info/bcp195"><front>anchor='BCP195' target='https://www.rfc-editor.org/info/bcp195'> <front> <title>Recommendations for Secure Use of Transport Layer Security (TLS) and Datagram Transport Layer Security (DTLS)</title> <authorinitials="Y." surname="Sheffer" fullname="Yaron Sheffer"/>initials='Y.' surname='Sheffer' fullname='Y. Sheffer'><organization /></author> <authorinitials="R." surname="Holz" fullname="Ralph Holz"/>initials='R.' surname='Holz' fullname='R. Holz'><organization /></author> <authorinitials="P." surname="Saint-Andre" fullname="Peter Saint-Andre"/>initials='P.' surname='Saint-Andre' fullname='P. 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From 2007 onwards, LLQ was implemented in Apple products including Mac OS X, Bonjour for Windows, and AirPort wireless base stations. In 2019, the LLQ protocol was superseded by the IETF Standards Track RFC "DNS Push Notifications", which builds on experience gained with the LLQ protocol to create a superior replacement.</t></abstract></front> <seriesInfoname='Internet-Draft' value='draft-sekar-dns-llq-03' /> <format type='TXT' target='http://www.ietf.org/internet-drafts/draft-sekar-dns-llq-03.txt'name='RFC' value='8764' /> <seriesInfo name="DOI" value="10.17487/RFC8764"/> </reference> <referenceanchor='DisProx'>anchor='RFC8766' target="https://www.rfc-editor.org/info/rfc8766"> <front> <title>Discovery Proxy for Multicast DNS-Based Service Discovery</title> <author initials='S' surname='Cheshire' fullname='Stuart Cheshire'> <organization /> </author> <datemonth='March' day='24' year='2019'month='June' year='2020' /><abstract><t>This document specifies a network proxy that uses Multicast DNS to automatically populate the wide-area unicast Domain Name System namespace with records describing devices and services found on the local link.</t></abstract></front> <seriesInfoname='Internet-Draft' value='draft-ietf-dnssd-hybrid-10' /> <format type='TXT' target='http://www.ietf.org/internet-drafts/draft-ietf-dnssd-hybrid-10.txt' />name="RFC" value="8766"/> <seriesInfo name="DOI" value="10.17487/RFC8766"/> </reference> <referenceanchor='SYN'>anchor="SYN" target="https://www.cisco.com/web/about/ac123/ac147/archived_issues/ipj_9-4/ipj_9-4.pdf"> <front> <title>Defenses Against TCP SYN Flooding Attacks</title> <authorinitials='W.' surname='Eddy' fullname='Wesley Eddy'>initials="W." surname="Eddy" fullname="Wesley Eddy"> <organization>Verizon Federal Network Systems</organization> <address> <email>weddy@grc.nasa.gov</email> </address> </author> <dateyear='2006' month='December' />year="2006" month="December"/> <keyword>TCP</keyword> </front><seriesInfo name="The<refcontent>The Internet ProtocolJournal," value='Cisco Systems' /> <seriesInfo name='Volume' value='9' /> <seriesInfo name='Number' value='4' /> <format type='PDF' octets='882020' target="http://www.cisco.com/web/about/ac123/ac147/archived_issues/ipj_9-4/ipj_9-4.pdf" /> <format type='HTML' octets='65566' target="http://www.cisco.com/web/about/ac123/ac147/archived_issues/ipj_9-4/syn_flooding_attacks.html" />Journal</refcontent> <refcontent>Cisco Systems</refcontent> <refcontent>Volume 9</refcontent> <refcontent>Number 4</refcontent> </reference> <referenceanchor='obs' target="https://en.wikipedia.org/wiki/Observer_pattern">anchor="OBS" target="https://en.wikipedia.org/w/index.php?title=Observer_pattern&oldid=939702131"> <front> <title>ObserverPattern</title> <author/> <date/>pattern</title> <author> <organization>Wikipedia </organization> </author> <date month="February" year="2020"/> </front> </reference> <referenceanchor='SD-API'anchor="SD-API" target="https://opensource.apple.com/source/mDNSResponder/mDNSResponder-878.70.2/mDNSShared/dns_sd.h.auto.html"> <front><title>dns_sd.h API</title> <author/> <date/><title>dns_sd.h</title> <author> <organization>Apple Inc. </organization> </author> </front> </reference> <referenceanchor="XEP0060">anchor="XEP0060" target="https://xmpp.org/extensions/xep-0060.html"> <front> <title>Publish-Subscribe</title> <author initials="P." surname="Millard" fullname="Peter Millard"> <organization/> <address> <email/> </address> </author> <author initials="P." surname="Saint-Andre" fullname="Peter Saint-Andre"> <organization/> <address> <email>peter@andyet.net</email> </address> </author> <author initials="R." surname="Meijer" fullname="Ralph Meijer"> <organization/> <address> <email>ralphm@ik.nu</email> </address> </author> <dateday="01" month="July" year="2010"/>month="October" year="2019"/> </front><seriesInfo name="XSF XEP" value="0060"/> <format type="HTML" target="http://xmpp.org/extensions/xep-0060.html"/><refcontent>XSF XEP 0060 </refcontent> </reference> </references> </references> <section anchor="Acknowledgments" numbered="false" toc="default"> <name>Acknowledgments</name> <t>The authors would like to thank <contact fullname="Kiren Sekar"/> and <contact fullname="Marc Krochmal"/> for previous work completed in this field.</t> <t>This document has been improved due to comments from <contact fullname="Ran Atkinson"/>, <contact fullname="Tim Chown"/>, <contact fullname="Sara Dickinson"/>, <contact fullname="Mark Delany"/>, <contact fullname="Ralph Droms"/>, <contact fullname="Jan Komissar"/>, <contact fullname="Eric Rescorla"/>, <contact fullname="Michael Richardson"/>, <contact fullname="David Schinazi"/>, <contact fullname="Manju Shankar Rao"/>, <contact fullname="Robert Sparks"/>, <contact fullname="Markus Stenberg"/>, <contact fullname="Andrew Sullivan"/>, <contact fullname="Michael Sweet"/>, <contact fullname="Dave Thaler"/>, <contact fullname="Brian Trammell"/>, <contact fullname="Bernie Volz"/>, <contact fullname="Éric Vyncke"/>, <contact fullname="Christopher Wood"/>, <contact fullname="Liang Xia"/>, and <contact fullname="Soraia Zlatkovic"/>. <contact fullname="Ted Lemon"/> provided clarifying text that was greatly appreciated.</t> </section> </back> </rfc>