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Internet Engineering Task Force                              T. Pusateri
Internet-Draft                                       Seeking affiliation
Intended status: Standards Track                             S. Cheshire
Expires: September 22, 2016                                   Apple Inc.
                                                          March 21, 2016


                         DNS Push Notifications
                        draft-ietf-dnssd-push-06

Abstract

   The Domain Name System (DNS) was designed to return matching records
   efficiently for queries for data that is relatively static.  When
   those records change frequently, DNS is still efficient at returning
   the updated results when polled.  But 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.

Status of This Memo

   This Internet-Draft is submitted in full conformance with the
   provisions of BCP 78 and BCP 79.

   Internet-Drafts are working documents of the Internet Engineering
   Task Force (IETF).  Note that other groups may also distribute
   working documents as Internet-Drafts.  The list of current Internet-
   Drafts is at http://datatracker.ietf.org/drafts/current/.

   Internet-Drafts are draft documents valid for a maximum of six months
   and may be updated, replaced, or obsoleted by other documents at any
   time.  It is inappropriate to use Internet-Drafts as reference
   material or to cite them other than as "work in progress."

   This Internet-Draft will expire on September 22, 2016.

Copyright Notice

   Copyright (c) 2016 IETF Trust and the persons identified as the
   document authors.  All rights reserved.

   This document is subject to BCP 78 and the IETF Trust's Legal
   Provisions Relating to IETF Documents
   (http://trustee.ietf.org/license-info) in effect on the date of
   publication of this document.  Please review these documents
   carefully, as they describe your rights and restrictions with respect
   to this document.  Code Components extracted from this document must



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   include Simplified BSD License text as described in Section 4.e of
   the Trust Legal Provisions and are provided without warranty as
   described in the Simplified BSD License.

Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   2
     1.1.  Requirements Language . . . . . . . . . . . . . . . . . .   3
   2.  Motivation  . . . . . . . . . . . . . . . . . . . . . . . . .   3
   3.  Overview  . . . . . . . . . . . . . . . . . . . . . . . . . .   5
   4.  Transport . . . . . . . . . . . . . . . . . . . . . . . . . .   6
     4.1.  Client-Initiated Termination  . . . . . . . . . . . . . .   7
     4.2.  Server-Initiated Termination  . . . . . . . . . . . . . .   7
   5.  State Considerations  . . . . . . . . . . . . . . . . . . . .   9
   6.  Protocol Operation  . . . . . . . . . . . . . . . . . . . . .  10
     6.1.  Discovery . . . . . . . . . . . . . . . . . . . . . . . .  11
     6.2.  DNS Push Notification SUBSCRIBE . . . . . . . . . . . . .  13
     6.3.  DNS Push Notification UNSUBSCRIBE . . . . . . . . . . . .  18
     6.4.  DNS Push Notification Update Messages . . . . . . . . . .  19
     6.5.  DNS RECONFIRM . . . . . . . . . . . . . . . . . . . . . .  22
     6.6.  DNS Push Notification Termination Message . . . . . . . .  24
   7.  Security Considerations . . . . . . . . . . . . . . . . . . .  26
     7.1.  Security Services . . . . . . . . . . . . . . . . . . . .  26
     7.2.  TLS Name Authentication . . . . . . . . . . . . . . . . .  27
     7.3.  TLS Compression . . . . . . . . . . . . . . . . . . . . .  27
     7.4.  TLS Session Resumption  . . . . . . . . . . . . . . . . .  27
   8.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .  27
   9.  Acknowledgements  . . . . . . . . . . . . . . . . . . . . . .  28
   10. References  . . . . . . . . . . . . . . . . . . . . . . . . .  28
     10.1.  Normative References . . . . . . . . . . . . . . . . . .  28
     10.2.  Informative References . . . . . . . . . . . . . . . . .  30
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  31

1.  Introduction

   DNS records may be updated using DNS Update [RFC2136].  Other
   mechanisms such as a Hybrid Proxy [I-D.ietf-dnssd-hybrid] can also
   generate changes to a DNS zone.  This document specifies a protocol
   for Unicast DNS clients to subscribe to receive asynchronous
   notifications of changes to RRSets of interest.  It is immediately
   relevant in the case of DNS Service Discovery [RFC6763] but is not
   limited to that use case, and provides a general DNS mechanism for
   DNS record change notifications.  Familiarity with the DNS protocol
   and DNS packet formats is assumed [RFC1034] [RFC1035] [RFC6195].







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1.1.  Requirements Language

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   "SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and
   "OPTIONAL" in this document are to be interpreted as described in
   "Key words for use in RFCs to Indicate Requirement Levels" [RFC2119].

2.  Motivation

   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 to state changes
   following the Observer design pattern.  XMPP Publish-Subscribe
   [XEP0060] and Atom [RFC4287] 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 result in more timely notification of changes with
   reduced CPU usage and lower network traffic.

   Multicast DNS [RFC6762] implementations always listen on a well known
   link-local IP multicast group, and new services and updates are sent
   for all group members to receive.  Therefore, Multicast DNS already
   has asynchronous change notification capability.  However, when DNS
   Service Discovery [RFC6763] is used across a wide area network using
   Unicast DNS (possibly facilitated via a Hybrid Proxy
   [I-D.ietf-dnssd-hybrid]) it would be beneficial to have an equivalent
   capability for Unicast DNS, to allow clients to learn about DNS
   record changes in a timely manner without polling.

   DNS Long-Lived Queries (LLQ) [I-D.sekar-dns-llq] is an existing
   deployed solution to provide asynchronous change notifications.  Even
   though it can be used over TCP, LLQ is defined primarily as a UDP-
   based protocol, and as such it defines its own equivalents of
   existing TCP features like the three-way handshake.  This document
   builds on experience gained with the LLQ protocol, with an improved
   design that uses long-lived TCP connections instead of UDP (and
   therefore doesn't need to duplicate existing TCP functionality), and
   adopts the syntax and semantics of DNS Update messages [RFC2136]
   instead of inventing a new vocabulary of messages to communicate DNS
   zone changes.










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   Because DNS Push Notifications impose a certain load on the
   responding server (though less load than rapid polling of that
   server) DNS Push Notification clients SHOULD exercise restraint in
   issuing DNS Push Notification subscriptions.  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 of
   that subscription to the user) and the subscription SHOULD be
   cancelled as soon as the need for that data ends (for example, when
   the user dismisses that display).  Implementations MAY want to
   implement idle timeouts, so that if the user ceases interacting with
   the device, the display showing the result of the DNS Push
   Notification subscription is automatically dismissed after a certain
   period of inactivity.  For example, if a user presses the "Print"
   button on their phone, and then leaves the phone showing the printer
   discovery screen until the phone goes to sleep, then the printer
   discovery screen should be automatically dismissed as the device goes
   to sleep.  If the user does still intend to print, this will require
   them to press the "Print" button again when they wake their phone up.

   A DNS Push Notification client MUST NOT 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 it will be really fast if
   the user does choose to bring up an on-screen display of that data.
   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
   needed later.

























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3.  Overview

   The existing DNS Update protocol [RFC2136] provides a mechanism for
   clients to add or delete individual resource records (RRs) or entire
   resource record sets (RRSets) on the zone's server.

   This specification adopts a simplified subset of these existing
   syntax and semantics, and uses them for DNS Push Notification
   messages going in the opposite direction, from server to client, to
   communicate changes to a zone.  The client subscribes for Push
   Notifications by connecting to the server and sending DNS message(s)
   indicating the RRSet(s) of interest.  When the client loses interest
   in updates to these records, it unsubscribes.

   The DNS Push Notification server for a zone is any server capable
   of generating the correct change notifications for a name.
   It may be a master, slave, or stealth name server [RFC1996].
   Consequently, the "_dns-push-tls._tcp.<zone>" SRV record for a
   zone MAY reference the same target host and port as that zone's
   "_dns-update-tls._tcp.<zone>" SRV record.  When the same target host
   and port is offered for both DNS Updates and DNS Push Notifications,
   a client MAY use a single TCP connection to that server for both DNS
   Updates and DNS Push Notification Queries.

   Supporting DNS Updates and DNS Push Notifications on the same server
   is OPTIONAL.  A DNS Push Notification server is not REQUIRED to
   support DNS Update.

   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.

   Standard DNS Queries MAY be sent over a DNS Push Notification
   connection, provided that these are queries for names falling within
   the server's zone (the <zone> in the "_dns-push-tls._tcp.<zone>" SRV
   record).  The RD (Recursion Desired) bit MUST be zero.

   DNS Push Notification clients are NOT required to implement DNS
   Update Prerequisite processing.  Prerequisites are used to perform
   tentative atomic test-and-set type operations when a client updates
   records on a server, and that concept has no applicability when it
   comes to an authoritative server informing a client of changes to DNS
   records.

   This DNS Push Notification specification includes support for DNS
   classes, for completeness.  However, in practice, it is anticipated
   that for the foreseeable future the only DNS class in use will be DNS



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   class "IN", as it is today with existing DNS servers and clients.  A
   DNS Push Notification server MAY choose to implement only DNS class
   "IN".

4.  Transport

   Implementations of DNS Update [RFC2136] MAY use either User Datagram
   Protocol (UDP) [RFC0768] or Transmission Control Protocol (TCP)
   [RFC0793] as the transport protocol, in keeping with the historical
   precedent that DNS queries must first be sent over UDP [RFC1123].
   This requirement to use UDP has subsequently been relaxed
   [RFC5966][I-D.ietf-dnsop-5966bis].

   In keeping with the more recent precedent, DNS Push Notification is
   defined only for TCP.  DNS Push Notification clients MUST use TLS
   over TCP.

   Connection setup over TCP ensures return reachability and alleviates
   concerns of state overload at the server through anonymous
   subscriptions.  All subscribers are guaranteed to be reachable by the
   server by virtue of the TCP three-way handshake.  Because TCP SYN
   flooding attacks are possible with any protocol over TCP,
   implementers are encouraged to use industry best practices to guard
   against such attacks [IPJ.9-4-TCPSYN] [RFC4953].

   Transport Layer Security (TLS) [RFC5246] is well understood and
   deployed across many protocols running over TCP.  It is designed to
   prevent eavesdropping, tampering, or message forgery.  TLS is
   REQUIRED 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.
   Additional authentication of the SRV target using DNSSEC verification
   and DANE TLSA records [RFC7673] is strongly encouraged.  See below in
   Section 7.2 for details.

   A DNS Push Notification session begins with a client connecting to a
   DNS Push Notification server.  Over that connection the client then
   issues DNS operation requests, such as SUBSCRIBE.












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4.1.  Client-Initiated Termination

   An individual subscription is terminated by sending an UNSUBSCRIBE
   message for that specific subscription, or all subscriptions can be
   cancelled at once by the client closing the connection with a TCP
   RST.  When a client terminates an individual subscription (via
   UNSUBSCRIBE) or all subscriptions on that connection (by closing the
   connection) it is signalling to the server that it is 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.

   After terminating its last subscription on a connection via
   UNSUBSCRIBE, a client MAY close the connection immediately with a TCP
   FIN, or it may keep it open if it anticipates performing further
   operations on that connection in the future.  If a client wishes to
   keep an idle connection open, it MUST meet its keepalive obligations
   [I-D.ietf-dnsop-edns-tcp-keepalive] or the server is entitled to
   close the connection (see below).

   If a client plans to terminate one or more subscriptions on a
   connection and doesn't intend to keep that connection open, then as
   an efficiency optimization it MAY instead choose to simply close the
   connection with a TCP RST, which implicitly terminates all
   subscriptions on that connection.  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 connection has been cancelled.

4.2.  Server-Initiated Termination

   If a client makes a connection and then fails to send any DNS message
   that uses EDNS(0) TCP Keepalive [I-D.ietf-dnsop-edns-tcp-keepalive]
   (either SUBSCRIBE, where Keepalive is implicit, or some other DNS
   message, with an explicit an EDNS(0) TCP Keepalive option) then after
   30 seconds of inactivity the server SHOULD close the connection.  If
   no data has been sent on the connection the server MAY abort the
   connection with a TCP RST.  If data has been sent on the connection
   then the server SHOULD close the connection gracefully with a TCP FIN
   so that the data is reliably delivered.

   In the response to the first successful SUBSCRIBE, the included
   EDNS(0) TCP Keepalive option specifies the idle timeout so that the
   client knows the frequency of traffic it must generate to keep the
   connection alive.  If the idle timeout for that connection changes,
   then the server communicates this by placing an updated EDNS(0) TCP
   Keepalive option in a subsequent message to the client.




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   At both servers and clients, the generation or reception of any
   request, response, update, or keepalive message resets the keepalive
   timer for that connection.

   In the absence of any requests, responses, or update messages on a
   connection, a client MUST generate keepalive traffic before the idle
   timeout expires, or the server is entitled to close the connection.

   If a client disconnects from the network abruptly, without closing
   its connection, the server learns of this after failing to receive
   further traffic from that client.  If no requests, responses, update
   messages or keepalive traffic occurs on a connection for 1.5 times
   the idle timeout, then this indicates that the client is probably no
   longer on the network, and the server SHOULD abort the connection
   with a TCP RST.

   [We need to discuss the nature of "the required keepalives".  Are
   they TCP-layer keepalives?  DNS-layer keepalives?  There is currently
   no DNS-layer keepalive or 'no-op' operation defined.  What would that
   operation be?  A DNS QUERY containing zero questions?  A DNS
   SUBSCRIBE containing zero questions?  An "empty" DNS message over the
   TCP connection (just a pair of zero bytes, signifying a zero-length
   message)?  One benefit of TCP-layer keepalives is that they transmit
   fewer bytes, and involve less software overhead for processing those
   bytes.  Another benefit is that it is more feasible to implement
   these in networking offload hardware, which can allow devices to meet
   their TCP keepalive obligations while sleeping.  This is particularly
   important for battery-powered devices like mobile phones and tablets.
   On the other hand, using TCP-layer keepalives requires an API for a
   client to tell the networking stack at what frequency to perform TCP-
   layer keepalives, and an API for a server to request the networking
   stack to inform it when TCP-layer keepalives are not received by the
   required deadline.  TCP-layer keepalives also only verify liveness of
   the remote networking stack, whereas DNS-layer keepalives provide
   higher assurance of liveness of the remote server application
   software -- though this a limited benefit, since there is no reason
   to expect that DNS Push Notification server software will routinely
   become wedged and unresponsive.]

   After sending an error response to a client, the server MAY close the
   connection with a TCP FIN.

   If the server is overloaded and needs to shed load, it MAY send a
   Termination Message to the client and close the connection with a TCP
   FIN.

   Apart from the cases described above, a server MUST NOT close a
   connection with a DNS Push Notification client, except in



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   extraordinary error conditions.  Closing the connection is the
   client's responsibility, to be done at the client's discretion, when
   it so chooses.  A DNS Push Notification server only closes a DNS Push
   Notification connection under exceptional circumstances, such as when
   the server application software or underlying operating system is
   restarting, the server application terminated unexpectedly (perhaps
   due to a bug that makes it crash), or the server is undergoing
   maintenance procedures.  When possible, a DNS Push Notification
   server SHOULD send a Termination Message (Section 6.6 ) informing the
   client of the reason for the connection being closed.

   After a connection is closed by the server, the client SHOULD try to
   reconnect, to that server, or to another server supporting DNS Push
   Notifications for the zone.  If reconnecting to the same server, and
   there was a Termination Message or error response containing a
   EDNS(0) TCP Keepalive option, the client MUST respect the indicated
   delay before attempting to reconnect.

5.  State Considerations

   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, network bandwidth, and operating system resource
   allocation.  After a client establishes a connection to a DNS server,
   each record 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 connections to alternate DNS servers that support DNS
   Push Notifications for the zone and distribute record subscriptions
   at its discretion.  In this way, both clients and servers can react
   to resource constraints.  Token bucket rate limiting schemes are also
   effective in providing fairness by a server across numerous client
   requests.

















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6.  Protocol Operation

   A DNS Push Notification exchange begins with the client discovering
   the appropriate server, and then making a TLS/TCP connection to it.
   The client may then add and remove Push Notification subscriptions
   over this connection.  In accordance with the current set of active
   subscriptions the server sends relevant asynchronous Push
   Notifications to the client.  Note that a client MUST 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 message cancelling that
   subscription is simultaneously in flight from client to server.

   The exchange between client and server terminates when either end
   closes the TCP connection with a TCP FIN or RST.

   A client SHOULD NOT make multiple TLS/TCP connections to the same DNS
   Push Notification server.  A client SHOULD share a single TLS/TCP
   connection for all requests to the same DNS Push Notification server.
   This shared connection should be used for all DNS Queries and DNS
   Push Notification Queries queries to that server, and for DNS Update
   requests too when the "_dns-update-tls._tcp.<zone>" SRV record
   indicates that the same server also handles DNS Update requests.
   This is to reduce unnecessary load on the DNS Push Notification
   server.

   For the purposes here, the determination of "same server" is made by
   inspecting the target hostname and port, regardless of the name being
   queried, or what zone if falls within.  A given server may support
   Push Notifications (and possibly DNS Updates too) for multiple DNS
   zones.  When a client discovers that the DNS Push Notification server
   (and/or DNS Update server) for several different names (including
   names that fall within different zones) is the same target hostname
   and port, the client SHOULD use a single shared TCP connection for
   all relevant operations on those names.  A client SHOULD NOT open
   multiple TCP connections to the same target host and port just
   because the names being queried (or updated) happen to fall within
   different zones.

   Note that the "same server" determination described here is made
   using the target hostname given in the SRV record, not the IP
   address(es) that the hostname resolves to.  If two different target
   hostnames happen to resolve to the same IP address(es), then the
   client SHOULD NOT recognize these as the "same server" for the
   purposes of using a single shared connection to that server.  If an
   administrator wishes to use a single server for multiple zones and/or
   multiple roles (e.g., both DNS Push Notifications and DNS Updates),



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   and wishes to have clients use a single shared connection for
   operations on that server, then the administrator MUST use the same
   target hostname in the appropriate SRV records.

   However, server implementers and operators should be aware that this
   connection sharing may not be possible in all cases.  A single client
   device may be home to multiple independent client software instances
   that don't know about each other, so a DNS Push Notification server
   MUST be prepared to accept multiple connections from the same client
   IP address.  This is undesirable from an efficiency standpoint, but
   may be unavoidable in some situations, so a DNS Push Notification
   server MUST be prepared to accept multiple connections from the same
   client IP address.

6.1.  Discovery

   The first step in DNS Push Notification subscription is to discover
   an appropriate DNS server that supports DNS Push Notifications for
   the desired zone.  The client MUST also determine which TCP port on
   the server is listening for connections, which need not be (and often
   is not) the typical TCP port 53 used for conventional DNS.

   1.  The client begins the discovery by sending a DNS query to the
       local resolver with record type SOA [RFC1035] for the name of the
       record it wishes to subscribe.

   2.  If the SOA record exists, it MUST be returned in the Answer
       Section of the reply.  If not, the local resolver SHOULD include
       the SOA record for the zone of the requested name in the
       Authority Section.

   3.  If no SOA record is returned, the client then strips off the
       leading label from the requested name.  If the resulting name has
       at least one label in it, the client sends a new SOA query and
       processing continues at step 2 above.  If the resulting name is
       empty (the root label) then this is a network configuration error
       and the client gives up.  The client MAY retry the operation at a
       later time.

   4.  Once the SOA is known, the client sends a DNS query with type SRV
       [RFC2782] for the record name "_dns-push-tls._tcp.<zone>", where
       <zone> is the owner name of the discovered SOA record.

   5.  If the zone in question does not offer DNS Push Notifications
       then SRV record MUST NOT exist and the SRV query will return a
       negative answer.





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   6.  If the zone in question is set up to offer DNS Push Notifications
       then this SRV record MUST exist.  The SRV "target" 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 "port" field.  The address(es) of the target host MAY be
       included in the Additional Section, however, the address records
       SHOULD be authenticated before use as described below in
       Section 7.2 [RFC7673].

   7.  More than one SRV record may be returned.  In this case, the
       "priority" and "weight" values in the returned SRV records are
       used to determine the order in which to contact the servers for
       subscription requests.  As described in the SRV specification
       [RFC2782], the server with the lowest "priority" is first
       contacted.  If more than one server has the same "priority", the
       "weight" indicates the weighted probability that the client
       should contact that server.  Higher weights have higher
       probabilities of being selected.  If a server is not reachable or
       is not willing to accept a subscription request, then a
       subsequent server is to be contacted.

   Each time a client makes a new DNS Push Notification subscription
   connection, it SHOULD repeat the discovery process in order to
   determine the preferred DNS server for subscriptions at that time.



























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6.2.  DNS Push Notification SUBSCRIBE

   A DNS Push Notification client indicates its desire to receive DNS
   Push Notifications for a given domain name by sending a SUBSCRIBE
   request over the established TCP connection to the server.  A
   SUBSCRIBE request is formatted identically to a conventional DNS
   QUERY request [RFC1035], except that the opcode is SUBSCRIBE (6)
   instead of QUERY (0).  If neither QTYPE nor QCLASS are ANY (255) then
   this is a specific subscription to changes for the given name, type
   and class.  If one or both of QTYPE or QCLASS are ANY (255) then this
   subscription matches any type and/or any class, as appropriate.

   In a SUBSCRIBE request the DNS Header QR bit MUST be zero.
   If the QR bit is not zero the message is not a SUBSCRIBE request.

   The AA, TC, RD, RA, Z, AD, and CD bits, the ID field, and the RCODE
   field, MUST be zero on transmission, and MUST be silently ignored on
   reception.

   Like a DNS QUERY request, a SUBSCRIBE request MUST contain exactly
   one question.  Since SUBSCRIBE requests are sent over TCP, multiple
   SUBSCRIBE requests can be concatenated in a single TCP stream and
   packed efficiently into TCP segments, so the ability to pack multiple
   SUBSCRIBE operations into a single DNS message within that TCP stream
   would add extra complexity for little benefit.

   ANCOUNT MUST be zero, and the Answer Section MUST be empty.
   Any records in the Answer Section MUST be silently ignored.

   NSCOUNT MUST be zero, and the Authority Section MUST be empty.
   Any records in the Authority Section MUST be silently ignored.

   ARCOUNT specifies the number of records in the Additional Data
   Section.  Typically this is zero, but it may be nonzero in some
   cases, such as when the request includes an EDNS(0) OPT record.

   If accepted, the subscription will stay in effect until the client
   revokes the subscription or until the connection between the client
   and the server is closed.

   SUBSCRIBE requests on a given connection MUST be unique.  A client
   MUST NOT send a SUBSCRIBE message that duplicates the name, type and
   class of an existing active subscription on that TLS/TCP connection.
   For the purpose of this matching, the established DNS case-
   insensitivity for US-ASCII letters applies (e.g., "foo.com" and
   "Foo.com" are the same).  If a server receives such a duplicate
   SUBSCRIBE message this is an error and the server MUST immediately
   close the TCP connection.



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   DNS wildcarding is not supported.  That is, a wildcard ("*") in a
   SUBSCRIBE message matches only a literal wildcard character ("*") in
   the zone, and nothing else.

   Aliasing is not supported.  That is, a CNAME in a SUBSCRIBE message
   matches only a literal CNAME record in the zone, and nothing else.

   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 responsible for) and this is not an error.
   The server MUST accept these requests and send Push Notifications if
   and when matches are found in the future.

   Since all SUBSCRIBE operations are implicitly long-lived operations,
   the server MUST interpret a SUBSCRIBE request as if it contained an
   EDNS(0) TCP Keepalive option [I-D.ietf-dnsop-edns-tcp-keepalive].  A
   client MUST NOT include an actual EDNS(0) TCP Keepalive option in the
   request, since it is automatic, and implied by the semantics of
   SUBSCRIBE.  If a server receives a SUBSCRIBE request that does
   contain an actual EDNS(0) TCP Keepalive option this is an error and
   the server MUST immediately close the TCP connection.

   A SUBSCRIBE operation MAY include an explicit EDNS(0) [RFC6891] OPT
   record where necessary to carry additional information.

   The presence of a SUBSCRIBE operation on a connection indicates to
   the server that the client fully implements EDNS(0) [RFC6891], and
   can correctly understand any response that conforms to that
   specification.  After receiving a SUBSCRIBE request, the server MAY
   include OPT record in any of its responses, as needed.





















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   Each SUBSCRIBE request generates exactly one SUBSCRIBE response from
   the server.

   In a SUBSCRIBE response the DNS Header QR bit MUST be one.
   If the QR bit is not one the message is not a SUBSCRIBE response.

   The AA, TC, RD, RA, Z, AD, and CD bits, and the ID field, MUST be
   zero on transmission, and MUST be silently ignored on reception.

   The Question Section MUST echo back the values provided by the client
   in the SUBSCRIBE request that generated this SUBSCRIBE response.

   ANCOUNT MUST be zero, and the Answer Section MUST be empty.
   Any records in the Answer Section MUST be silently ignored.
   If the subscription was accepted and there are positive answers for
   the requested name, type and class, then these positive answers MUST
   be communicated to the client in an immediately following Push
   Notification Update, not in the Answer Section of the SUBSCRIBE
   response.  This simplifying requirement is made so that there is only
   a single way that information is communicated to a DNS Push
   Notification client.  Since a DNS Push Notification client has to
   parse information received via Push Notification Updates anyway, it
   is simpler if it does not also have to parse information received via
   the Answer Section of a SUBSCRIBE response.

   NSCOUNT MUST be zero, and the Authority Section MUST be empty.
   Any records in the Authority Section MUST be silently ignored.

   ARCOUNT specifies the number of records in the Additional Data
   Section, e.g., the EDNS(0) OPT record.





















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   In the SUBSCRIBE response the RCODE indicates whether or not the
   subscription was accepted.  Supported RCODEs are as follows:

   +----------+-------+------------------------------------------------+
   | Mnemonic | Value | Description                                    |
   +----------+-------+------------------------------------------------+
   | NOERROR  |   0   | SUBSCRIBE successful.                          |
   | FORMERR  |   1   | Server failed to process request due to a      |
   |          |       | malformed request.                             |
   | SERVFAIL |   2   | Server failed to process request due to        |
   |          |       | resource exhaustion.                           |
   | NXDOMAIN |   3   | NOT APPLICABLE. DNS Push Notification MUST NOT |
   |          |       | return NXDOMAIN errors in response to          |
   |          |       | SUBSCRIBE requests.                            |
   | NOTIMP   |   4   | Server does not implement DNS Push             |
   |          |       | Notifications.                                 |
   | REFUSED  |   5   | Server refuses to process request for policy   |
   |          |       | or security reasons.                           |
   | NOTAUTH  |   9   | Server is not authoritative for the requested  |
   |          |       | name.                                          |
   +----------+-------+------------------------------------------------+

                         SUBSCRIBE Response codes

   This document specifies only these RCODE values for SUBSCRIBE
   Responses.  Servers sending SUBSCRIBE Responses SHOULD use one of
   these values.  However, future circumstances may create situations
   where other RCODE values are appropriate in SUBSCRIBE Responses, so
   clients MUST be prepared to accept SUBSCRIBE Responses with any RCODE
   value.

   In the first SUBSCRIBE response on a connection, the server MUST
   include an explicit EDNS(0) TCP Keepalive option.  If the first
   SUBSCRIBE response does not include an explicit EDNS(0) TCP Keepalive
   option this is an error and the client MUST immediately close the TCP
   connection.  In this case the client should act as if the response
   contained an EDNS(0) TCP Keepalive option with a value of one hour,
   and not attempt any further DNS Push Notification requests to that
   server until one hour has passed.  This situation may occur if a
   client connects to a server that doesn't implement DNS Push
   Notifications at all, and it is important not to burden such servers
   with continuous retries.

   The server MAY include EDNS(0) TCP Keepalive options in subsequent
   messages, if the idle timeout changes.  If the client receives
   subsequent messages that do not contain an explicit EDNS(0) TCP
   Keepalive option then the idle timeout for that connection remains
   unchanged at that time.



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   In an error response, with nonzero RCODE, the server MUST contain an
   EDNS(0) TCP Keepalive option specifying the delay before the client
   tries again:

      For RCODE = 1 (FORMERR) the delay may be any value selected by the
      implementer.  A value of one minute is RECOMMENDED, to avoid high
      load from defective clients.

      For RCODE = 2 (SERVFAIL), which occurs due to resource exhaustion,
      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 is RECOMMENDED.

      For RCODE = 4 (NOTIMP), which occurs on a server that 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 delay SHOULD be one hour.  Note that a
      server that doesn't implement DNS Push Notifications will most
      likely not implement this retry delay mechanism using the EDNS(0)
      TCP Keepalive option either, and in this case the client will fall
      back to the case described above specifying how to handle
      SUBSCRIBE responses that do not contain an EDNS(0) TCP Keepalive
      option.

      For RCODE = 5 (REFUSED), which occurs on a server that implements
      DNS Push 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 the operator.
      This is a misconfiguration, since this server is listed in a
      "_dns-push-tls._tcp.<zone>" SRV record, but the server itself is
      not currently configured to support DNS Push Notifications.  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 is RECOMMENDED.

      For RCODE = 9 (NOTAUTH), which occurs on a server that implements
      DNS Push 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 the operator.
      This is a misconfiguration, since this server is listed in a
      "_dns-push-tls._tcp.<zone>" SRV record, but the server itself is
      not currently configured to support DNS Push Notifications for
      that zone.  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 is RECOMMENDED.






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      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 is RECOMMENDED.

   After sending an error response the server MAY close the TCP
   connection with a FIN, or MAY allow it to remain open.  Clients MUST
   correctly handle both cases.

6.3.  DNS Push Notification UNSUBSCRIBE

   To cancel an individual subscription without closing the entire
   connection, the client sends an UNSUBSCRIBE message over the
   established TCP connection to the server.  The UNSUBSCRIBE message is
   formatted identically to the SUBSCRIBE message which created the
   subscription, with the exact same name, type and class, except that
   the opcode is UNSUBSCRIBE (7) instead of SUBSCRIBE (6).

   A client MUST NOT send an UNSUBSCRIBE message that does not exactly
   match the name, type and class of an existing active subscription on
   that TLS/TCP connection.  If a server receives such an UNSUBSCRIBE
   message this is an error and the server MUST immediately close the
   connection.

   No response message is generated as a result of processing an
   UNSUBSCRIBE message.

   Having being successfully revoked with a correctly-formatted
   UNSUBSCRIBE message, the previously referenced subscription is no
   longer active and the server MAY discard the state associated with it
   immediately, or later, at the server's discretion.





















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6.4.  DNS Push Notification Update Messages

   Once a subscription has been successfully established, the server
   generates Push Notification Updates to send to the client as
   appropriate.  An initial Push Notification Update will be sent
   immediately in the case that the answer set was non-empty at the
   moment the subscription was established.  Subsequent changes to the
   answer set are then communicated to the client in subsequent Push
   Notification Updates.

   The format of Push Notification Updates borrows from the existing DNS
   Update [RFC2136] protocol, with some simplifications.

   The following figure shows the existing DNS Update header format:

                                             1  1  1  1  1  1
               0  1  2  3  4  5  6  7  8  9  0  1  2  3  4  5
             +--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+
             |                      ID                       |
             +--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+
             |QR|   Opcode  |          Z         |   RCODE   |
             +--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+
             |                    ZOCOUNT                    |
             +--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+
             |                    PRCOUNT                    |
             +--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+
             |                    UPCOUNT                    |
             +--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+
             |                    ADCOUNT                    |
             +--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+

                                 Figure 1

   For DNS Push Notifications the following rules apply:

   The QR bit MUST be zero, and the Opcode MUST be UPDATE (5).
   Messages received where this is not true are not Push Notification
   Update Messages and should be silently ignored for the purposes of
   Push Notification Update Message handling.

   ID, the Z bits, and RCODE MUST be zero on transmission,
   and MUST be silently ignored on reception.

   ZOCOUNT MUST be zero, and the Zone Section MUST be empty.
   Any records in the Zone Section MUST be silently ignored.

   PRCOUNT MUST be zero, and the Prerequisite Section MUST be empty.
   Any records in the Prerequisite Section MUST be silently ignored.



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   UPCOUNT specifies the number of records in the Update Section.

   ADCOUNT specifies the number of records in the Additional Data
   Section.  Typically this is zero, but it may be nonzero in some
   cases, such as when the message includes an EDNS(0) OPT record.

   The Update Section contains the relevant change information for the
   client, formatted identically to a DNS Update [RFC2136].  To recap:

      Delete all RRsets from a name:
      TTL=0, CLASS=ANY, RDLENGTH=0, TYPE=ANY.

      Delete an RRset from a name:
      TTL=0, CLASS=ANY, RDLENGTH=0;
      TYPE specifies the RRset being deleted.

      Delete an individual RR from a name:
      TTL=0, CLASS=NONE;
      TYPE, RDLENGTH and RDATA specifies the RR being deleted.

      Add to an RRset:
      TTL, CLASS, TYPE, RDLENGTH and RDATA specifies the RR being added.

   When processing the records received in a Push Notification Update
   Message, the receiving client MUST validate that the records being
   added or deleted correspond with at least one currently active
   subscription on that connection.  Specifically, the record name MUST
   match the name given in the SUBSCRIBE request, subject to the usual
   established DNS case-insensitivity for US-ASCII letters.  If the
   QTYPE in the SUBSCRIBE request was not ANY (255) then the TYPE of the
   record must match the QTYPE given in the SUBSCRIBE request.  If the
   QCLASS in the SUBSCRIBE request was not ANY (255) then the CLASS of
   the record must match the QCLASS given in the SUBSCRIBE request.  If
   a matching active subscription on that connection is not found, then
   that individual record addition/deletion is silently ignored.
   Processing of other additions and deletions in this message is not
   affected.  The TCP connection is not closed.  This is to allow for
   the race condition where a client sends an outbound UNSUBSCRIBE while
   inbound Push Notification Updates for that subscription from the
   server are still in flight.

   In the case where a single change affects more than one active
   subscription, only one update is sent.  For example, an update adding
   a given record may match both a SUBSCRIBE request with the same QTYPE
   and a different SUBSCRIBE request with QTYPE=ANY.  It is not the case
   that two updates are sent because the new record matches two active
   subscriptions.




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   The server SHOULD encode change notifications in the most efficient
   manner possible.  For example, when three AAAA records are deleted
   from a given name, and no other AAAA records exist for that name, the
   server SHOULD send a "delete an RRset from a name" update, not three
   separate "delete an individual RR from a name" updates.  Similarly,
   when both an SRV and a TXT record are deleted from a given name, and
   no other records of any kind exist for that name, the server SHOULD
   send a "delete all RRsets from a name" update, not two separate
   "delete an RRset from a name" updates.

   A server SHOULD combine multiple change notifications in a single
   Update Message when possible, even if those change notifications
   apply to different subscriptions.  Conceptually, a Push Notification
   Update Message is a connection-level concept, not a subscription-
   level concept.

   Push Notification Update Messages MAY contain an EDNS(0) TCP
   Keepalive option [I-D.ietf-dnsop-edns-tcp-keepalive] if the idle
   timeout has changed since the last time the server sent an EDNS(0)
   TCP Keepalive option on this connection.

   In the event that the server wishes to inform a client of a new idle
   timeout for the connection, the server MAY combine that with the next
   message it sends to the client, or the server MAY send an empty Push
   Notification Update Message (zero records in the Update Section) to
   carry the EDNS(0) TCP Keepalive option.  Clients MUST correctly
   receive and process the EDNS(0) TCP Keepalive option in both cases.

   Reception of a Push Notification Update Message does not directly
   generate a response back to the server.  (Updates may indirectly
   generate other operations; e.g., a Push Notification Update Message
   declaring the appearance of a PTR record could lead to a query for
   the SRV record named in the rdata of that PTR record[RFC6763].

   The TTL of an added record is stored by the client and decremented as
   time passes, with the caveat that for as long as a relevant
   subscription is active, the TTL does not decrement below 1 second.
   For as long as a relevant subscription remains active, the client
   SHOULD assume that when a record goes away 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 is cancelled
   (individually, or as a result of the TCP connection being closed)
   record aging resumes and records are removed from the local cache
   when their TTL reaches zero.







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6.5.  DNS RECONFIRM

   Sometimes, particularly when used with a Hybrid Proxy
   [I-D.ietf-dnssd-hybrid], a DNS Zone may contain stale data.  When a
   client encounters data that it believe may be stale (e.g., an SRV
   record referencing a target host+port that is not responding to
   connection requests) the client sends a DNS RECONFIRM message to
   request that the server re-verify that the data is still valid.  For
   a Hybrid Proxy, this causes it to issue new Multicast DNS requests to
   ascertain whether the target device is still present.  For other
   kinds of DNS server the RECONFIRM operation is currently undefined
   and SHOULD be silently ignored.

   A RECONFIRM request is formatted similarly to a conventional DNS
   QUERY request [RFC1035], except that the opcode is RECONFIRM (8)
   instead of QUERY (0).  QTYPE MUST NOT be the value ANY (255).  QCLASS
   MUST NOT be the value ANY (255).

   In a RECONFIRM request the DNS Header QR bit MUST be zero.
   If the QR bit is not zero the message is not a RECONFIRM request.

   The AA, TC, RD, RA, Z, AD, and CD bits, the ID field, and the RCODE
   field, MUST be zero on transmission, and MUST be silently ignored on
   reception.

   Like a DNS QUERY request, a RECONFIRM request MUST contain exactly
   one question.  Since RECONFIRM requests are sent over TCP, multiple
   RECONFIRM requests can be concatenated in a single TCP stream and
   packed efficiently into TCP segments, so the ability to pack multiple
   RECONFIRM operations into a single DNS message within that TCP stream
   would add extra complexity for little benefit.

   ANCOUNT MUST be nonzero, and the Answer Section MUST contain the
   rdata for the record(s) that the client believes to be in doubt.

   NSCOUNT MUST be zero, and the Authority Section MUST be empty.
   Any records in the Authority Section MUST be silently ignored.

   ARCOUNT specifies the number of records in the Additional Data
   Section.  Typically this is zero, but it may be nonzero in some
   cases, such as when the request includes an EDNS(0) OPT record.

   DNS wildcarding is not supported.  That is, a wildcard ("*") in a
   SUBSCRIBE message matches only a wildcard ("*") in the zone, and
   nothing else.

   Aliasing is not supported.  That is, a CNAME in a SUBSCRIBE message
   matches only a CNAME in the zone, and nothing else.



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   No response message is generated as a result of processing a
   RECONFIRM message.

   If the server receiving the RECONFIRM request determines that the
   records are in fact no longer valid, then subsequent DNS Push
   Notification Update Messages will be generated to inform interested
   clients.  Thus, one client discovering that a previously-advertised
   printer is no longer present has the side effect of informing all
   other interested clients that the printer in question is now gone.










































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6.6.  DNS Push Notification Termination Message

   If a server is low on resources it MAY simply terminate a client
   connection with a TCP RST.  However, the likely behaviour of the
   client may be simply to reconnect immediately, putting more burden on
   the server.  Therefore, a server SHOULD instead choose to shed client
   load by (a) sending a DNS Push Notification Termination Message and
   then (b) immediately closing the client connection with a TCP FIN
   instead of RST, thereby facilitating reliable delivery of the
   Termination Message.

   The format of a Termination Message is similar to a Push Notification
   Update.

   The following figure shows the existing DNS Update header format:

                                             1  1  1  1  1  1
               0  1  2  3  4  5  6  7  8  9  0  1  2  3  4  5
             +--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+
             |                      ID                       |
             +--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+
             |QR|   Opcode  |          Z         |   RCODE   |
             +--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+
             |                    ZOCOUNT                    |
             +--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+
             |                    PRCOUNT                    |
             +--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+
             |                    UPCOUNT                    |
             +--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+
             |                    ADCOUNT                    |
             +--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+

                                 Figure 2

   For Termination Messages the following rules apply:

   The QR bit MUST be zero, and the Opcode MUST be UPDATE (5).
   Messages received where this is not true are not Termination Messages
   and should be silently ignored.

   ID and the Z bits MUST be zero on transmission,
   and MUST be silently ignored on reception.

   ZOCOUNT MUST be zero, and the Zone Section MUST be empty.
   Any records in the Zone Section MUST be silently ignored.

   PRCOUNT MUST be zero, and the Prerequisite Section MUST be empty.
   Any records in the Prerequisite Section MUST be silently ignored.



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   UPCOUNT MUST be zero, and the Update Section MUST be empty.
   Any records in the Update Section MUST be silently ignored.

   ADCOUNT specifies the number of records in the Additional Data
   Section, e.g., the EDNS(0) OPT record..

   The RCODE MUST contain a nonzero code giving the reason for
   termination, as indicated below:

   +----------+-------+------------------------------------------------+
   | Mnemonic | Value | Description                                    |
   +----------+-------+------------------------------------------------+
   | SERVFAIL |   2   | The server is overloaded due to resource       |
   |          |       | exhaustion.                                    |
   | REFUSED  |   5   | The server has been reconfigured and is no     |
   |          |       | longer accepting DNS Push Notification         |
   |          |       | requests for one or more of the currently      |
   |          |       | subscribed names.                              |
   +----------+-------+------------------------------------------------+

                        Termination Response codes

   This document specifies only these two RCODE values for Termination
   Messages.  Servers sending Termination Messages SHOULD use one of
   these two values.  However, future circumstances may create
   situations where other RCODE values are appropriate in Termination
   Messages, so clients MUST be prepared to accept Termination Messages
   with any RCODE value.  In particular, a Termination Message with
   RCODE value zero (NOERROR) is still a Termination Message and should
   be treated as such.

   The Termination Message MUST contain an EDNS(0) TCP Keepalive option
   [I-D.ietf-dnsop-edns-tcp-keepalive].  The client MUST wait for the
   time indicated in the EDNS(0) TCP Keepalive option's idle timeout
   before attempting any new connections to this server.  A client that
   receives a Termination Message without an EDNS(0) TCP Keepalive
   option SHOULD treat it as equivalent to a TCP Keepalive option with a
   zero timeout value.

   In the case where the server is rejecting some, but not all, of the
   existing subscriptions (perhaps because it has been reconfigured and
   is no longer authoritative for those names) with a REFUSED (5) RCODE,
   the EDNS(0) TCP Keepalive option's idle timeout MAY be zero,
   indicating that the client SHOULD attempt to re-establish its
   subscriptions immediately.

   In the case where a server is terminating a large number of
   connections at once (e.g., if the system is restarting) and the



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   server doesn't want to be inundated with a flood of simultaneous
   retries, it SHOULD send different EDNS(0) TCP Keepalive values to
   each client.  These adjustments MAY be selected randomly,
   pseudorandomly, or deterministically (e.g., incrementing the time
   value by one for each successive client, yielding a post-restart
   reconnection rate of ten clients per second).

7.  Security Considerations

   TLS support is REQUIRED in DNS Push Notifications.  There is no
   provision for opportunistic encryption using a mechanism like
   "STARTTLS".

   DNSSEC is RECOMMENDED for DNS Push Notifications.  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 is subverted
   then the client may have a secure connection to a rogue server.
   DNSSEC can provided added confidence that the SRV query has not been
   subverted.

7.1.  Security Services

   It is the goal of using TLS to provide the following security
   services:

   Confidentiality:  All application-layer communication is encrypted
      with the goal that no party should be able to decrypt it except
      the intended receiver.

   Data integrity protection:  Any changes made to the communication in
      transit are detectable by the receiver.

   Authentication:  An end-point of the TLS communication is
      authenticated as the intended entity to communicate with.

   Deployment recommendations on the appropriate key lengths and cypher
   suites are beyond the scope of this document.  Please refer to TLS
   Recommendations [RFC7525] for the best current practices.  Keep in
   mind that best practices only exist for a snapshot in time and
   recommendations will continue to change.  Updated versions or errata
   may exist for these recommendations.








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7.2.  TLS Name Authentication

   As described in Section 6.1, the client discovers the DNS Push
   Notification server using an SRV lookup for the record name
   "_dns-push-tls._tcp.<zone>".  The server connection endpoint SHOULD
   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 [RFC7673].  This procedure uses the TLS Sever
   Name Indication (SNI) extension [RFC6066] 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 is useable, 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.

7.3.  TLS Compression

   In order to reduce the chances of compression related attacks, TLS-
   level compression SHOULD be disabled when using TLS versions 1.2 and
   earlier.  In the draft version of TLS 1.3 [I-D.ietf-tls-tls13], TLS-
   level compression has been removed completely.

7.4.  TLS Session Resumption

   TLS Session Resumption is permissible on DNS Push Notification
   servers.  The server may keep TLS state with Session IDs [RFC5246] or
   operate in stateless mode by sending a Session Ticket [RFC5077] to
   the client for it to store.  However, once the connection is closed,
   any existing subscriptions will be dropped.  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 connection.
   Use of TLS Session Resumption allows a new TLS connection to be set
   up more quickly, but the client will still have to recreate any
   desired subscriptions.

8.  IANA Considerations

   This document defines the service name: "_dns-push-tls._tcp".
   It is only applicable for the TCP protocol.
   This name is to be published in the IANA Service Name Registry.

   This document defines three DNS OpCodes: SUBSCRIBE with (tentative)
   value 6, UNSUBSCRIBE with (tentative) value 7, and RECONFIRM with
   (tentative) value 8.






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9.  Acknowledgements

   The authors would like to thank Kiren Sekar and Marc Krochmal for
   previous work completed in this field.

   This draft has been improved due to comments from Ran Atkinson, Tim
   Chown, Mark Delany, Ralph Droms, Bernie Holz, Jan Komissar, Manju
   Shankar Rao, Markus Stenberg, and Dave Thaler.

10.  References

10.1.  Normative References

   [I-D.ietf-dnsop-5966bis]
              Dickinson, J., Dickinson, S., Bellis, R., Mankin, A., and
              D. Wessels, "DNS Transport over TCP - Implementation
              Requirements", draft-ietf-dnsop-5966bis-06 (work in
              progress), January 2016.

   [I-D.ietf-dnsop-edns-tcp-keepalive]
              Wouters, P., Abley, J., Dickinson, S., and R. Bellis, "The
              edns-tcp-keepalive EDNS0 Option", draft-ietf-dnsop-edns-
              tcp-keepalive-06 (work in progress), February 2016.

   [I-D.ietf-tls-tls13]
              Rescorla, E., "The Transport Layer Security (TLS) Protocol
              Version 1.3", draft-ietf-tls-tls13-11 (work in progress),
              December 2015.

   [RFC0768]  Postel, J., "User Datagram Protocol", STD 6, RFC 768, DOI
              10.17487/RFC0768, August 1980,
              <http://www.rfc-editor.org/info/rfc768>.

   [RFC0793]  Postel, J., "Transmission Control Protocol", STD 7, RFC
              793, DOI 10.17487/RFC0793, September 1981,
              <http://www.rfc-editor.org/info/rfc793>.

   [RFC1034]  Mockapetris, P., "Domain names - concepts and facilities",
              STD 13, RFC 1034, DOI 10.17487/RFC1034, November 1987,
              <http://www.rfc-editor.org/info/rfc1034>.

   [RFC1035]  Mockapetris, P., "Domain names - implementation and
              specification", STD 13, RFC 1035, DOI 10.17487/RFC1035,
              November 1987, <http://www.rfc-editor.org/info/rfc1035>.







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   [RFC1123]  Braden, R., Ed., "Requirements for Internet Hosts -
              Application and Support", STD 3, RFC 1123, DOI 10.17487/
              RFC1123, October 1989,
              <http://www.rfc-editor.org/info/rfc1123>.

   [RFC2119]  Bradner, S., "Key words for use in RFCs to Indicate
              Requirement Levels", BCP 14, RFC 2119, DOI 10.17487/
              RFC2119, March 1997,
              <http://www.rfc-editor.org/info/rfc2119>.

   [RFC2136]  Vixie, P., Ed., Thomson, S., Rekhter, Y., and J. Bound,
              "Dynamic Updates in the Domain Name System (DNS UPDATE)",
              RFC 2136, DOI 10.17487/RFC2136, April 1997,
              <http://www.rfc-editor.org/info/rfc2136>.

   [RFC2782]  Gulbrandsen, A., Vixie, P., and L. Esibov, "A DNS RR for
              specifying the location of services (DNS SRV)", RFC 2782,
              DOI 10.17487/RFC2782, February 2000,
              <http://www.rfc-editor.org/info/rfc2782>.

   [RFC4953]  Touch, J., "Defending TCP Against Spoofing Attacks", RFC
              4953, DOI 10.17487/RFC4953, July 2007,
              <http://www.rfc-editor.org/info/rfc4953>.

   [RFC5246]  Dierks, T. and E. Rescorla, "The Transport Layer Security
              (TLS) Protocol Version 1.2", RFC 5246, DOI 10.17487/
              RFC5246, August 2008,
              <http://www.rfc-editor.org/info/rfc5246>.

   [RFC5966]  Bellis, R., "DNS Transport over TCP - Implementation
              Requirements", RFC 5966, DOI 10.17487/RFC5966, August
              2010, <http://www.rfc-editor.org/info/rfc5966>.

   [RFC6066]  Eastlake 3rd, D., "Transport Layer Security (TLS)
              Extensions: Extension Definitions", RFC 6066, DOI
              10.17487/RFC6066, January 2011,
              <http://www.rfc-editor.org/info/rfc6066>.

   [RFC6195]  Eastlake 3rd, D., "Domain Name System (DNS) IANA
              Considerations", RFC 6195, DOI 10.17487/RFC6195, March
              2011, <http://www.rfc-editor.org/info/rfc6195>.

   [RFC6891]  Damas, J., Graff, M., and P. Vixie, "Extension Mechanisms
              for DNS (EDNS(0))", STD 75, RFC 6891, DOI 10.17487/
              RFC6891, April 2013,
              <http://www.rfc-editor.org/info/rfc6891>.





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   [RFC7673]  Finch, T., Miller, M., and P. Saint-Andre, "Using DNS-
              Based Authentication of Named Entities (DANE) TLSA Records
              with SRV Records", RFC 7673, DOI 10.17487/RFC7673, October
              2015, <http://www.rfc-editor.org/info/rfc7673>.

10.2.  Informative References

   [I-D.ietf-dnssd-hybrid]
              Cheshire, S., "Hybrid Unicast/Multicast DNS-Based Service
              Discovery", draft-ietf-dnssd-hybrid-03 (work in progress),
              November 2015.

   [I-D.sekar-dns-llq]
              Sekar, K., "DNS Long-Lived Queries", draft-sekar-dns-
              llq-01 (work in progress), August 2006.

   [IPJ.9-4-TCPSYN]
              Eddy, W., "Defenses Against TCP SYN Flooding Attacks", The
              Internet Protocol Journal, Cisco Systems, Volume 9, Number
              4, December 2006.

   [RFC1996]  Vixie, P., "A Mechanism for Prompt Notification of Zone
              Changes (DNS NOTIFY)", RFC 1996, DOI 10.17487/RFC1996,
              August 1996, <http://www.rfc-editor.org/info/rfc1996>.

   [RFC4287]  Nottingham, M., Ed. and R. Sayre, Ed., "The Atom
              Syndication Format", RFC 4287, DOI 10.17487/RFC4287,
              December 2005, <http://www.rfc-editor.org/info/rfc4287>.

   [RFC5077]  Salowey, J., Zhou, H., Eronen, P., and H. Tschofenig,
              "Transport Layer Security (TLS) Session Resumption without
              Server-Side State", RFC 5077, DOI 10.17487/RFC5077,
              January 2008, <http://www.rfc-editor.org/info/rfc5077>.

   [RFC6762]  Cheshire, S. and M. Krochmal, "Multicast DNS", RFC 6762,
              DOI 10.17487/RFC6762, February 2013,
              <http://www.rfc-editor.org/info/rfc6762>.

   [RFC6763]  Cheshire, S. and M. Krochmal, "DNS-Based Service
              Discovery", RFC 6763, DOI 10.17487/RFC6763, February 2013,
              <http://www.rfc-editor.org/info/rfc6763>.

   [RFC7525]  Sheffer, Y., Holz, R., and P. Saint-Andre,
              "Recommendations for Secure Use of Transport Layer
              Security (TLS) and Datagram Transport Layer Security
              (DTLS)", BCP 195, RFC 7525, DOI 10.17487/RFC7525, May
              2015, <http://www.rfc-editor.org/info/rfc7525>.




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   [XEP0060]  Millard, P., Saint-Andre, P., and R. Meijer, "Publish-
              Subscribe", XSF XEP 0060, July 2010.

Authors' Addresses

   Tom Pusateri
   Seeking affiliation
   Hilton Head Island, SC
   USA

   Phone: +1 843 473 7394
   Email: pusateri@bangj.com


   Stuart Cheshire
   Apple Inc.
   1 Infinite Loop
   Cupertino, CA  95014
   USA

   Phone: +1 408 974 3207
   Email: cheshire@apple.com





























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