Internet Engineering Task Force                              T. Pusateri
Internet-Draft                                       Seeking affiliation
Intended status: Standards Track                             S. Cheshire
Expires: January 9, May 4, 2017                                          Apple Inc.
                                                            July 8,
                                                        October 31, 2016

                         DNS Push Notifications
                        draft-ietf-dnssd-push-08
                        draft-ietf-dnssd-push-09

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
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   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 January 9, May 4, 2017.

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
   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  . . . . . . . . . . . . . . . . . . . . . . . . . .   4
   4.  Transport . . . . . . . . . . . . . . . . . . . . . . . . . .   6
     4.1.  Client-Initiated Termination
   5.  State Considerations  . . . . . . . . . . . . . .   7
     4.2.  Server-Initiated Termination . . . . . .   6
   6.  Protocol Operation  . . . . . . . .   9
   5.  State Considerations . . . . . . . . . . . . .   7
     6.1.  Discovery . . . . . . .  11
   6.  Protocol Operation . . . . . . . . . . . . . . . . .   8
     6.2.  DNS Push Notification SUBSCRIBE . . . .  12
     6.1.  Discovery . . . . . . . . .  10
       6.2.1.  SUBSCRIBE Request . . . . . . . . . . . . . . .  13
     6.2.  DNS Push Notification . . .  11
       6.2.2.  SUBSCRIBE Response  . . . . . . . . . . . . . . .  15 . .  14
     6.3.  DNS Push Notification UNSUBSCRIBE Update Messages . . . . . . . . . .  18
       6.3.1.  PUSH Message format . . . . . . . . . . . . . . .  20 . .  18
     6.4.  DNS Push Notification Update Messages UNSUBSCRIBE . . . . . . . . . . . .  21
     6.5.  DNS RECONFIRM
       6.4.1.  UNSUBSCRIBE Request . . . . . . . . . . . . . . . . .  22
       6.4.2.  UNSUBSCRIBE Response  . . . . . . . . . . . . . . . .  24
     6.6.
     6.5.  DNS Session Signaling Push Notification RECONFIRM . . . .  26
     6.6.  Client-Initiated Termination Message  . . . . . . . .  25 . . . . . .  28
   7.  Security Considerations . . . . . . . . . . . . . . . . . . .  28
     7.1.  Security Services . . . . . . . . . . . . . . . . . . . .  28  29
     7.2.  TLS Name Authentication . . . . . . . . . . . . . . . . .  28  29
     7.3.  TLS Compression . . . . . . . . . . . . . . . . . . . . .  29  30
     7.4.  TLS Session Resumption  . . . . . . . . . . . . . . . . .  29  30
   8.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .  29  30
   9.  Acknowledgements  . . . . . . . . . . . . . . . . . . . . . .  29  30
   10. References  . . . . . . . . . . . . . . . . . . . . . . . . .  30  31
     10.1.  Normative References . . . . . . . . . . . . . . . . . .  30  31
     10.2.  Informative References . . . . . . . . . . . . . . . . .  31  32
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  33  34

1.  Introduction

   IMPORTANT NOTE: This document currently references the EDNS(0) TCP
   Keepalive option [RFC7828].  As a result of discussions about this
   document, the community came to the realization that DNS needs
   explicit session-level signaling, to complement the current EDNS(0)
   per-message signaling.  As a result, work on DNS Session Signaling
   [I-D.bellis-dnsop-session-signal] is underway, and this document will
   be updated shortly to make use of those new Session Signaling
   mechanisms once they are agreed.

   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] [RFC6895].

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. pattern [obs].  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. handshake, flow control, and
   reliability.  This document builds on experience gained with the LLQ
   protocol, with an improved
   design that design.  Instead of using UDP, this
   specification uses long-lived TCP connections instead of UDP (and
   [I-D.ietf-dnsop-session-signal], and therefore doesn't need to duplicate
   reinvent existing TCP functionality), and
   adopts the syntax and semantics of DNS Update messages [RFC2136]
   instead functionality.  Instead of inventing a new
   vocabulary of messages to communicate DNS zone changes.

   Because changes, this
   specification adopts the syntax and semantics of DNS Update messages
   [RFC2136].

   DNS Push Notifications impose a certain less load on the responding server (though less load than
   rapid polling of that
   server) would, but Push Notifications do still have a cost, so
   DNS Push Notification clients SHOULD exercise restraint in
   issuing DNS MUST NOT recklessly create an excessive
   number of 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, smartphone,
   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.

   Generally, a client SHOULD NOT keep a connection to a server open
   indefinitely if it has no active subscriptions on that connection.
   After 30 seconds with no active subscriptions the client SHOULD close
   the idle connection, and, if needed in the future, open a new
   connection.

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 does NOT also have 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
   class "IN", as it is the reality 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 [RFC7766].

   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  Flooding attacks
   are possible with any protocol over TCP,
   implementers protocol, and a benefit of TCP is that there
   are encouraged to use already established industry best practices to guard against such SYN
   flooding and similar attacks [IPJ.9-4-TCPSYN] [RFC4953].

   Use of TCP also allows DNS Push Notifications to take advantage of
   current and future developments in TCP, such as Multipath TCP (MPTCP)
   [RFC6824], TCP Fast Open (TFO) [RFC7413], Tail Loss Probe (TLP)
   [I-D.dukkipati-tcpm-tcp-loss-probe], and so on.

   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

5.  State Considerations

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

4.1.  Client-Initiated Termination

   An individual server,
   each record subscription is terminated by sending an UNSUBSCRIBE
   message for that specific subscription, individually accepted or all rejected.
   Servers may employ various techniques to limit subscriptions can be
   cancelled at once by the client closing the connection.  When to a
   client terminates an individual subscription (via UNSUBSCRIBE) or all
   subscriptions on that connection (by closing
   manageable level.  Correspondingly, the connection) it client is
   signaling free to the server that it is longer interested in receiving
   those particular updates.  It is informing the server establish
   simultaneous connections to alternate DNS servers that support DNS
   Push Notifications for the server
   may release any state information it has been keeping with regards to
   these particular subscriptions.

   After terminating zone and distribute record subscriptions
   at its last subscription on a connection via
   UNSUBSCRIBE, 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 client MAY close server across numerous client
   requests.

6.  Protocol Operation

   The DNS Push Notification protocol is a session-oriented protocol,
   and makes use of DNS Session Signaling
   [I-D.ietf-dnsop-session-signal].

   DNS Push Notification clients and servers MUST support DNS Session
   Signaling, but the connection immediately, or it may
   keep it open if it anticipates performing further server must not issue any DNS Session Signaling
   operations on that
   connection in until after the future.  If a client wishes to keep an idle
   connection open, it MUST continue to meet has first initiated a DNS Session
   Signaling operation of its keepalive obligations
   [RFC7828] or own.  A single server can support DNS
   Queries, DNS Updates, and DNS Push Notifications (using DNS Session
   Signaling) on the same TCP port, and until the client has sent at
   least one DNS Session Signaling operation the server is entitled does not know
   what kind of client has connected to close it.  Once the connection (see
   below).

   If a client plans has
   indicated willingness to terminate use DNS Session Signaling operations by
   sending one or more subscriptions on a
   connection and doesn't intend to keep that of its own, either side of the connection open, may then as
   an efficiency optimization it MAY instead choose to simply close
   initiate further Session Signaling operations at any time.

   A DNS Push Notification exchange begins with the
   connection, which implicitly terminates all subscriptions on that
   connection.  This may occur because client discovering
   the appropriate server, using the procedure described in Section 6.1,
   and then making a TLS/TCP connection to it.

   A typical DNS Push Notification client computer is being shut
   down, is going will immediately issue a DNS
   Session Signaling Idle Timeout operation to sleep, request a session timeout
   longer than the application requiring the subscriptions
   has terminated, or simply because the last active subscription 30-second default, but this is NOT REQUIRED.  A
   DNS Push Notification client MAY issue other requests on
   that the
   connection has been cancelled.

   When closing a connection, a client will generally do an abortive
   disconnect, sending a TCP RST.  This immediately discards all
   remaining inbound first, and outbound data, which is appropriate only issue a DNS Session Signaling Idle Timeout
   operation later if it determines that to be necessary.

   Once the
   client no longer has any interest in this data.  In the BSD sockets
   API, sending a TCP RST connection is achieved by setting the SO_LINGER option
   with a time of 0 seconds and then closing made, the socket.

   If a client has performed operations on this connection that it would
   not want lost (like DNS updates) may then the client SHOULD do an orderly
   disconnect, sending a TCP FIN.  In the BSD sockets API, sending a TCP
   FIN is achieved by calling "shutdown(s,SHUT_WR)" add and keeping the
   socket open until all remaining data has been read from it. remove Push
   Notification subscriptions.  In accordance with the first SUBSCRIBE response on a connection, current set of
   active subscriptions 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 and not attempt any further DNS sends relevant asynchronous Push Notification requests
   Notifications to the client.  Note that server until one hour has passed.  This situation may occur
   if a client connects MUST be prepared to a server that doesn't implement DNS
   receive (and silently ignore) Push Notifications at all, and for subscriptions it
   has previously removed, since there is important not no way to burden such servers
   with continuous retries.

   Upon receiving an error response from prevent the server,
   situation where a Push Notification is in flight from server to
   client SHOULD NOT
   close while the connection.  An error relating to one particular operation
   on a connection does not necessarily imply that all other operations
   on 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 have also failed, with a TCP FIN or RST.

6.1.  Discovery

   The first step in DNS Push Notification subscription is to discover
   an appropriate DNS server that future operations will
   fail. supports DNS Push Notifications for
   the desired zone.  The client should assume that MUST also determine which TCP port on
   the server will make its own
   decision about whether or is listening for connections, which need not be (and often
   is not) the typical TCP port 53 used for conventional DNS, or TCP
   port 853 used for DNS over TLS [RFC7858].

   1.  The client begins the discovery by sending a DNS query to close the connection, based on
       local resolver with record type SOA [RFC1035] for the
   server's determination name of whether the error condition pertains to
   this particular operation, or would also apply
       record it wishes to any subsequent
   operations. subscribe.

   2.  If the server does not close SOA record exists, it MUST be returned in the connection then Answer
       Section of the
   client response.  If not, the local resolver SHOULD continue to use that connection
       include the SOA record for subsequent
   operations.

   Upon receiving a Termination Message from the server (see below), a
   client MUST immediately close zone of the connection.

4.2.  Server-Initiated Termination requested name in the
       Authority Section.

   3.  If a no SOA record is returned, the client makes a connection and then fails to send any DNS message
   that uses EDNS(0) TCP Keepalive [RFC7828] (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 strips off the
   server SHOULD close
       leading label from the connection. requested name.  If no data has been sent on the
   connection the server MAY abort resulting name has
       at least one label in it, the connection with client sends a TCP RST. new SOA query and
       processing continues at step 2 above.  If
   data has been sent on the connection resulting name is
       empty (the root label) then this is a network configuration error
       and the server SHOULD close client gives up.  The client MAY retry the
   connection gracefully with operation at a TCP FIN so that
       later time.

   4.  Once the data SOA is reliably
   delivered.

   In the response to the first successful SUBSCRIBE, known (either by virtue of being seen in the included
   EDNS(0) TCP Keepalive option specifies
       Answer Section, or in the idle timeout so that Authority Section), the client knows sends a
       DNS query with type SRV [RFC2782] for the record name
       "_dns-push-tls._tcp.<zone>", where <zone> is the frequency owner name of traffic it must generate to keep
       the
   connection alive. discovered SOA record.

   5.  If the idle timeout for that connection changes, zone in question does not offer DNS Push Notifications
       then SRV record MUST NOT exist and the server communicates this by placing an updated EDNS(0) TCP
   Keepalive option in SRV query will return a subsequent message to
       negative answer.

   6.  If the client.

   At both servers and clients, zone in question is set up to offer DNS Push Notifications
       then this SRV record MUST exist.  The SRV "target" contains the generation or reception
       name of any
   complete request, response, update, or keepalive message resets the
   keepalive timer server providing DNS Push Notifications for that connection.

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

   If a client disconnects from SRV
       record "port" field.  The address(es) of the network abruptly, without closing
   its connection, target host MAY be
       included in 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 Additional Section, however, the idle timeout, then 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 indicates that the client is probably no
   longer on case, the network,
       "priority" and "weight" values in the server SHOULD abort returned SRV records are
       used to determine the connection
   with a TCP RST.  The time before order in which to contact the servers for
       subscription requests.  As described in the SRV specification
       [RFC2782], the server closes with the connection lowest "priority" is
   intentionally 50% longer first
       contacted.  If more than one server has the time before same "priority", the client is required
   to generate keepalive traffic, to allow for differences in clock rate
   and network propagation delays.

   [We need to discuss
       "weight" indicates 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 weighted probability 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 client
       should contact that server.  Higher weights have higher
       probabilities of zero bytes, signifying being selected.  If 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 server is that it not reachable or
       is more feasible to implement
   these in networking offload hardware, which can allow devices not willing 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 accept a
   client to tell the networking stack at what frequency to perform TCP-
   layer keepalives, and an API for subscription request, then a
       subsequent 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 be contacted.

   Each time a client makes a new 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, or may allow the connection to remain
   open.  For error conditions that only affect subscription
   connection, it SHOULD repeat the single operation discovery process in
   question, the server SHOULD return an error response order to
   determine the client
   and leave the connection open preferred DNS server for further operations.  For error
   conditions subscriptions at that are likely to make all operations unsuccessful time.

   Note that this repeated discovery step is typically very fast and
   typically results in no queries on the
   immediate future, the server SHOULD return an error response to the network.  The client and then close the connection with a TCP FIN.

   If device
   MUST respect the server is overloaded and needs to shed load, DNS TTL values on records it SHOULD send a
   Termination Message to the client receives, and close the connection store
   them in its local cache with a TCP
   FIN.

   Apart from this lifetime.  This means that, as long
   as the cases described above, a server MUST NOT close a
   connection with a DNS Push Notification client, except in
   extraordinary error conditions.  Closing the connection is TTL values on the
   client's responsibility, authoritative records were set to
   reasonable values, repeated application of this discovery process can
   be done at completed nearly instantaneously by the client's discretion, when
   it so chooses.  A client, using only
   locally-stored data.

6.2.  DNS Push Notification server only closes SUBSCRIBE

   After connecting, and requesting a longer idle timeout if necessary,
   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 client then indicates its desire to receive
   DNS Push Notifications for a bug that makes it crash), or given domain name by sending a SUBSCRIBE
   request over the server established TLS connection to the server.  A
   SUBSCRIBE request is undergoing
   maintenance procedures.  When possible, encoded in a DNS Session Signaling
   [I-D.ietf-dnsop-session-signal] message.  This specification defines
   a new DNS Session Signaling TLV for DNS Push Notification SUBSCRIBE
   Requests/Responses (tentatively Session Signaling Type Code 64).

   A server SHOULD send may not initiate a Termination Message (Section 6.6 ) informing SUBSCRIBE request.

6.2.1.  SUBSCRIBE Request

   A SUBSCRIBE request message begins with the
   client of standard DNS Session
   Signaling 4-byte header [I-D.ietf-dnsop-session-signal], followed by
   the reason SUBSCRIBE TLV.

                                             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  |          Z         |   RCODE   |
             +--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+
             |             SSOP-TYPE (SUBSCRIBE)             |
             +--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+
             |                  SSOP-LENGTH                  |
             +--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+
             |                                               |
             \                     QNAME                     \
             \                                               \
             +--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+
             |                     QTYPE                     |
             +--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+
             |                    QCLASS                     |
             +--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+

                                 Figure 1

   The MESSAGE ID field MUST be set to a unique value, that the client
   is not using for any other active operation on this connection.  For
   the connection being closed.

   After purposes here, a connection MESSAGE ID is closed by the server, in use on this connection if the
   client SHOULD try to
   reconnect, to that server, or to another server supporting DNS Push
   Notifications has used it in a request for the zone.  If reconnecting to the same server, and
   there was which it has not yet received a Termination Message
   response, or error response containing a
   EDNS(0) TCP Keepalive option, if if 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 has used it for a DNS server,
   each record subscription is individually accepted or rejected.
   Servers may employ various techniques to limit subscriptions to which it
   has not yet cancelled using UNSUBSCRIBE.  In the SUBSCRIBE response
   the server MUST echo back the MESSAGE ID value unchanged.

   In a
   manageable level.  Correspondingly, request the client is free to establish
   simultaneous connections to alternate DNS servers that support Header QR bit MUST be zero.

   The DNS
   Push Notifications for Header Opcode field holds the zone and distribute record subscriptions
   at its discretion.  In this way, both clients Session Signaling Opcode value
   (tentatively 6).

   The Z bits MUST be zero on transmission, and servers can react MUST be silently ignored
   on reception.

   The return code (RCODE) field MUST be set to resource constraints.  Token bucket rate limiting schemes are also
   effective 0 in providing fairness by a server across numerous client
   requests.

6.  Protocol Operation

   A DNS Push Notification exchange begins with request.

   In the client discovering SUBSCRIBE TLV the appropriate server, and then making a TLS/TCP connection to it. SSOP-TYPE is SUBSCRIBE (tentatively 64).
   The client may then add and remove Push Notification subscriptions
   over this connection.  In accordance with SSOP-LENGTH is the current set length of active
   subscriptions the server sends relevant asynchronous Push
   Notifications to the client.  Note data that a client follows, which
   specifies the name, type, and class of the record(s) being sought.

   A SUBSCRIBE request MUST be prepared to
   receive (and silently ignore) Push Notifications for subscriptions it
   has previously removed, since there contain exactly one question.  There is no way
   QCOUNT field to prevent the
   situation where a Push Notification is specify more than one question.  Since SUBSCRIBE
   requests are sent over TCP, multiple SUBSCRIBE requests can be
   concatenated in flight from server to
   client while a single TCP stream and packed efficiently into TCP
   segments.

   If accepted, the client's UNSUBSCRIBE message cancelling that subscription is simultaneously will stay in flight from effect until the client to server.

   The exchange
   cancels the subscription using UNSUBSCRIBE or until the connection
   between the client and server terminates when either end
   closes the TCP connection with server is closed.

   SUBSCRIBE requests on a TCP FIN or RST. given connection MUST be unique.  A client SHOULD
   MUST NOT make multiple TLS/TCP connections to the same DNS
   Push Notification server.  A client SHOULD share send 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 SUBSCRIBE message that duplicates the same server also handles DNS Update requests.
   This is to reduce unnecessary load QNAME, QTYPE
   and QCLASS of an existing active subscription on the DNS Push Notification
   server. that TLS/TCP
   connection.  For the purposes here, the determination purpose of "same server" is made by
   inspecting this matching, the target hostname established DNS
   case-insensitivity for US-ASCII letters applies (e.g., "foo.com" and port, regardless of
   "Foo.com" are the name being
   queried, or what zone if falls within.  A given same).  If a server may support
   Push Notifications (and possibly DNS Updates too) for multiple DNS
   zones.  When receives such a client discovers that duplicate
   SUBSCRIBE message this is an error and the DNS Push Notification server
   (and/or MUST immediately
   close the TCP connection.

   DNS Update server) for several different names (including
   names that fall within different zones) wildcarding is not supported.  That is, a wildcard ("*") in a
   SUBSCRIBE message matches only a literal wildcard character ("*") in
   the same target hostname zone, and port, the client SHOULD use nothing else.

   Aliasing is not supported.  That is, a single shared TCP connection for
   all relevant operations on those names.  A client SHOULD NOT open
   multiple TCP connections to CNAME in a SUBSCRIBE message
   matches only a literal CNAME record in the same target host zone, and port just
   because the names being queried (or updated) happen nothing else.

   A client may SUBSCRIBE to fall within
   different zones.

   Note records that are unknown to the "same server" determination described here is made
   using the target hostname given in server at
   the SRV record, not time of the IP
   address(es) request (providing 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 name falls within one of
   the "same server" for zone(s) the
   purposes of using a single shared connection to that server.  If server is responsible for) and this is not an
   administrator wishes to use a single error.
   The server for multiple zones and/or
   multiple roles (e.g., both DNS MUST accept these requests and send Push Notifications if
   and DNS Updates),
   and wishes to have clients use when matching records are found in the future.

   If neither QTYPE nor QCLASS are ANY (255) then this is a single shared connection specific
   subscription to changes for
   operations on that server, then the administrator MUST use the same
   target hostname in the appropriate SRV records.

   However, server implementers given QNAME, QTYPE 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.

   Clients SHOULD silently ignore unrecognized messages (both requests
   and responses) over the TLS/TCP connection.  For example, UNSUBSCRIBE
   and RECONFIRM currently generate no response, but if future versions
   of this specification change that, existing clients SHOULD silently
   ignore these unexpected responses.  This allows for backwards
   compatibility with future enhancements.

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, or TCP
   port 853 used for DNS over TLS [I-D.ietf-dprive-dns-over-tls].

   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 response.  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 (either by virtue of being seen in the
       Answer Section, or in the Authority Section), 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.

   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.

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.

   NOTE: A little-known quirk of DNS is that in DNS QUERY requests,
   QTYPE and QCLASS 255 mean "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, were a query returns some valid answers but
   not all of them, and makes QTYPE=ANY queries less useful than people
   sometimes imagine.

   When used in conjunction with DNS SUBSCRIBE, QTYPE and QCLASS 255
   should be interpreted to mean "ALL", not "ANY".  After accepting a
   subscription where one or both of QTYPE or QCLASS are 255, the server
   MUST send Push Notification Updates for ALL record changes that match
   the subscription, not just some of them.

   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, and the RCODE field, MUST be
   zero on transmission, and MUST be silently ignored on reception.

   The ID field may be set to any value of the client's choosing, and
   the server MUST echo this value back in the response message.  The
   client is not required to select unique ID values; it is permissible
   to use the same value (e.g., zero) for all operations.  Since the
   name, qtype, and qclass are sufficient to uniquely identify a
   SUBSCRIBE operation on a connection, the name, qtype, and qclass in a
   SUBSCRIBE response are sufficient to correlate a response with its
   corresponding request.  However, for convenience, the client may put
   any value it chooses in the ID field of the SUBSCRIBE request, and
   the server MUST echo that value back unchanged in the SUBSCRIBE
   response.  Note that the ID field of Push Notification Update
   Messages is always zero, since a Push Notification Update Message
   could potentially match more than one subscription, or could relate
   to a subscription that the client has just cancelled with an
   UNSUBSCRIBE message.

   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.

   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 QCLASS.  If
   one or both of
   the zone(s) the server is responsible for) and QTYPE or QCLASS are ANY (255) then this is not an error.
   The server MUST accept these requests and send Push Notifications if
   and when subscription
   matches are found in the future.

   Since all SUBSCRIBE operations are implicitly long-lived operations,
   the server MUST interpret a SUBSCRIBE request any type and/or any class, as if it contained an
   EDNS(0) TCP Keepalive option [RFC7828]. appropriate.

   NOTE: A client MUST NOT include an
   actual EDNS(0) TCP Keepalive option in the request, since it is
   automatic, and implied by the semantics little-known quirk of SUBSCRIBE.  If a server
   receives a SUBSCRIBE request that does contain an actual EDNS(0) TCP
   Keepalive option this DNS is an error that in DNS QUERY requests,
   QTYPE and QCLASS 255 mean "ANY" not "ALL".  They indicate that the
   server MUST immediately
   close the TCP connection.

   A SUBSCRIBE operation MAY include an explicit EDNS(0) [RFC6891] OPT
   record where necessary should respond with ANY matching records of its choosing, not
   necessarily ALL matching records.  This can lead to carry additional EDNS(0) information other
   than some surprising
   and unexpected results, were a TCP Keepalive option.

   The presence query returns some valid answers but
   not all of a SUBSCRIBE operation on a connection indicates to
   the server that the client fully implements EDNS(0) [RFC6891], them, and
   can correctly understand any response that conforms makes QTYPE=ANY queries less useful than people
   sometimes imagine.

   When used in conjunction with SUBSCRIBE, QTYPE and QCLASS 255 should
   be interpreted to that
   specification. mean "ALL", not "ANY".  After receiving accepting a SUBSCRIBE request,
   subscription where one or both of QTYPE or QCLASS are 255, the server MAY
   include OPT
   MUST send Push Notification Updates for ALL record in any changes that match
   the subscription, not just some of its responses, as needed. them.

6.2.2.  SUBSCRIBE Response

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

   In a

   A SUBSCRIBE response message begins with the standard DNS Header QR bit MUST be one.
   If the QR bit is not Session
   Signaling 4-byte header [I-D.ietf-dnsop-session-signal], possibly
   followed by one the message is not or more optional modifier TLVs such as a SUBSCRIBE response.

   The AA, TC, RD, RA, Z, AD, and CD bits, MUST be zero on transmission,
   and MUST be silently ignored on reception. Terminate
   modifier TLV [I-D.ietf-dnsop-session-signal].

                                             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  |          Z         |   RCODE   |
             +--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+

                                 Figure 2

   The MESSAGE ID field MUST echo the value given in the ID field of the
   SUBSCRIBE request.

   The Question Section MUST echo back the values provided by  This is how the client
   in the SUBSCRIBE knows which request that generated this SUBSCRIBE response.

   ANCOUNT MUST be zero, and the Answer Section MUST be empty.
   Any records in is
   being responded to.

   In a response the Answer Section DNS Header QR bit MUST be silently ignored. one.
   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, QR bit is not in the Answer Section of one 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 message 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 DNS Header Opcode field holds the Authority Section Session Signaling Opcode value
   (tentatively 6).

   The Z bits MUST be empty.
   Any records in the Authority Section zero on transmission, and MUST be silently ignored.

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

   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        |
   |            |       | servers MUST NOT return NXDOMAIN errors in response to   |
   |            |       | response to SUBSCRIBE requests.              |
   | NOTIMP     |   4   | Server does not implement recognize DNS Push Session        |
   |            |       | Notifications. Signaling Opcode.                            |
   | REFUSED    |   5   | Server refuses to process request for policy |
   |            |       | or security reasons.                         |
   | NOTAUTH    |   9   | Server is not authoritative for the requested          |
   |            |       | requested name.                              |
   +----------+-------+------------------------------------------------+
   | SSOPNOTIMP |   11  | SUBSCRIBE operation not supported.           |
   +------------+-------+----------------------------------------------+

                         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,

   If the server MUST
   include an explicit EDNS(0) TCP Keepalive option.  If sends a nonzero RCODE in the first SUBSCRIBE response does not include an explicit EDNS(0) TCP Keepalive
   option this is an error and response, either
   the client MUST immediately close is (at least partially) misconfigured or the TCP
   connection. server
   resources are exhausted.  In this case either case, the client should act as if shouldn't retry
   the subscription right away.  Either end can terminate the
   connection, but 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 may want to a server try this subscription again or
   it may have other successful subscriptions that doesn't implement DNS Push
   Notifications at all, and it is important not doesn't want to burden such servers
   with continuous retries.

   The server MAY include EDNS(0) TCP Keepalive options in subsequent
   messages, if the idle timeout changes.
   abandon.  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.

   In an error response, with server sends a nonzero RCODE, RCODE then it SHOULD append a
   Terminate modifier TLV [I-D.ietf-dnsop-session-signal] to the server MUST contain an
   EDNS(0) TCP Keepalive option
   response specifying the a delay before the client
   submits further requests to attempts this server: operation
   again.  Recommended values for the delay for different RCODE values
   are given below:

      For RCODE = 1 (FORMERR) the delay may be any value selected by the
      implementer.  A value of one minute five minutes 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, Session Signaling [I-D.ietf-dnsop-session-signal],
      it is unlikely that the server will begin supporting DNS Push Notifications Session
      Signaling 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.

      For RCODE = 11 (DNS Push SUBSCRIBE operation not supported), 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.

      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.

   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 other
   RCODEs the time delay applies to all subsequent requests to this
   server.

   After sending an error response the server MAY close the TCP
   connection with a FIN, or MAY allow it to remain open, depending on
   the nature of the error.  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 remain open, or MAY discard send a DNS Push Notification Terminate Session
   operation TLV and then close the state associated with it
   immediately, or later, at TCP connection, as described in the server's discretion.

6.4.
   DNS Session Signaling specification [I-D.ietf-dnsop-session-signal].
   Clients MUST correctly handle both cases.

6.3.  DNS Push Notification Update Messages

   Once a subscription has been successfully established, the server
   generates Push Notification Updates PUSH messages to send to the client as appropriate.  An
   initial Push Notification Update PUSH message 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. PUSH messages.

6.3.1.  PUSH Message format

   A PUSH message begins with the standard DNS Session Signaling 4-byte
   header [I-D.ietf-dnsop-session-signal], followed by the PUSH TLV.

   The format of Push Notification Updates PUSH messages 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
             +--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+
             |                  MESSAGE ID                   |
             +--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+
             |QR|   Opcode  |          Z         |   RCODE   |
             +--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+
             |                    ZOCOUNT               SSOP-TYPE (PUSH)                |
             +--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+
             |                    PRCOUNT                  SSOP-LENGTH                  |
             +--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+
             |                    UPCOUNT                    |
             +--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+
             |                    ADCOUNT                                               |
             \              Resource Records...              \
             \                                               \
             +--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+

                                 Figure 1

   For DNS Push Notifications the following rules apply: 3

   The QR bit MUST be zero, and the Opcode MESSAGE ID field MUST be UPDATE (5).
   Messages received where this is not true are not Push Notification
   Update Messages set to zero on transmission, and should be
   silently ignored for on reception.  A PUSH message could potentially
   match more than one subscription, or could relate to a subscription
   that the purposes of
   Push Notification Update Message handling.

   ID, client has just cancelled with an UNSUBSCRIBE message, so
   the MESSAGE ID field serves no useful purpose.

   In a PUSH message the DNS Header QR bit MUST be zero.

   The DNS Header Opcode field holds the Session Signaling Opcode value
   (tentatively 6).

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

   ZOCOUNT MUST be zero, and the Zone Section

   The return code (RCODE) field MUST be empty.
   Any records set to 0 in a request.

   In the Zone Section MUST be silently ignored.

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

   UPCOUNT specifies SSOP-TYPE is PUSH (tentatively 65).  The
   SSOP-LENGTH is the number length of records in the Update Section.

   ADCOUNT specifies SSOP-DATA that follows.

   The SSOP-DATA contains a two-byte count of the number of records in the Additional Data
   Section.  Typically this is zero, but it may be nonzero in some
   cases, such as when that
   follow, followed by the message includes an EDNS(0) OPT record.

   The records, in customary Resource Record format
   (as used in DNS Update Section [RFC2136] messages).

   The SSOP-DATA 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 PUSH 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 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 unavoidable race
   condition where a client sends an outbound UNSUBSCRIBE while inbound Push Notification Updates
   PUSH messages 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 PUSH message is sent.  For example, an update a PUSH
   message 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 PUSH messages are sent because the new
   record matches two active subscriptions.

   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, PUSH message, not
   three separate "delete an individual RR from a name" updates. PUSH messages.
   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, PUSH message,
   not two separate "delete an RRset from a name" updates. PUSH messages.

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

   Reception of a PUSH message does not directly generate a response
   back to the server.  (Updates may indirectly generate other
   operations; e.g., a Push Notification Update Messages MAY contain an EDNS(0) TCP
   Keepalive option [RFC7828] if Message declaring the idle timeout has changed since
   appearance of a PTR record could lead to a query for the
   last time SRV record
   named in the server sent rdata of that PTR record [RFC6763].)

   The TTL of an EDNS(0) TCP Keepalive option on this
   connection.

   In added record is stored by the event client and decremented as
   time passes, with the caveat that for as long as a relevant
   subscription is active, the server wishes to inform TTL does not decrement below 1 second.
   For as long as a relevant subscription remains active, the client of
   SHOULD assume that when a new idle
   timeout for the connection, record goes away the server MAY combine that with the next
   message will notify it sends
   of that fact.  Consequently, a client does not have to poll to verify
   that the client, record is still there.  Once a subscription is cancelled
   (individually, or as a result of the server MAY send an empty TCP connection being closed)
   record ageing resumes and records are removed from the local cache
   when their TTL reaches zero.

6.4.  DNS Push Notification Update Message (zero records in UNSUBSCRIBE

   To cancel an individual subscription without closing the Update Section) to
   carry entire
   connection, the EDNS(0) TCP Keepalive option.  Clients MUST correctly
   receive and process client sends an UNSUBSCRIBE message over the EDNS(0)
   established TCP Keepalive option in both cases.

   Reception of a Push Notification Update Message does not directly
   generate a response back connection to the server.  (Updates may indirectly
   generate other operations; e.g.,  The UNSUBSCRIBE message is
   encoded in a DNS Session Signaling [I-D.ietf-dnsop-session-signal]
   message.  This specification defines a new DNS Session Signaling TLV
   for DNS Push Notification Update Message
   declaring UNSUBSCRIBE Requests/Responses (tentatively
   Session Signaling Type Code 66).

   A server may not initiate an UNSUBSCRIBE request.

6.4.1.  UNSUBSCRIBE Request

   An UNSUBSCRIBE request message begins with the appearance of a PTR record could lead to a query for standard DNS Session
   Signaling 4-byte header [I-D.ietf-dnsop-session-signal], followed by
   the SRV record named UNSUBSCRIBE TLV.

                                             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  |          Z         |   RCODE   |
             +--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+
             |            SSOP-TYPE (UNSUBSCRIBE)            |
             +--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+
             |                SSOP-LENGTH (0)                |
             +--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+

                                 Figure 4

   The MESSAGE ID field MUST match the value given in the rdata of that PTR record[RFC6763].

   The TTL ID field of an added record
   active SUBSCRIBE request.  This is stored by how the client and decremented as
   time passes, with server knows which
   SUBSCRIBE request is being cancelled.  After receipt of the caveat that for as long as a relevant
   subscription
   UNSUBSCRIBE request, the SUBSCRIBE request is active, no longer active.  If a
   server receives an UNSUBSCRIBE message where the TTL MESSAGE ID does not decrement below 1 second.
   For as long as a relevant subscription remains active,
   match the client
   SHOULD assume that when ID of an active SUBSCRIBE request this is an error and the
   the server MUST return a record goes away response containing RCODE = 1 (FORMERR).  In
   the UNSUBSCRIBE response the server MUST echo back the MESSAGE ID
   value unchanged.  It is allowable for the server will notify it
   of that fact.  Consequently, client to issue an
   UNSUBSCRIBE request for a previous SUBSCRIBE request for which the
   client does has not have to poll yet received a SUBSCRIBE response.  This is to verify
   that allow
   for the record is still there.  Once case where a subscription is cancelled
   (individually, or as client starts and stops a result of subscription in less
   than the TCP connection being closed)
   record aging resumes and records are removed from round-trip time to the local cache
   when their TTL reaches zero.

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 server.  The client encounters data that it believe may be stale (e.g., an SRV
   record referencing a target host+port that is not responding NOT required
   to
   connection requests) wait for the client sends SUBSCRIBE response before issuing the UNSUBSCRIBE
   request.

   In a request the DNS RECONFIRM message Header QR bit MUST be zero.

   The DNS Header Opcode field holds the Session Signaling Opcode value
   (tentatively 6).

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

   The return code (RCODE) field MUST be set to 0 in a request.

   In the UNSUBSCRIBE TLV the SSOP-TYPE is UNSUBSCRIBE (tentatively 66).

   The SSOP-LENGTH is zero.

6.4.2.  UNSUBSCRIBE Response

   Each UNSUBSCRIBE request that generates exactly one UNSUBSCRIBE response
   from the server re-verify that server.

   An UNSUBSCRIBE response message contains with the standard DNS
   Session Signaling 4-byte header [I-D.ietf-dnsop-session-signal].

                                             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  |          Z         |   RCODE   |
             +--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+

                                 Figure 5

   The MESSAGE ID field MUST echo the data is still valid.  For
   a Hybrid Proxy, this causes it to issue new Multicast DNS requests to
   ascertain whether value given in the target device is still present.  For other
   kinds ID field of DNS server the RECONFIRM operation is currently undefined
   and SHOULD be silently ignored.

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

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

   The AA, TC, RD, RA, Z, AD, and CD bits, the ID field, and DNS Header Opcode field holds the RCODE
   field, Session Signaling Opcode value
   (tentatively 6).

   The Z bits 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

   In 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 UNSUBSCRIBE response the zone, and
   nothing else.

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

   No response message is generated
   unsubscribe request was successful.  Supported RCODEs are as follows:

   +------------+-------+----------------------------------------------+
   | Mnemonic   | Value | Description                                  |
   +------------+-------+----------------------------------------------+
   | NOERROR    |   0   | UNSUBSCRIBE successful.                      |
   | FORMERR    |   1   | Server failed to process request due to a result    |
   |            |       | malformed request.                           |
   | NOTIMP     |   4   | Server does not recognize DNS Session        |
   |            |       | Signaling Opcode.                            |
   | SSOPNOTIMP |   11  | UNSUBSCRIBE operation not supported.         |
   +------------+-------+----------------------------------------------+

                        UNSUBSCRIBE Response codes

   This document specifies only these RCODE values for UNSUBSCRIBE
   Responses.  Servers sending UNSUBSCRIBE Responses SHOULD use one of processing a
   RECONFIRM message.

   If the server receiving the RECONFIRM request determines that the
   records
   these values.  However, future circumstances may create situations
   where other RCODE values are appropriate in fact no longer valid, then subsequent DNS Push
   Notification Update Messages will UNSUBSCRIBE Responses, so
   clients MUST be generated prepared to inform interested
   clients.  Thus, one client discovering that accept UNSUBSCRIBE Responses with any
   RCODE value.

   Having being successfully revoked with a previously-advertised
   printer correctly-formatted
   UNSUBSCRIBE message (resulting in a response with RCODE NOERROR) the
   previously referenced subscription is no longer present has active and the side effect server
   MAY discard the state associated with it immediately, or later, at
   the server's discretion.

   Nonzero RCODE values signal some kind of informing all error.

   RCODE value FORMERR indicates an incorrect MESSAGE ID or other interested clients that
   message format error.

   RCODE values NOTIMP and SSOPNOTIMP should not occur in practice.

   A server would only generate NOTIMP if it did not support Session
   Signaling, and if the printer in question is now gone.

6.6.  DNS Push Notification Termination Message

   If a server is low on resources does not support Session Signaling then
   it MAY simply terminate should not be possible for a client
   connection with to have an active subscription
   to cancel.

   Similarly, a TCP RST.  However, the likely behaviour of server would only generate SSOPNOTIMP if it did not
   support Push Notifications, and if the
   client may server does not support Push
   Notifications then it should not be simply possible for a client to reconnect immediately, putting more burden on
   the server.  Therefore, have an
   active subscription to cancel.

   All nonzero RCODE values indicate a serious problem with the client.
   After sending an error response, the server SHOULD instead choose to shed client
   load by (a) sending send a DNS Push
   Notification Termination Message Terminate Session operation TLV and then (b) immediately closing close the client connection with a TCP FIN
   instead of RST, thereby facilitating reliable delivery of the
   Termination Message.  Upon successful reception of the Termination
   Message the client is expected to close
   connection, as described in the connection.  The server
   SHOULD set DNS Session Signaling specification
   [I-D.ietf-dnsop-session-signal].

6.5.  DNS Session Signaling Push Notification RECONFIRM

   Sometimes, particularly when used with a Hybrid Proxy
   [I-D.ietf-dnssd-hybrid], a DNS Zone may contain stale data.  When a timer and, if the
   client has encounters data that it believe may be stale (e.g., an SRV
   record referencing a target host+port that is not closed the responding to
   connection
   within a reasonable time, the server SHOULD then terminate requests) the TCP
   connection with client can send a TCP RST.  The RECOMMENDED time RECONFIRM message to
   request that the server should
   wait before terminating re-verify that the TCP connection with a TCP RST data is ten
   seconds.

   The format of still valid.  For
   a Termination Message is similar Hybrid Proxy, this causes it to a Push Notification
   Update.

   The following figure shows the existing issue new Multicast 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 requests to
   ascertain whether the following rules apply:

   The QR bit MUST be zero, and target device is still present.  For other
   types of DNS server, the Opcode MUST be UPDATE (5).
   Messages received where this RECONFIRM operation is not true are not Termination Messages currently undefined
   and should 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

   A RECONFIRM request is formatted identically to a SUBSCRIBE request,
   except that the Zone Section TLV type is RECONFIRM (tentatively 67) instead of
   SUBSCRIBE.  Additionally, QTYPE MUST NOT be empty.
   Any records in the Zone Section MUST be silently ignored.

   PRCOUNT MUST be zero, value ANY (255) and the Prerequisite Section MUST be empty.
   Any records in the Prerequisite Section MUST be silently ignored.

   UPCOUNT
   QCLASS MUST NOT be zero, and the Update Section value ANY (255).

   Like all DNS Session Signaling [I-D.ietf-dnsop-session-signal]
   requests, a RECONFIRM request MUST be empty.
   Any records contain a unique MESSAGE ID, not
   currently in use in this session.

   A RECONFIRM request generates exactly one RECONFIRM response from the Update Section MUST be silently ignored.

   ADCOUNT specifies
   server, formatted identically to a SUBSCRIBE response, which echoes
   back the number of records in unique MESSAGE ID from the Additional Data
   Section, e.g., RECONFIRM request.

   In the EDNS(0) OPT record..

   The RCODE MUST contain a nonzero code giving RECONFIRM response the reason for
   termination, RCODE indicates whether or not the
   request was successful.  Supported RCODEs are as indicated below:

   +----------+-------+------------------------------------------------+ follows:

   +------------+-------+----------------------------------------------+
   | Mnemonic   | Value | Description                                  |
   +----------+-------+------------------------------------------------+
   +------------+-------+----------------------------------------------+
   | SERVFAIL NOERROR    |   2   0   | The server is overloaded due to resource RECONFIRM successful.                        |
   | FORMERR    |   1   | exhaustion. Server failed to process request due to a    |
   | REFUSED            |   5       | The server has been reconfigured and is no malformed request.                           |
   | NOTIMP     |   4   | longer accepting Server does not recognize DNS Push Notification Session        |
   |            |       | requests for one or more of the currently Signaling Opcode.                            |
   | SSOPNOTIMP |   11  | subscribed names. RECONFIRM operation not supported.           |
   +----------+-------+------------------------------------------------+

                        Termination
   +------------+-------+----------------------------------------------+

                         RECONFIRM Response codes

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

   A correctly-formatted RECONFIRM message results in a Termination Message response with
   RCODE NOERROR.

   Nonzero RCODE values signal some kind of error.  If the server sends
   a nonzero RCODE then it SHOULD append a Terminate modifier TLV
   [I-D.ietf-dnsop-session-signal] to the response specifying a delay
   before the client attempts this operation again.  The RECOMMENDED
   value zero (NOERROR) for the delay is still five minutes.  For serious errors, after
   sending the error response, the server SHOULD send a DNS Push
   Notification Terminate Session operation TLV and then close the TCP
   connection, as described in the DNS Session Signaling specification
   [I-D.ietf-dnsop-session-signal].

   If, after receiving a valid RECONFIRM request, the server determines
   that the records are in fact no longer valid, then subsequent DNS
   PUSH 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.

6.6.  Client-Initiated Termination

   An individual subscription is terminated by sending an UNSUBSCRIBE
   TLV for that specific subscription, or all subscriptions can be
   cancelled at once by the client closing the connection.  When a Termination Message and should
   be treated as such.

   The Termination Message MUST contain an EDNS(0) TCP Keepalive option
   [RFC7828].  The
   client MUST wait for terminates an individual subscription (via UNSUBSCRIBE) or all
   subscriptions on that connection (by closing the time indicated connection) it is
   signaling to the server that it is longer interested in receiving
   those particular updates.  It is informing the
   EDNS(0) TCP Keepalive option's idle timeout before attempting server that the server
   may release any new
   connections state information it has been keeping with regards to this server.  A
   these particular subscriptions.

   After terminating its last subscription on a connection via
   UNSUBSCRIBE, a client MAY close the connection immediately, or it may
   keep it open if it anticipates performing further operations on that receives
   connection in the future.  If a Termination
   Message without client wishes to keep an EDNS(0) TCP Keepalive option SHOULD treat idle
   connection open, it as
   equivalent MUST continue to meet its keepalive obligations
   [I-D.ietf-dnsop-session-signal] or the server is entitled to close
   the connection (see below).

   If a TCP Keepalive option with client plans to terminate one or more subscriptions on a zero timeout value.

   In
   connection and doesn't intend to keep that connection open, then as
   an efficiency optimization it MAY instead choose to simply close the case where
   connection, which implicitly terminates all subscriptions on that
   connection.  This may occur because the server client computer is rejecting some, but not all, of being shut
   down, is going to sleep, the application requiring the
   existing subscriptions (perhaps
   has terminated, or simply because it the last active subscription on
   that connection has been reconfigured cancelled.

   When closing a connection, a client will generally do an abortive
   disconnect, sending a TCP RST.  This immediately discards all
   remaining inbound and outbound data, which 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 appropriate if the
   client SHOULD attempt to re-establish its
   subscriptions immediately. no longer has any interest in this data.  In the case where BSD Sockets
   API, sending a server TCP RST is terminating achieved by setting the SO_LINGER option
   with a large number time of
   connections at once (e.g., if the system is restarting) 0 seconds and then closing the
   server doesn't want to be inundated with socket.

   If a flood of simultaneous
   retries, client has performed operations on this connection that it would
   not want lost (like DNS updates) then the client SHOULD send different EDNS(0) do an orderly
   disconnect, sending a TCP Keepalive values to
   each client.  These adjustments MAY be selected randomly,
   pseudorandomly, or deterministically (e.g., incrementing FIN.  In the time
   value by one tenth of a second for each successive client, yielding BSD Sockets API, sending a
   post-restart reconnection rate of ten clients per second). TCP
   FIN is achieved by calling "shutdown(s,SHUT_WR)" and keeping the
   socket open until all remaining data has been read from it.

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.

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 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: Session Signaling TLV types:
   SUBSCRIBE with (tentative) value 6, 64, PUSH with (tentative) value 65,
   UNSUBSCRIBE with (tentative) value 7, 66, and RECONFIRM with (tentative)
   value 8. 67.

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 Volz, Jan Komissar, Manju
   Shankar Rao, Markus Stenberg, Dave Thaler, and Soraia Zlatkovic.

10.  References

10.1.  Normative References

   [I-D.bellis-dnsop-session-signal]

   [I-D.ietf-dnsop-session-signal]
              Bellis, R., Cheshire, S., Marcon, Dickinson, J., Dickinson, S.,
              Mankin, A., and T. Pusateri, "DNS Session Signaling", draft-bellis-dnsop-
              session-signal-00
              draft-ietf-dnsop-session-signal-00 (work in progress), July
              August 2016.

   [I-D.ietf-tls-tls13]
              Rescorla, E., "The Transport Layer Security (TLS) Protocol
              Version 1.3", draft-ietf-tls-tls13-13 draft-ietf-tls-tls13-18 (work in progress),
              May
              October 2016.

   [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>.

   [RFC1123]  Braden, R., Ed., "Requirements for Internet Hosts -
              Application and Support", STD 3, RFC 1123,
              DOI 10.17487/
              RFC1123, 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, 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>.

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

   [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>.

   [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>.

   [RFC6895]  Eastlake 3rd, D., "Domain Name System (DNS) IANA
              Considerations", BCP 42, RFC 6895, DOI 10.17487/RFC6895,
              April 2013, <http://www.rfc-editor.org/info/rfc6895>.

   [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>.

   [RFC7766]  Dickinson, J., Dickinson, S., Bellis, R., Mankin, A., and
              D. Wessels, "DNS Transport over TCP - Implementation
              Requirements", RFC 7766, DOI 10.17487/RFC7766, March 2016,
              <http://www.rfc-editor.org/info/rfc7766>.

   [RFC7828]  Wouters, P., Abley, J., Dickinson, S., and R. Bellis, "The
              edns-tcp-keepalive EDNS0 Option", RFC 7828, DOI 10.17487/
              RFC7828, April 2016,
              <http://www.rfc-editor.org/info/rfc7828>.

10.2.  Informative References

   [I-D.dukkipati-tcpm-tcp-loss-probe]
              Dukkipati, N., Cardwell, N., Cheng, Y., and M. Mathis,
              "Tail Loss Probe (TLP): An Algorithm for Fast Recovery of
              Tail Losses", draft-dukkipati-tcpm-tcp-loss-probe-01 (work
              in progress), February 2013.

   [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.ietf-dprive-dns-over-tls]
              Zi, Z., Zhu, L., Heidemann, J., Mankin, A., Wessels, D.,
              and P. Hoffman, "Specification for DNS over TLS", draft-
              ietf-dprive-dns-over-tls-09 (work in progress), March
              February 2016.

   [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.

   [obs]      "Observer Pattern", <https://en.wikipedia.org/wiki/
              Observer_pattern>.

   [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>.

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

   [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>.

   [RFC6824]  Ford, A., Raiciu, C., Handley, M., and O. Bonaventure,
              "TCP Extensions for Multipath Operation with Multiple
              Addresses", RFC 6824, DOI 10.17487/RFC6824, January 2013,
              <http://www.rfc-editor.org/info/rfc6824>.

   [RFC7413]  Cheng, Y., Chu, J., Radhakrishnan, S., and A. Jain, "TCP
              Fast Open", RFC 7413, DOI 10.17487/RFC7413, December 2014,
              <http://www.rfc-editor.org/info/rfc7413>.

   [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>.

   [RFC7858]  Hu, Z., Zhu, L., Heidemann, J., Mankin, A., Wessels, D.,
              and P. Hoffman, "Specification for DNS over Transport
              Layer Security (TLS)", RFC 7858, DOI 10.17487/RFC7858, May
              2016, <http://www.rfc-editor.org/info/rfc7858>.

   [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