[Docs] [txt|pdf|xml|html] [Tracker] [WG] [Email] [Diff1] [Diff2] [Nits]

Versions: 00 01 02 03 04 05 06 07 08 09 10 11 12 13 14

Internet Engineering Task Force                              T. Pusateri
Internet-Draft                                              Unaffiliated
Intended status: Standards Track                             S. Cheshire
Expires: September 19, 2018                                   Apple Inc.
                                                          March 18, 2018


                         DNS Push Notifications
                        draft-ietf-dnssd-push-14

Abstract

   The Domain Name System (DNS) was designed to return matching records
   efficiently for queries for data that are relatively static.  When
   those records change frequently, DNS is still efficient at returning
   the updated results when polled, as long as the polling rate is not
   too high.  But there exists no mechanism for a client to be
   asynchronously notified when these changes occur.  This document
   defines a mechanism for a client to be notified of such changes to
   DNS records, called DNS Push Notifications.

Status of This Memo

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

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

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

   This Internet-Draft will expire on September 19, 2018.

Copyright Notice

   Copyright (c) 2018 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
   (https://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



Pusateri & Cheshire    Expires September 19, 2018               [Page 1]


Internet-Draft           DNS Push Notifications               March 2018


   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  . . . . . . . . . . . . . . . . . . . . . . . .   3
     1.1.  Requirements Language . . . . . . . . . . . . . . . . . .   3
   2.  Motivation  . . . . . . . . . . . . . . . . . . . . . . . . .   4
   3.  Overview  . . . . . . . . . . . . . . . . . . . . . . . . . .   5
   4.  Transport . . . . . . . . . . . . . . . . . . . . . . . . . .   7
   5.  State Considerations  . . . . . . . . . . . . . . . . . . . .   8
   6.  Protocol Operation  . . . . . . . . . . . . . . . . . . . . .   9
     6.1.  Discovery . . . . . . . . . . . . . . . . . . . . . . . .  10
     6.2.  DNS Push Notification SUBSCRIBE . . . . . . . . . . . . .  13
       6.2.1.  SUBSCRIBE Request . . . . . . . . . . . . . . . . . .  13
       6.2.2.  SUBSCRIBE Response  . . . . . . . . . . . . . . . . .  16
     6.3.  DNS Push Notification Updates . . . . . . . . . . . . . .  19
       6.3.1.  PUSH Message  . . . . . . . . . . . . . . . . . . . .  19
     6.4.  DNS Push Notification UNSUBSCRIBE . . . . . . . . . . . .  22
       6.4.1.  UNSUBSCRIBE Request . . . . . . . . . . . . . . . . .  22
     6.5.  DNS Push Notification RECONFIRM . . . . . . . . . . . . .  24
       6.5.1.  RECONFIRM Request . . . . . . . . . . . . . . . . . .  24
       6.5.2.  RECONFIRM Response  . . . . . . . . . . . . . . . . .  26
     6.6.  Client-Initiated Termination  . . . . . . . . . . . . . .  28
   7.  Security Considerations . . . . . . . . . . . . . . . . . . .  29
     7.1.  Security Services . . . . . . . . . . . . . . . . . . . .  29
     7.2.  TLS Name Authentication . . . . . . . . . . . . . . . . .  29
     7.3.  TLS Compression . . . . . . . . . . . . . . . . . . . . .  30
     7.4.  TLS Session Resumption  . . . . . . . . . . . . . . . . .  30
   8.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .  30
   9.  Acknowledgements  . . . . . . . . . . . . . . . . . . . . . .  31
   10. References  . . . . . . . . . . . . . . . . . . . . . . . . .  31
     10.1.  Normative References . . . . . . . . . . . . . . . . . .  31
     10.2.  Informative References . . . . . . . . . . . . . . . . .  33
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  35














Pusateri & Cheshire    Expires September 19, 2018               [Page 2]


Internet-Draft           DNS Push Notifications               March 2018


1.  Introduction

   Domain Name System (DNS) records may be updated using DNS Update
   [RFC2136].  Other mechanisms such as a Discovery Proxy [DisProx] can
   also generate changes to a DNS zone.  This document specifies a
   protocol for DNS clients to subscribe to receive asynchronous
   notifications of changes to RRSets of interest.  It is immediately
   relevant in the case of 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",
   when, and only when, they appear in all capitals, as shown here
   [RFC2119] [RFC8174].































Pusateri & Cheshire    Expires September 19, 2018               [Page 3]


Internet-Draft           DNS Push Notifications               March 2018


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 following the
   Observer design 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
   deliver 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 record changes are sent to that
   multicast group address 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 Discovery Proxy [DisProx]) 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.

   The DNS Long-Lived Queries (LLQ) mechanism [LLQ] is an existing
   deployed solution to provide asynchronous change notifications, used
   by Apple's Back to My Mac Service [RFC6281] introduced in Mac OS X
   10.5 Leopard in 2007.  Back to My Mac was designed in an era when the
   data center operations staff asserted that it was impossible for a
   server to handle large numbers of mostly-idle TCP connections, so LLQ
   was defined as a UDP-based protocol, effectively replicating much of
   TCP's connection state management logic in user space, and creating
   its own poor imitations of existing TCP features like the three-way
   handshake, flow control, and reliability.

   This document builds on experience gained with the LLQ protocol, with
   an improved design.  Instead of using UDP, this specification uses
   DNS Stateful Operations (DSO) [DSO] running over TLS over TCP, and
   therefore doesn't need to reinvent existing TCP functionality.  Using
   TCP also gives long-lived low-traffic connections better longevity
   through NAT gateways without resorting to excessive keepalive
   traffic.  Instead of inventing a new vocabulary of messages to
   communicate DNS zone changes as LLQ did, this specification borrows
   the established syntax and semantics of DNS Update messages
   [RFC2136].







Pusateri & Cheshire    Expires September 19, 2018               [Page 4]


Internet-Draft           DNS Push Notifications               March 2018


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 receiving further 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 [RFC7719].
   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 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.  If a query is
   received with the RD bit set, matching records for names falling
   within the server's zones should be returned with the RA (Recursion
   Available) bit clear.  If the query is for a name not in the server's
   zone, an error with RCODE NOTAUTH (Not Authoritative) should be
   returned.

   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




Pusateri & Cheshire    Expires September 19, 2018               [Page 5]


Internet-Draft           DNS Push Notifications               March 2018


   comes to an authoritative server unilaterally 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 is the reality today with existing DNS servers and
   clients.  A DNS Push Notification server MAY choose to implement only
   DNS class "IN".  If messages are received for a class other than
   "IN", and that class is not supported, an error with RCODE NOTIMPL
   (Not Implemented) should be returned.

   DNS Push Notifications impose less load on the responding server than
   rapid polling would, but Push Notifications do still have a cost, so
   DNS Push Notification clients must not recklessly create an excessive
   number of Push Notification subscriptions.  Specifically:

   (a) A subscription should only be active when there is a valid reason
   to need live data (for example, an on-screen display is currently
   showing the results to 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 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.

   (b) A DNS Push Notification client SHOULD 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 if the user does
   choose to bring up an on-screen display of that data, it can be
   displayed really fast.  DNS Push Notifications are designed to be
   fast enough that there is no need to pre-load a "warm" list in memory
   just in case it might be needed later.

   Generally, as described in the DNS Stateful Operations specification
   [DSO], a client must not keep a session to a server open indefinitely
   if it has no subscriptions (or other operations) active on that
   session.  A client MAY close a session as soon as it becomes idle,
   and then if needed in the future, open a new session when required.
   Alternatively, a client MAY speculatively keep an idle session open
   for some time, subject to the constraint that it MUST NOT keep a



Pusateri & Cheshire    Expires September 19, 2018               [Page 6]


Internet-Draft           DNS Push Notifications               March 2018


   session open that has been idle for more than the session's idle
   timeout (15 seconds by default).

4.  Transport

   Other DNS operations like 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 DNS
   Stateful Operations (DSO) [DSO] running over TLS over TCP [RFC7858].

   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.  Flooding attacks
   are possible with any protocol, and a benefit of TCP is that there
   are already established industry best practices to guard against SYN
   flooding and similar attacks [SYN] [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, and 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.















Pusateri & Cheshire    Expires September 19, 2018               [Page 7]


Internet-Draft           DNS Push Notifications               March 2018


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 session to a DNS server,
   each subscription is individually accepted or rejected.  Servers may
   employ various techniques to limit subscriptions to a manageable
   level.  Correspondingly, the client is free to establish simultaneous
   sessions to alternate DNS servers that support DNS Push Notifications
   for the zone and distribute subscriptions at the client's discretion.
   In this way, both clients and servers can react to resource
   constraints.  Token bucket rate limiting schemes are also effective
   in providing fairness by a server across numerous client requests.




































Pusateri & Cheshire    Expires September 19, 2018               [Page 8]


Internet-Draft           DNS Push Notifications               March 2018


6.  Protocol Operation

   The DNS Push Notification protocol is a session-oriented protocol,
   and makes use of DNS Stateful Operations (DSO) [DSO].

   For details of the DSO message format refer to the DNS Stateful
   Operations specification [DSO].  Those details are not repeated here.

   DNS Push Notification clients and servers MUST support DSO, but (as
   stated in the DSO specification [DSO]) the server SHOULD NOT issue
   any DSO messages until after the client has first initiated an
   acknowledged DSO message of its own.  A single server can support DNS
   Queries, DNS Updates, and DNS Push Notifications (using DSO) on the
   same TCP port, and until the client has sent at least one DSO
   message, the server does not know what kind of client has connected
   to it.  Once the client has indicated willingness to use DSO by
   sending one of its own, either side of the session may then initiate
   further DSO messages at any time.

   A DNS Push Notification exchange begins with the 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 will immediately issue a DSO
   Keepalive operation to request a session timeout or keepalive
   interval longer than the the 15-second defaults, but this is not
   required.  A DNS Push Notification client MAY issue other requests on
   the session first, and only issue a DSO Keepalive operation later if
   it determines that to be necessary.

   Once the session is made, the client may then add and remove Push
   Notification subscriptions.  In accordance with the current set of
   active subscriptions the server sends relevant asynchronous Push
   Notifications to the client.  Note that a client MUST be prepared to
   receive (and silently ignore) Push Notifications for subscriptions it
   has previously removed, since there is no way to prevent the
   situation where a Push Notification is in flight from server to
   client while the client's UNSUBSCRIBE message cancelling that
   subscription is simultaneously in flight from client to server.












Pusateri & Cheshire    Expires September 19, 2018               [Page 9]


Internet-Draft           DNS Push Notifications               March 2018


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 begins by opening a DSO Session to its normal configured
   DNS recursive resolver and requesting a Push Notification
   subscription.  If this is successful, then the recursive resolver
   will make appropriate Push Notification subscriptions on the client's
   behalf, and the client will receive appropriate results.  If the
   recursive resolver does not support Push Notification subscriptions,
   then it will return an error code, and the client should proceed to
   discover the appropriate server for direct communication.  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 [RFC7858].

   The algorithm described here is an iterative algorithm, which starts
   with the full name of the record to which the client wishes to
   subscribe.  Successive SOA queries are then issued, trimming one
   label each time, until the closest enclosing authoritative server is
   discovered.  There is also an optimization to enable the client to
   take a "short cut" directly to the SOA record of the closest
   enclosing authoritative server in many cases.

   1.  The client begins the discovery by sending a DNS query to its
       local resolver, with record type SOA [RFC1035] for the record
       name to which it wishes to subscribe.  As an example, suppose the
       client wishes to subscribe to PTR records with the name
       _ipp._tcp.foo.example.com (to discover Internet Printing Protocol
       (IPP) printers [RFC8010] [RFC8011] being advertised at
       "foo.example.com").  The client begins by sending an SOA query
       for _ipp._tcp.foo.example.com to the local recursive resolver.
       The goal is to determine the server authoritative for the name
       _ipp._tcp.foo.example.com.  The DNS zone containing the name
       _ipp._tcp.foo.example.com could be example.com, or
       foo.example.com, or _tcp.foo.example.com, or even
       _ipp._tcp.foo.example.com.  The client does not know in advance
       where the closest enclosing zone cut occurs, which is why it uses
       the procedure described here to discover this information.

   2.  If the requested SOA record exists, it will be returned in the
       Answer section with a NOERROR response code, and the client has
       succeeded in discovering the information it needs.  (This text is
       not placing any new requirements on DNS recursive resolvers.  It




Pusateri & Cheshire    Expires September 19, 2018              [Page 10]


Internet-Draft           DNS Push Notifications               March 2018


       is merely describing the existing operation of the DNS protocol
       [RFC1034] [RFC1035].)

   3.  If the requested SOA record does not exist, the client will get
       back a NOERROR/NODATA response or an NXDOMAIN/Name Error
       response.  In either case, the local resolver SHOULD include the
       SOA record for the zone of the requested name in the Authority
       Section.  If the SOA record is received in the Authority Section,
       then the client has succeeded in discovering the information it
       needs.  (This text is not placing any new requirements on DNS
       recursive resolvers.  It is merely describing the existing
       operation of the DNS protocol regarding negative responses
       [RFC2308].)

   4.  If the client receives a response containing no SOA record, then
       it proceeds with the iterative approach.  The client strips the
       leading label from the current query name and if the resulting
       name has at least one label in it, the client sends a new SOA
       query, and processing continues at step 2 above, repeating the
       iterative search until either an SOA is received, or the query
       name is empty.  In the case of an empty name, this is a network
       configuration error which should not happen and the client gives
       up.  The client may retry the operation at a later time, of the
       client's choosing, such after a change in network attachment.

   5.  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.

   6.  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.  (The "_dns-push-tls._tcp" service type is
       allocated by IANA for this purpose, and, like any allocated IANA
       service type, MUST NOT be used for other services.  Other
       services that require an IANA service type should use a unique
       service type allocated by IANA for that service [RFC6335][ST].)

   7.  If the zone in question is set up to offer DNS Push Notifications
       then this SRV record MUST exist.  (If this SRV record does not
       exist then the zone is not correctly configured for DNS Push
       Notifications as specified in this document.)  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



Pusateri & Cheshire    Expires September 19, 2018              [Page 11]


Internet-Draft           DNS Push Notifications               March 2018


       below in Section 7.2 and in the specification for using DANE TLSA
       Records with SRV Records [RFC7673].

   8.  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 willing to
       accept a subscription request, or is not reachable within a
       reasonable time, as determined by the client, then a subsequent
       server is to be contacted.

   Each time a client makes a new DNS Push Notification subscription
   session, it SHOULD repeat the discovery process in order to determine
   the preferred DNS server for subscriptions at that time.  However,
   the client device MUST respect the DNS TTL values on records it
   receives, and store them in its local cache with this lifetime.  This
   means that, as long as the DNS TTL values on the authoritative
   records were set to reasonable values, repeated application of this
   discovery process can be completed nearly instantaneously by the
   client, using only locally-stored cached data.


























Pusateri & Cheshire    Expires September 19, 2018              [Page 12]


Internet-Draft           DNS Push Notifications               March 2018


6.2.  DNS Push Notification SUBSCRIBE

   After connecting, and requesting a longer idle timeout and/or
   keepalive interval if necessary, a DNS Push Notification client then
   indicates its desire to receive DNS Push Notifications for a given
   domain name by sending a SUBSCRIBE request over the established DSO
   session to the server.  A SUBSCRIBE request is encoded in a DSO [DSO]
   message.  This specification defines a DSO TLV for DNS Push
   Notification SUBSCRIBE Requests/Responses (tentatively DSO Type Code
   0x40).

   The entity that initiates a SUBSCRIBE request is by definition the
   client.  A server MUST NOT send a SUBSCRIBE request over an existing
   session from a client.  If a server does send a SUBSCRIBE request
   over a DSO session initiated by a client, this is a fatal error and
   the client should immediately abort the connection with a TCP RST (or
   equivalent for other protocols).

6.2.1.  SUBSCRIBE Request

   A SUBSCRIBE request begins with the standard DSO 12-byte header
   [DSO], followed by the SUBSCRIBE TLV.  A SUBSCRIBE request message is
   illustrated in 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 session.  For the
   purposes here, a MESSAGE ID is in use on this session if the client
   has used it in a request for which it has not yet received a
   response, or if the client has used it for a subscription which it
   has not yet cancelled using UNSUBSCRIBE.  In the SUBSCRIBE response
   the server MUST echo back the MESSAGE ID value unchanged.

   The other header fields MUST be set as described in the DSO
   specification [DSO].  The DNS Opcode is the DSO Opcode (tentatively
   6).  The four count fields MUST be zero, and the corresponding four
   sections MUST be empty (i.e., absent).

   The DSO-TYPE is SUBSCRIBE (tentatively 0x40).  The DSO-LENGTH is the
   length of the DSO-DATA that follows, which specifies the name, type,
   and class of the record(s) being sought.











Pusateri & Cheshire    Expires September 19, 2018              [Page 13]


Internet-Draft           DNS Push Notifications               March 2018


                                     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   |    |
     +--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+    |
     |             QDCOUNT (MUST BE ZERO)            |    |
     +--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+     > HEADER
     |             ANCOUNT (MUST BE ZERO)            |    |
     +--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+    |
     |             NSCOUNT (MUST BE ZERO)            |    |
     +--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+    |
     |             ARCOUNT (MUST BE ZERO)            |   /
     +--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+  /
     |    DSO-TYPE = SUBSCRIBE (tentatively 0x40)    |
     +--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+
     |   DSO-LENGTH (number of octets in DSO-DATA)   |
     +--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+  \
     |                                               |   \
     \                     NAME                      \    |
     \                                               \    |
     +--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+     > DSO-DATA
     |                     TYPE                      |    |
     +--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+    |
     |                     CLASS                     |   /
     +--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+  /

                        Figure 1: SUBSCRIBE Request

   The DSO-DATA for a SUBSCRIBE request MUST contain exactly one
   question.  The DSO-DATA for a SUBSCRIBE request has no QDCOUNT field
   to specify more than one question.  Since SUBSCRIBE requests are sent
   over TCP, multiple SUBSCRIBE request messages can be concatenated in
   a single TCP stream and packed efficiently into TCP segments.

   If accepted, the subscription will stay in effect until the client
   cancels the subscription using UNSUBSCRIBE or until the DSO session
   between the client and the server is closed.

   SUBSCRIBE requests on a given session 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 DSO session.  For the
   purpose of this matching, the established DNS case-insensitivity for
   US-ASCII letters applies (e.g., "example.com" and "Example.com" are
   the same).  If a server receives such a duplicate SUBSCRIBE message
   this is an error and the server MUST immediately terminate the
   connection with a TCP RST (or equivalent for other protocols).



Pusateri & Cheshire    Expires September 19, 2018              [Page 14]


Internet-Draft           DNS Push Notifications               March 2018


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

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

   A client may SUBSCRIBE to records that are unknown to the server at
   the time of the request (providing that the name falls within one of
   the zone(s) the server is responsible for) and this is not an error.
   The server MUST accept these requests and send Push Notifications if
   and when matching records are found in the future.

   If neither TYPE nor CLASS are ANY (255) then this is a specific
   subscription to changes for the given NAME, TYPE and CLASS.  If one
   or both of TYPE or CLASS 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, where 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 SUBSCRIBE, TYPE and CLASS 255 should be
   interpreted to mean "ALL", not "ANY".  After accepting a subscription
   where one or both of TYPE or CLASS are 255, the server MUST send Push
   Notification Updates for ALL record changes that match the
   subscription, not just some of them.




















Pusateri & Cheshire    Expires September 19, 2018              [Page 15]


Internet-Draft           DNS Push Notifications               March 2018


6.2.2.  SUBSCRIBE Response

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

   A SUBSCRIBE response message begins with the standard DSO 12-byte
   header [DSO], possibly followed by one or more optional TLVs, such as
   a Retry Delay TLV.

   The MESSAGE ID field MUST echo the value given in the ID field of the
   SUBSCRIBE request.  This is how the client knows which request is
   being responded to.

   A SUBSCRIBE response message MUST NOT include a SUBSCRIBE TLV.  If a
   client receives a SUBSCRIBE response message containing a SUBSCRIBE
   TLV then the response message is processed but the SUBSCRIBE TLV MUST
   be silently ignored.

   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 a     |
   |           |       | problem with the server.                      |
   | NXDOMAIN  |   3   | NOT APPLICABLE. DNS Push Notification servers |
   |           |       | MUST NOT return NXDOMAIN errors in response   |
   |           |       | to SUBSCRIBE requests.                        |
   | NOTIMP    |   4   | Server does not implement DSO.                |
   | REFUSED   |   5   | Server refuses to process request for policy  |
   |           |       | or security reasons.                          |
   | NOTAUTH   |   9   | Server is not authoritative for the requested |
   |           |       | name.                                         |
   | DSOTYPENI |   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.



Pusateri & Cheshire    Expires September 19, 2018              [Page 16]


Internet-Draft           DNS Push Notifications               March 2018


   If the server sends a nonzero RCODE in the SUBSCRIBE response, that
   means (a) the client is (at least partially) misconfigured, (b) the
   server resources are exhausted, or (c) there is some other unknown
   failure on the server.  In any case, the client shouldn't retry the
   subscription right away.  Either end can terminate the session, but
   the client may want to try this subscription again, or it may have
   other successful subscriptions that it doesn't want to abandon.  If
   the server sends a nonzero RCODE then it SHOULD append a Retry Delay
   TLV [DSO] to the response specifying a delay before the client
   attempts this operation again.  Recommended values for the delay for
   different RCODE values are given below.  These recommended values
   apply both to the default values a server should place in the Retry
   Delay TLV, and the default values a client should assume if the
   server provides no Retry Delay TLV.

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

      For RCODE = 2 (SERVFAIL) the delay should be chosen according to
      the level of server overload and the anticipated duration of that
      overload.  By default, a value of one minute is RECOMMENDED.  If a
      more serious server failure occurs, the delay may be longer in
      accordance with the specific problem encountered.

      For RCODE = 4 (NOTIMP), which occurs on a server that doesn't
      implement DSO [DSO], it is unlikely that the server will begin
      supporting DSO in the next few minutes, so the retry delay SHOULD
      be one hour.  Note that in such a case, a server that doesn't
      implement DSO is unlikely to place a Retry Delay TLV in its
      response, so this recommended value in particular applies to what
      a client should assume by default.

      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.



Pusateri & Cheshire    Expires September 19, 2018              [Page 17]


Internet-Draft           DNS Push Notifications               March 2018


      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 allow the session to
   remain open, or MAY send a DNS Push Notification Retry Delay
   Operation TLV instructing the client to close the session, as
   described in the DSO specification [DSO].  Clients MUST correctly
   handle both cases.























Pusateri & Cheshire    Expires September 19, 2018              [Page 18]


Internet-Draft           DNS Push Notifications               March 2018


6.3.  DNS Push Notification Updates

   Once a subscription has been successfully established, the server
   generates PUSH messages to send to the client as appropriate.  In the
   case that the answer set was non-empty at the moment the subscription
   was established, an initial PUSH message will be sent immediately
   following the SUBSCRIBE Response.  Subsequent changes to the answer
   set are then communicated to the client in subsequent PUSH messages.

6.3.1.  PUSH Message

   A PUSH message begins with the standard DSO 12-byte header [DSO],
   followed by the PUSH TLV.  A PUSH message is illustrated in Figure 2.

   The MESSAGE ID field MUST be zero.  There is no client response to a
   PUSH message.

   The other header fields MUST be set as described in the DSO
   specification [DSO].  The DNS Opcode is the DSO Opcode (tentatively
   6).  The four count fields MUST be zero, and the corresponding four
   sections MUST be empty (i.e., absent).

   The DSO-TYPE is PUSH (tentatively 0x41).  The DSO-LENGTH is the
   length of the DSO-DATA that follows, which specifies the changes
   being communicated.

   The DSO-DATA contains one or more Update records.  A PUSH Message
   MUST contain at least one Update record.  If a PUSH Message is
   received that contains no Update records, this is a fatal error, and
   the receiver MUST immediately terminate the connection with a TCP RST
   (or equivalent for other protocols).  The Update records are
   formatted in the customary way for Resource Records in DNS messages.
   Update records in a PUSH Message are interpreted according to the
   same rules as for DNS Update [RFC2136] messages, namely:

      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.



Pusateri & Cheshire    Expires September 19, 2018              [Page 19]


Internet-Draft           DNS Push Notifications               March 2018


                                     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   |    |
     +--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+    |
     |             QDCOUNT (MUST BE ZERO)            |    |
     +--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+     > HEADER
     |             ANCOUNT (MUST BE ZERO)            |    |
     +--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+    |
     |             NSCOUNT (MUST BE ZERO)            |    |
     +--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+    |
     |             ARCOUNT (MUST BE ZERO)            |   /
     +--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+  /
     |      DSO-TYPE = PUSH (tentatively 0x41)       |
     +--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+
     |   DSO-LENGTH (number of octets in DSO-DATA)   |
     +--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+  \
     \                     NAME                      \   \
     +--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+    |
     |                     TYPE                      |    |
     +--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+    |
     |                     CLASS                     |    |
     +--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+    |
     |                      TTL                      |    |
     |                   (32 bits)                   |     > DSO-DATA
     +--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+    |
     |                     RDLEN                     |    |
     +--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+    |
     \                     RDATA                     \    |
     +--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+    |
     :     NAME, TYPE, CLASS, TTL, RDLEN, RDATA      :    |
     :             Repeated As Necessary             :   /
     +--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+  /

                          Figure 2: PUSH Message

   When processing the records received in a PUSH Message, the receiving
   client MUST validate that the records being added or deleted
   correspond with at least one currently active subscription on that
   session.  Specifically, the record name MUST match the name given in
   the SUBSCRIBE request, subject to the usual established DNS case-
   insensitivity for US-ASCII letters.  If the TYPE in the SUBSCRIBE
   request was not ANY (255) then the TYPE of the record must match the
   TYPE given in the SUBSCRIBE request.  If the CLASS in the SUBSCRIBE
   request was not ANY (255) then the CLASS of the record must match the
   CLASS given in the SUBSCRIBE request.  If a matching active



Pusateri & Cheshire    Expires September 19, 2018              [Page 20]


Internet-Draft           DNS Push Notifications               March 2018


   subscription on that session 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 DSO
   session is not closed.  This is to allow for the unavoidable race
   condition where a client sends an outbound UNSUBSCRIBE while inbound
   PUSH messages for that subscription from the server are still in
   flight.

   In the case where a single change affects more than one active
   subscription, only one PUSH message is sent.  For example, a PUSH
   message adding a given record may match both a SUBSCRIBE request with
   the same TYPE and a different SUBSCRIBE request with TYPE=ANY.  It is
   not the case that two 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" PUSH message, not
   three separate "delete an individual RR from a name" 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" PUSH message,
   not two separate "delete an RRset from a name" PUSH messages.

   A server SHOULD combine multiple change notifications in a single
   PUSH message when possible, even if those change notifications apply
   to different subscriptions.  Conceptually, a PUSH message is a
   session-level mechanism, not a subscription-level mechanism.

   The TTL of an added record is stored by the client and decremented as
   time passes, with the caveat that for as long as a relevant
   subscription is active, the TTL does not decrement below 1 second.
   For as long as a relevant subscription remains active, the client
   SHOULD assume that when a record goes away the server will notify it
   of that fact.  Consequently, a client does not have to poll to verify
   that the record is still there.  Once a subscription is cancelled
   (individually, or as a result of the DSO session being closed) record
   aging resumes and records are removed from the local cache when their
   TTL reaches zero.











Pusateri & Cheshire    Expires September 19, 2018              [Page 21]


Internet-Draft           DNS Push Notifications               March 2018


6.4.  DNS Push Notification UNSUBSCRIBE

   To cancel an individual subscription without closing the entire DSO
   session, the client sends an UNSUBSCRIBE message over the established
   DSO session to the server.  The UNSUBSCRIBE message is encoded in a
   DSO [DSO] message.  This specification defines a DSO TLV for DNS Push
   Notification UNSUBSCRIBE Requests/Responses (tentatively DSO Type
   Code 0x42).

   A server MUST NOT initiate an UNSUBSCRIBE request.  If a server does
   send an UNSUBSCRIBE request over a DSO session initiated by a client,
   this is a fatal error and the client should immediately abort the
   connection with a TCP RST (or equivalent for other protocols).

6.4.1.  UNSUBSCRIBE Request

   An UNSUBSCRIBE request begins with the standard DSO 12-byte header
   [DSO], followed by the UNSUBSCRIBE TLV.  An UNSUBSCRIBE request
   message is illustrated in Figure 3.

   The MESSAGE ID field MUST be zero.  There is no server response to a
   UNSUBSCRIBE message.

   The other header fields MUST be set as described in the DSO
   specification [DSO].  The DNS Opcode is the DSO Opcode (tentatively
   6).  The four count fields MUST be zero, and the corresponding four
   sections MUST be empty (i.e., absent).

   In the UNSUBSCRIBE TLV the DSO-TYPE is UNSUBSCRIBE (tentatively
   0x42).  The DSO-LENGTH is 2 octets.

   The DSO-DATA contains the MESSAGE ID field of the value given in the
   ID field of an active SUBSCRIBE request.  This is how the server
   knows which SUBSCRIBE request is being cancelled.  After receipt of
   the UNSUBSCRIBE request, the SUBSCRIBE request is no longer active.

   It is allowable for the client to issue an UNSUBSCRIBE request for a
   previous SUBSCRIBE request for which the client has not yet received
   a SUBSCRIBE response.  This is to allow for the case where a client
   starts and stops a subscription in less than the round-trip time to
   the server.  The client is NOT required to wait for the SUBSCRIBE
   response before issuing the UNSUBSCRIBE request.









Pusateri & Cheshire    Expires September 19, 2018              [Page 22]


Internet-Draft           DNS Push Notifications               March 2018


                                      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   |    |
      +--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+    |
      |             QDCOUNT (MUST BE ZERO)            |    |
      +--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+     > HEADER
      |             ANCOUNT (MUST BE ZERO)            |    |
      +--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+    |
      |             NSCOUNT (MUST BE ZERO)            |    |
      +--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+    |
      |             ARCOUNT (MUST BE ZERO)            |   /
      +--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+  /
      |   DSO-TYPE = UNSUBSCRIBE (tentatively 0x42)   |
      +--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+
      |              DSO-LENGTH (2 octets)            |
      +--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+  \
      |              SUBSCRIBE MESSAGE ID             |   > DSO-DATA
      +--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+  /

                       Figure 3: UNSUBSCRIBE Request




























Pusateri & Cheshire    Expires September 19, 2018              [Page 23]


Internet-Draft           DNS Push Notifications               March 2018


6.5.  DNS Push Notification RECONFIRM

   Sometimes, particularly when used with a Discovery Proxy [DisProx], a
   DNS Zone may contain stale data.  When a client encounters data that
   it believe may be stale (e.g., an SRV record referencing a target
   host+port that is not responding to connection requests) the client
   can send a RECONFIRM request to ask the server to re-verify that the
   data is still valid.  For a Discovery Proxy, this causes it to issue
   new Multicast DNS requests to ascertain whether the target device is
   still present.  For other types of DNS server, the RECONFIRM
   operation is currently undefined, and SHOULD result in a NOERROR
   response, but otherwise need not cause any action to occur.  Frequent
   RECONFIRM operations may be a sign of network unreliability, or some
   kind of misconfiguration, so RECONFIRM operations MAY be logged or
   otherwise communicated to a human administrator to assist in
   detecting, and remedying, such network problems.

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

6.5.1.  RECONFIRM Request

   A RECONFIRM request begins with the standard DSO 12-byte header
   [DSO], followed by the RECONFIRM TLV.  A RECONFIRM request message is
   illustrated in Figure 4.

   The MESSAGE ID field MUST be set to a unique value, that the client
   is not using for any other active operation on this DSO session.  For
   the purposes here, a MESSAGE ID is in use on this session if the
   client has used it in a request for which it has not yet received a
   response, or if the client has used it for a subscription which it
   has not yet cancelled using UNSUBSCRIBE.  In the RECONFIRM response
   the server MUST echo back the MESSAGE ID value unchanged.

   The other header fields MUST be set as described in the DSO
   specification [DSO].  The DNS Opcode is the DSO Opcode (tentatively
   6).  The four count fields MUST be zero, and the corresponding four
   sections MUST be empty (i.e., absent).

   The DSO-TYPE is RECONFIRM (tentatively 0x43).  The DSO-LENGTH is the
   length of the data that follows, which specifies the name, type,
   class, and content of the record being disputed.




Pusateri & Cheshire    Expires September 19, 2018              [Page 24]


Internet-Draft           DNS Push Notifications               March 2018


                                     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   |    |
     +--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+    |
     |             QDCOUNT (MUST BE ZERO)            |    |
     +--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+     > HEADER
     |             ANCOUNT (MUST BE ZERO)            |    |
     +--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+    |
     |             NSCOUNT (MUST BE ZERO)            |    |
     +--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+    |
     |             ARCOUNT (MUST BE ZERO)            |   /
     +--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+  /
     |    DSO-TYPE = RECONFIRM (tentatively 0x43)    |
     +--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+
     |   DSO-LENGTH (number of octets in DSO-DATA)   |
     +--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+  \
     \                     NAME                      \   \
     +--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+    |
     |                     TYPE                      |    |
     +--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+     > DSO-DATA
     |                     CLASS                     |    |
     +--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+    |
     \                     RDATA                     \   /
     +--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+  /

                        Figure 4: RECONFIRM Request

   The DSO-DATA for a RECONFIRM request MUST contain exactly one record.
   The DSO-DATA for a RECONFIRM request has no count field to specify
   more than one record.  Since RECONFIRM requests are sent over TCP,
   multiple RECONFIRM request messages can be concatenated in a single
   TCP stream and packed efficiently into TCP segments.

   TYPE MUST NOT be the value ANY (255) and CLASS MUST NOT be the value
   ANY (255).

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

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






Pusateri & Cheshire    Expires September 19, 2018              [Page 25]


Internet-Draft           DNS Push Notifications               March 2018


6.5.2.  RECONFIRM Response

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

   A RECONFIRM response message begins with the standard DSO 12-byte
   header [DSO], possibly followed by one or more optional TLVs, such as
   a Retry Delay TLV.  For suggested values for the Retry Delay TLV, see
   Section 6.2.2.

   The MESSAGE ID field MUST echo the value given in the ID field of the
   RECONFIRM request.  This is how the client knows which request is
   being responded to.

   A RECONFIRM response message MUST NOT include a DSO RECONFIRM TLV.
   If a client receives a RECONFIRM response message containing a
   RECONFIRM TLV then the response message is processed but the
   RECONFIRM TLV MUST be silently ignored.

   In the RECONFIRM response the RCODE confirms receipt of the
   reconfirmation request.  Supported RCODEs are as follows:

   +-----------+-------+-----------------------------------------------+
   | Mnemonic  | Value | Description                                   |
   +-----------+-------+-----------------------------------------------+
   | NOERROR   |   0   | RECONFIRM accepted.                           |
   | FORMERR   |   1   | Server failed to process request due to a     |
   |           |       | malformed request.                            |
   | SERVFAIL  |   2   | Server failed to process request due to a     |
   |           |       | problem with the server.                      |
   | NXDOMAIN  |   3   | NOT APPLICABLE. DNS Push Notification servers |
   |           |       | MUST NOT return NXDOMAIN errors in response   |
   |           |       | to RECONFIRM requests.                        |
   | NOTIMP    |   4   | Server does not implement DSO.                |
   | REFUSED   |   5   | Server refuses to process request for policy  |
   |           |       | or security reasons.                          |
   | NOTAUTH   |   9   | Server is not authoritative for the requested |
   |           |       | name.                                         |
   | DSOTYPENI |   11  | RECONFIRM operation not supported.            |
   +-----------+-------+-----------------------------------------------+

                         RECONFIRM Response codes

   This document specifies only these RCODE values for RECONFIRM
   Responses.  Servers sending RECONFIRM Responses SHOULD use one of
   these values.  However, future circumstances may create situations
   where other RCODE values are appropriate in RECONFIRM Responses, so




Pusateri & Cheshire    Expires September 19, 2018              [Page 26]


Internet-Draft           DNS Push Notifications               March 2018


   clients MUST be prepared to accept RECONFIRM Responses with any RCODE
   value.

   Nonzero RCODE values signal some kind of error.

   RCODE value FORMERR indicates a message format error, for example
   TYPE or CLASS being ANY (255).

   RCODE value SERVFAIL indicates that the server has exhausted its
   resources or other serious problem occurred.

   RCODE values NOTIMP indicates that the server does not support DSO,
   and DSO is required for RECONFIRM requests.

   RCODE value REFUSED indicates that the server supports RECONFIRM
   requests but is currently not configured to accept them from this
   client.

   RCODE value NOTAUTH indicates that the server is not authoritative
   for the requested name, and can do nothing to remedy the apparent
   error.  Note that there may be future cases in which a server is able
   to pass on the RECONFIRM request to the ultimate source of the
   information, and in these cases the server should return NOERROR.

   RCODE value DSOTYPENI indicates that the server does not support
   RECONFIRM requests.

   Nonzero RCODE values SERVFAIL, REFUSED and DSOTYPENI are benign from
   the client's point of view.  The client may log them to aid in
   debugging, but otherwise they require no special action.

   Nonzero RCODE values other than these three indicate a serious
   problem with the client.  After sending an error response other than
   one of these three, the server SHOULD send a DSO Retry Delay TLV to
   end the DSO session, as described in the DSO specification [DSO].
















Pusateri & Cheshire    Expires September 19, 2018              [Page 27]


Internet-Draft           DNS Push Notifications               March 2018


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 DSO session.  When a
   client terminates an individual subscription (via UNSUBSCRIBE) or all
   subscriptions on that DSO session (by ending the session) it is
   signaling to the server that it is longer interested in receiving
   those particular updates.  It is informing the server that the server
   may release any state information it has been keeping with regards to
   these particular subscriptions.

   After terminating its last subscription on a session via UNSUBSCRIBE,
   a client MAY close the session immediately, or it may keep it open if
   it anticipates performing further operations on that session in the
   future.  If a client wishes to keep an idle session open, it MUST
   respect the maximum idle time required by the server [DSO].

   If a client plans to terminate one or more subscriptions on a session
   and doesn't intend to keep that session open, then as an efficiency
   optimization it MAY instead choose to simply close the session, which
   implicitly terminates all subscriptions on that session.  This may
   occur because the client computer is being shut down, is going to
   sleep, the application requiring the subscriptions has terminated, or
   simply because the last active subscription on that session has been
   cancelled.

   When closing a session, a client will generally do an abortive
   disconnect, sending a TCP RST.  This immediately discards all
   remaining inbound and outbound data, which is appropriate if the
   client no longer has any interest in this data.  In the BSD Sockets
   API, sending a TCP RST is achieved by setting the SO_LINGER option
   with a time of 0 seconds and then closing the socket.

   If a client has performed operations on this session that it would
   not want lost (like DNS updates) then the client SHOULD do an orderly
   disconnect, sending a TLS close_notify followed by a TCP FIN.  (In
   the BSD Sockets API, sending a TCP FIN is achieved by calling
   "shutdown(s,SHUT_WR)" and keeping the socket open until all remaining
   data has been read from it.)











Pusateri & Cheshire    Expires September 19, 2018              [Page 28]


Internet-Draft           DNS Push Notifications               March 2018


7.  Security Considerations

   The Strict Privacy Usage Profile for DNS over TLS is strongly
   recommended for DNS Push Notifications as defined in "Authentication
   and (D)TLS Profile for DNS-over-(D)TLS"
   [I-D.ietf-dprive-dtls-and-tls-profiles].  The Opportunistic Privacy
   Usage Profile is permissible as a way to support incremental
   deployment of security capabilities.  Cleartext connections for DNS
   Push Notifications are not permissible.

   DNSSEC is RECOMMENDED for the authentication of DNS Push Notification
   servers.  TLS alone does not provide complete security.  TLS
   certificate verification can provide reasonable assurance that the
   client is really talking to the server associated with the desired
   host name, but since the desired host name is learned via a DNS SRV
   query, if the SRV query 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



Pusateri & Cheshire    Expires September 19, 2018              [Page 29]


Internet-Draft           DNS Push Notifications               March 2018


   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.

   See Authentication and (D)TLS Profile for DNS-over-(D)TLS
   [I-D.ietf-dprive-dtls-and-tls-profiles] for more information on
   authenticating domain names.  Also note that a DNS Push server is an
   authoritative server and a DNS Push client is a standard DNS client.
   While the terminology in Authentication and (D)TLS Profile for DNS-
   over-(D)TLS [I-D.ietf-dprive-dtls-and-tls-profiles] explicitly states
   it does not apply to authoritative servers, it does in this case
   apply to DNS Push Notification clients and servers.

7.3.  TLS Compression

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

7.4.  TLS Session Resumption

   TLS Session Resumption is permissible on DNS Push Notification
   servers.  The server may keep TLS state with Session IDs [RFC5246] or
   operate in stateless mode by sending a Session Ticket [RFC5077] to
   the client for it to store.  However, once the DSO session 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 DSO
   session.  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 Registry Service Types
   [RFC6335][ST].

   This document defines four DNS Stateful Operations TLV types:
   SUBSCRIBE with (tentative) value 0x40 (64), PUSH with (tentative)




Pusateri & Cheshire    Expires September 19, 2018              [Page 30]


Internet-Draft           DNS Push Notifications               March 2018


   value 0x41 (65), UNSUBSCRIBE with (tentative) value 0x42 (66), and
   RECONFIRM with (tentative) value 0x43 (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, Soraia Zlatkovic, Sara
   Dickinson, and Andrew Sullivan.

10.  References

10.1.  Normative References

   [DSO]      Bellis, R., Cheshire, S., Dickinson, J., Dickinson, S.,
              Mankin, A., and T. Pusateri, "DNS Stateful Operations",
              draft-ietf-dnsop-session-signal-05 (work in progress),
              January 2018.

   [I-D.ietf-tls-tls13]
              Rescorla, E., "The Transport Layer Security (TLS) Protocol
              Version 1.3", draft-ietf-tls-tls13-26 (work in progress),
              March 2018.

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

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

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

   [RFC1035]  Mockapetris, P., "Domain names - implementation and
              specification", STD 13, RFC 1035, DOI 10.17487/RFC1035,
              November 1987, <https://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, October 1989,
              <https://www.rfc-editor.org/info/rfc1123>.




Pusateri & Cheshire    Expires September 19, 2018              [Page 31]


Internet-Draft           DNS Push Notifications               March 2018


   [RFC2119]  Bradner, S., "Key words for use in RFCs to Indicate
              Requirement Levels", BCP 14, RFC 2119,
              DOI 10.17487/RFC2119, March 1997,
              <https://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,
              <https://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,
              <https://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, August 2008,
              <https://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,
              <https://www.rfc-editor.org/info/rfc6066>.

   [RFC6335]  Cotton, M., Eggert, L., Touch, J., Westerlund, M., and S.
              Cheshire, "Internet Assigned Numbers Authority (IANA)
              Procedures for the Management of the Service Name and
              Transport Protocol Port Number Registry", BCP 165,
              RFC 6335, DOI 10.17487/RFC6335, August 2011,
              <https://www.rfc-editor.org/info/rfc6335>.

   [RFC6895]  Eastlake 3rd, D., "Domain Name System (DNS) IANA
              Considerations", BCP 42, RFC 6895, DOI 10.17487/RFC6895,
              April 2013, <https://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, <https://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,
              <https://www.rfc-editor.org/info/rfc7766>.






Pusateri & Cheshire    Expires September 19, 2018              [Page 32]


Internet-Draft           DNS Push Notifications               March 2018


   [RFC8174]  Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC
              2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174,
              May 2017, <https://www.rfc-editor.org/info/rfc8174>.

   [ST]       "Service Name and Transport Protocol Port Number
              Registry", <http://www.iana.org/assignments/
              service-names-port-numbers/>.

10.2.  Informative References

   [DisProx]  Cheshire, S., "Discovery Proxy for Multicast DNS-Based
              Service Discovery", draft-ietf-dnssd-hybrid-08 (work in
              progress), March 2018.

   [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-dprive-dtls-and-tls-profiles]
              Dickinson, S., Gillmor, D., and T. Reddy, "Usage and
              (D)TLS Profiles for DNS-over-(D)TLS", draft-ietf-dprive-
              dtls-and-tls-profiles-11 (work in progress), September
              2017.

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

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

   [RFC2308]  Andrews, M., "Negative Caching of DNS Queries (DNS
              NCACHE)", RFC 2308, DOI 10.17487/RFC2308, March 1998,
              <https://www.rfc-editor.org/info/rfc2308>.

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

   [RFC4953]  Touch, J., "Defending TCP Against Spoofing Attacks",
              RFC 4953, DOI 10.17487/RFC4953, July 2007,
              <https://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, <https://www.rfc-editor.org/info/rfc5077>.



Pusateri & Cheshire    Expires September 19, 2018              [Page 33]


Internet-Draft           DNS Push Notifications               March 2018


   [RFC6281]  Cheshire, S., Zhu, Z., Wakikawa, R., and L. Zhang,
              "Understanding Apple's Back to My Mac (BTMM) Service",
              RFC 6281, DOI 10.17487/RFC6281, June 2011,
              <https://www.rfc-editor.org/info/rfc6281>.

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

   [RFC6763]  Cheshire, S. and M. Krochmal, "DNS-Based Service
              Discovery", RFC 6763, DOI 10.17487/RFC6763, February 2013,
              <https://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,
              <https://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,
              <https://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, <https://www.rfc-editor.org/info/rfc7525>.

   [RFC7719]  Hoffman, P., Sullivan, A., and K. Fujiwara, "DNS
              Terminology", RFC 7719, DOI 10.17487/RFC7719, December
              2015, <https://www.rfc-editor.org/info/rfc7719>.

   [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, <https://www.rfc-editor.org/info/rfc7858>.

   [RFC8010]  Sweet, M. and I. McDonald, "Internet Printing
              Protocol/1.1: Encoding and Transport", RFC 8010,
              DOI 10.17487/RFC8010, January 2017,
              <https://www.rfc-editor.org/info/rfc8010>.

   [RFC8011]  Sweet, M. and I. McDonald, "Internet Printing
              Protocol/1.1: Model and Semantics", RFC 8011,
              DOI 10.17487/RFC8011, January 2017,
              <https://www.rfc-editor.org/info/rfc8011>.





Pusateri & Cheshire    Expires September 19, 2018              [Page 34]


Internet-Draft           DNS Push Notifications               March 2018


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

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

Authors' Addresses

   Tom Pusateri
   Unaffiliated
   Raleigh, NC  27608
   USA

   Phone: +1 919 867 1330
   Email: pusateri@bangj.com


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

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

























Pusateri & Cheshire    Expires September 19, 2018              [Page 35]


Html markup produced by rfcmarkup 1.126, available from https://tools.ietf.org/tools/rfcmarkup/