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Internet Engineering Task Force                              T. Pusateri
Internet-Draft                                       Seeking affiliation
Intended status: Standards Track                             S. Cheshire
Expires: September 10, 2015                                   Apple Inc.
                                                           March 9, 2015


                         DNS Push Notifications
                        draft-ietf-dnssd-push-00

Abstract

   The Domain Name System (DNS) was designed to efficiently return
   matching records for queries for data that is relatively static.
   When those records change frequently, DNS is still efficient at
   returning the updated results when polled.  But there exists no
   mechanism for a client to be asynchronously notified when these
   changes occur.  This document defines a mechanism for a client to be
   notified of such changes to DNS records, called DNS Push
   Notifications.

Status of This Memo

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

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

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

   This Internet-Draft will expire on September 10, 2015.

Copyright Notice

   Copyright (c) 2015 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



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   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 . . . . . . . . . . . . . . . . . .   2
   2.  Motivation  . . . . . . . . . . . . . . . . . . . . . . . . .   3
   3.  Overview  . . . . . . . . . . . . . . . . . . . . . . . . . .   4
   4.  Transport . . . . . . . . . . . . . . . . . . . . . . . . . .   4
   5.  State Considerations  . . . . . . . . . . . . . . . . . . . .   5
   6.  Protocol Operation  . . . . . . . . . . . . . . . . . . . . .   6
     6.1.  Discovery . . . . . . . . . . . . . . . . . . . . . . . .   6
     6.2.  DNS Push Notification SUBSCRIBE . . . . . . . . . . . . .   8
     6.3.  DNS Push Notification UNSUBSCRIBE . . . . . . . . . . . .  10
     6.4.  DNS Push Notification Update Messages . . . . . . . . . .  11
   7.  Acknowledgements  . . . . . . . . . . . . . . . . . . . . . .  13
   8.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .  13
   9.  Security Considerations . . . . . . . . . . . . . . . . . . .  14
     9.1.  Security Services . . . . . . . . . . . . . . . . . . . .  14
     9.2.  TLS Name Authentication . . . . . . . . . . . . . . . . .  14
     9.3.  TLS Compression . . . . . . . . . . . . . . . . . . . . .  14
     9.4.  TLS Session Resumption  . . . . . . . . . . . . . . . . .  15
   10. References  . . . . . . . . . . . . . . . . . . . . . . . . .  15
     10.1.  Normative References . . . . . . . . . . . . . . . . . .  15
     10.2.  Informative References . . . . . . . . . . . . . . . . .  16
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  17

1.  Introduction

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

1.1.  Requirements Language

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   "SHOULD", "SHOULD 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].



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2.  Motivation

   As the domain name system continues to adapt to new uses and changes
   in deployment, polling has the potential to burden DNS servers at
   many levels throughout the network.  Other network protocols have
   successfully deployed a publish/subscribe model to state changes
   following the Observer design pattern.  XMPP Publish-Subscribe
   [XEP-0060] and Atom [RFC4287] are examples.  While DNS servers are
   generally highly tuned and capable of a high rate of query/response
   traffic, adding a publish/subscribe model for tracking changes to DNS
   records can result in more timely notification of changes with
   reduced CPU usage and lower network traffic.

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

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

















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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.  Adopting this
   existing syntax and semantics for DNS Push Notifications allows for
   messages going in the other direction, from server to client, to
   communicate changes to a zone.  The client first must subscribe 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].

   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 on the server, and that
   concept has no application when it comes to an authoritative server
   telling a client of changes to DNS records.

4.  Transport

   Implementations of DNS Update [RFC2136] MAY use either User Datagram
   Protocol (UDP) [RFC0768] or Transmission Control Protocol (TCP)
   [RFC0793] as the transport protocol, in keeping with the historical
   precedent that DNS queries must first be sent over UDP [RFC1123].
   This requirement to use UDP has subsequently been relaxed [RFC5966].
   DNS Push Notification is defined only for TCP.  DNS Push Notification
   clients MUST use TCP.

   Either end of the TCP connection can terminate all of the
   subscriptions on that connection by simply closing the connection
   abruptly with a TCP RST.  (An individual subscription is terminated
   by sending an UNSUBSCRIBE message for that specific subscription.)

   If a client closes the connection, it is signaling that it is no
   longer interested in receiving updates to any of the records it has
   subscribed.  It is informing the server that the server may release
   all state information it has been keeping with regards to this
   client.  This may occur because the client computer has been
   disconnected from the network, has gone to sleep, or the application
   requiring the records has terminated.

   If a server closes the connection, it is informing the client that it
   can no longer provide updates for the subscribed records.  This may
   occur because the server application software or operating system is
   restarting, the application terminated unexpectedly, the server is



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   undergoing maintenance procedures, or the server is overloaded and
   can no longer provide the information to all the clients that wish to
   receive it.  The client can try to re-subscribe at a later time or
   connect to another server supporting DNS Push Notifications for the
   zone.

   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.

   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.  Additional security
   measures such as authentication during TLS negotiation MAY also be
   employed to increase the trust relationship between client and
   server.  Because TCP SYN flooding attacks are possible with any
   protocol over TCP, implementers are encouraged to use industry best
   practices to guard against such attacks [IPJ.9-4-TCPSYN].

5.  State Considerations

   Each DNS Push Notification server is capable and handling some finite
   number of Push Notification subscriptions.  This number will vary
   from server to server and is based on physical machine
   characteristics, network bandwidth, and operating system resource
   allocation.  After a client establishes a connection to a DNS server,
   each record subscription is individually accepted or rejected.
   Servers may employ various techniques to limit subscriptions to a
   manageable level.  Correspondingly, the client is free to establish
   simultaneous connections to alternate DNS servers that support DNS
   Push Notifications for the zone and distribute record subscriptions
   at its discretion.  In this way, both clients and servers can react
   to resource constraints.  Token bucket rate limiting schemes are also
   effective in providing fairness by a server across numerous client
   requests.













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6.  Protocol Operation

   A DNS Push Notification exchange begins with the client discovering
   the appropriate server, and connecting to it.  The client may then
   add and remove Push Notification subscriptions over this connection.
   In accordance with the current set of active subscriptions the server
   sends relevant asynchronous Push Notifications to the client.  The
   exchange terminates when either end closes the TCP connection with a
   TCP RST.

6.1.  Discovery

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

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

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

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

   4.  Once the SOA is known, the client sends a DNS query with type SRV
       [RFC2782] for the record name "_dns-push._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



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       included in the Additional Section, however, the address records
       SHOULD be authenticated before use as described below in
       Section 9.2 [I-D.ietf-dane-srv].

   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" is 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.

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
































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6.2.  DNS Push Notification SUBSCRIBE

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

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

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

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

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

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

   ARCOUNT MUST be zero, and the Additional Section MUST be empty.
   Any records in the Additional Section MUST be silently ignored.
















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

   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                            |
   | NOTIMP   |   4   | Server does not implement DNS Push             |
   |          |       | Notifications                                  |
   | REFUSED  |   5   | Server refuses to process request for policy   |
   |          |       | or security reasons                            |
   +----------+-------+------------------------------------------------+

                          Table 1: Response codes

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

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

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

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

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

   ARCOUNT MUST be zero, and the Additional Section MUST be empty.



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   Any records in the Additional Section MUST be silently ignored.

   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.

   A client MUST not send a SUBSCRIBE message that duplicates the name,
   type and class of an existing active subscription.  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.

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

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

   A client may SUBSCRIBE to records that are unknown to the server at
   the time of the request and this is not an error.  The server MUST
   accept these requests and send Push Notifications if and when matches
   are found in the future.

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.
   If a server receives such an UNSUBSCRIBE message this is an error and
   the server MUST immediately close the TCP connection.

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

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





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6.4.  DNS Push Notification Update Messages

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

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

   The following figure shows the existing DNS Update header format:

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

                                 Figure 1

   For DNS Push Notifications the following rules apply:

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

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

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

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



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   ADCOUNT MUST be zero, and the Additional Data Section MUST be empty.
   Any records in the Additional Data Section MUST be silently ignored.

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

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

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

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

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

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

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

   The server SHOULD encode change notifications in the most efficient
   manner possible.  For example, when three AAAA records are deleted
   from a given name, and no other AAAA records exist for that name, the
   server SHOULD send a "delete an RRset from a name" update, not three



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   separate "delete an individual RR from a name" updates.  Similarly,
   when both an SRV and a TXT record are deleted from a given name, and
   no other records of any kind exist for that name, the server SHOULD
   send a "delete all RRsets from a name" update, not two separate
   "delete an RRset from a name" updates.

   Reception of a Push Notification Update Message results in no
   response back to the server.

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

7.  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.

8.  IANA Considerations

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

   This document defines two DNS OpCodes: SUBSCRIBE with (tentative)
   value 6 and UNSUBSCRIBE with (tentative) value 7.

















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9.  Security Considerations

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

9.1.  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 [I-D.ietf-uta-tls-bcp] 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.

9.2.  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._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 [I-D.ietf-dane-srv].  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.

9.3.  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.






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9.4.  TLS Session Resumption

   TLS Session Resumption MUST be disabled on DNS Push Notification
   servers.  It is not useful to have subscription state cached for long
   periods of time.  It is also not desirable for subscription state to
   be maintained while the client is not connected.

10.  References

10.1.  Normative References

   [I-D.ietf-dane-srv]
              Finch, T., Miller, M., and P. Saint-Andre, "Using DNS-
              Based Authentication of Named Entities (DANE) TLSA Records
              with SRV Records", draft-ietf-dane-srv-11 (work in
              progress), February 2015.

   [I-D.ietf-tls-tls13]
              Dierks, T. and E. Rescorla, "The Transport Layer Security
              (TLS) Protocol Version 1.3", draft-ietf-tls-tls13-04 (work
              in progress), January 2015.

   [I-D.ietf-uta-tls-bcp]
              Sheffer, Y., Holz, R., and P. Saint-Andre,
              "Recommendations for Secure Use of TLS and DTLS", draft-
              ietf-uta-tls-bcp-11 (work in progress), February 2015.

   [RFC0768]  Postel, J., "User Datagram Protocol", STD 6, RFC 768,
              August 1980.

   [RFC0793]  Postel, J., "Transmission Control Protocol", STD 7, RFC
              793, September 1981.

   [RFC1034]  Mockapetris, P., "Domain names - concepts and facilities",
              STD 13, RFC 1034, November 1987.

   [RFC1035]  Mockapetris, P., "Domain names - implementation and
              specification", STD 13, RFC 1035, November 1987.

   [RFC1123]  Braden, R., "Requirements for Internet Hosts - Application
              and Support", STD 3, RFC 1123, October 1989.

   [RFC2119]  Bradner, S., "Key words for use in RFCs to Indicate
              Requirement Levels", BCP 14, RFC 2119, March 1997.

   [RFC2136]  Vixie, P., Thomson, S., Rekhter, Y., and J. Bound,
              "Dynamic Updates in the Domain Name System (DNS UPDATE)",
              RFC 2136, April 1997.



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   [RFC2782]  Gulbrandsen, A., Vixie, P., and L. Esibov, "A DNS RR for
              specifying the location of services (DNS SRV)", RFC 2782,
              February 2000.

   [RFC5246]  Dierks, T. and E. Rescorla, "The Transport Layer Security
              (TLS) Protocol Version 1.2", RFC 5246, August 2008.

   [RFC5966]  Bellis, R., "DNS Transport over TCP - Implementation
              Requirements", RFC 5966, August 2010.

   [RFC6066]  Eastlake, D., "Transport Layer Security (TLS) Extensions:
              Extension Definitions", RFC 6066, January 2011.

   [RFC6195]  Eastlake, D., "Domain Name System (DNS) IANA
              Considerations", RFC 6195, March 2011.

10.2.  Informative References

   [I-D.ietf-dnssd-hybrid]
              Cheshire, S., "Hybrid Unicast/Multicast DNS-Based Service
              Discovery", draft-ietf-dnssd-hybrid-00 (work in progress),
              November 2014.

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

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

   [RFC1996]  Vixie, P., "A Mechanism for Prompt Notification of Zone
              Changes (DNS NOTIFY)", RFC 1996, August 1996.

   [RFC4287]  Nottingham, M., Ed. and R. Sayre, Ed., "The Atom
              Syndication Format", RFC 4287, December 2005.

   [RFC6762]  Cheshire, S. and M. Krochmal, "Multicast DNS", RFC 6762,
              February 2013.

   [RFC6763]  Cheshire, S. and M. Krochmal, "DNS-Based Service
              Discovery", RFC 6763, February 2013.

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




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Authors' Addresses

   Tom Pusateri
   Seeking affiliation
   Hilton Head Island, SC
   USA

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


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

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
































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