DNS Long-Lived Queries

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Last updated 2019-03-01
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Network Working Group                                        S. Cheshire
Internet-Draft                                               M. Krochmal
Intended status: Informational                                Apple Inc.
Expires: September 2, 2019                                 March 1, 2019

                         DNS Long-Lived Queries


   DNS Long-Lived Queries (LLQ) is a protocol for extending the DNS
   protocol to support change notification, thus allowing clients to
   learn about changes to DNS data without polling the server.  From
   2007 onwards, LLQ was implemented in Apple products including Mac OS
   X, Bonjour for Windows, and AirPort wireless base stations.  In 2019,
   the LLQ protocol was superseded by the IETF Standards Track RFC "DNS
   Push Notifications", which builds on experience gained with the LLQ
   protocol to create a superior replacement.

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1.  Introduction

   In dynamic environments, DNS Service Discovery [RFC6763] benefits
   significantly from clients being able to learn about changes to DNS
   information via a mechanism that is both more timely and more
   efficient than simple polling.  Such a mechanism enables "live
   browses" that learn when a new instance of a service appears, or when
   an existing service disappears from the network, and allows clients
   to monitor changes to a service.  Multicast DNS [RFC6762] supports
   this natively.  When a host on the network publishes or deletes DNS
   records, these records are multicast to other hosts on the network.
   These hosts deliver the records to interested clients (applications
   running on the host).  Hosts also send occasional queries to the
   network in case gratuitous announcements are not received due to
   packet loss, and to detect records lost due to their publishers
   crashing or having become disconnected from the network.

   There is currently no equivalent in traditional unicast DNS.  Queries
   are "one-shot" -- a name server will answer a query once, returning
   the results available at that instant in time.  Changes could be
   inferred via polling of the name server.  This solution is not
   scalable, however, as a low polling rate could leave the client with
   stale information, and a high polling rate would have an adverse
   impact on the network and server.

   Therefore, an extension to DNS is required that enables a client to
   issue long-lived queries.  This extension would allow a DNS server to
   notify clients about changes to DNS data.

1.1.  Transition to DNS Push Notifications

   The LLQ protocol enjoyed over a decade of useful operation, enabling
   timely live updates for the service discovery user interface in
   Apple's Back to My Mac [RFC6281] service.

   Operational experience with LLQ informed the design of its IETF
   Standards Track successor, DNS Push Notifications [Push].

   Because of the significant enhancements in DNS Push Notifications,
   all existing LLQ implementations are encouraged to migrate to using
   DNS Push Notifications instead.

   For existing LLQ servers, they are encouraged to implement and
   support DNS Push Notifications, so that clients can begin migrating
   to the newer protocol.

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   For existing LLQ clients, they are encouraged to query for the
   "_dns-push-tls._tcp.<zone>" SRV record first, and only if DNS Push
   fails, then fall back to query for "_dns-llq._udp.<zone>" instead.

   This will cause clients to prefer the newer protocol when possible.
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