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A 'Time Since Registration' Resource Record for Multicast DNS

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This is an older version of an Internet-Draft whose latest revision state is "Expired".
Authors Ted Lemon , Liang Qin
Last updated 2021-10-22
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Internet Engineering Task Force                                 T. Lemon
Internet-Draft                                            秦 良 (L. Qin)
Intended status: Standards Track                              Apple Inc.
Expires: 25 April 2022                                   22 October 2021

     A 'Time Since Registration' Resource Record for Multicast DNS


   This document defines a new DNS Resource Record (RR) to be used with
   multicast DNS.  The new RR is used to communicate the time at which
   the set of RRsets on a domain name were first registered.

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

   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 25 April 2022.

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   Copyright (c) 2021 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
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   license-info) in effect on the date of publication of this document.
   Please review these documents carefully, as they describe your rights
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   as described in Section 4.e of the Trust Legal Provisions and are
   provided without warranty as described in the Simplified BSD License.

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Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   2
   2.  Time Since Registered Resource Record . . . . . . . . . . . .   3
   3.  mDNS Registrar Behavior . . . . . . . . . . . . . . . . . . .   4
   4.  Internal Handling of TSR records  . . . . . . . . . . . . . .   5
   5.  Timeliness of Conflict Resolution . . . . . . . . . . . . . .   5
   6.  Legacy Registrars . . . . . . . . . . . . . . . . . . . . . .   5
   7.  When to Use TSR . . . . . . . . . . . . . . . . . . . . . . .   5
   8.  Registrant API considerations . . . . . . . . . . . . . . . .   5
   9.  Security Considerations . . . . . . . . . . . . . . . . . . .   6
   10. IANA Considerations . . . . . . . . . . . . . . . . . . . . .   6
   11. Informative References  . . . . . . . . . . . . . . . . . . .   6
   12. Normative References  . . . . . . . . . . . . . . . . . . . .   6
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .   7

1.  Introduction

   Unlike the Internet Domain Name service, with its authority servers
   and delegation of authority, Multicast DNS has no single source of
   authority.  Because of this, mDNS has a mechanism, conflict
   resolution Section 9 of [RFC6762], for detecting and fixing conflicts
   in mDNS advertisements.

   The current mechanism for conflict resolution is simple: when a new
   service is to be advertised, the server that wishes to advertise its
   service typically registers the service with a central mDNS registrar
   on the host on which it is running.

   This mDNS registrar may have an internal database of services already
   registered, and may detect a conflict with one of those services.  In
   this case, no network transaction is required: the mDNS registrar
   immediately detects the conflict and addresses it in one of two ways,
   depending on what the service requested.  The first alternative is
   that the registrar will report the conflict to the server an error,
   which the server must fix.  Alternatively, the server may have
   indicated that the mDNS registrar should automatically choose a new
   name for it, in which case the mDNS registrar does so automatically.

   Once any local conflicts have been resolved, the mDNS registrar sends
   a series of multicast probes to the local network to see if any other
   host has already registered a service the conflicts with the proposed
   new service.  If such a service is present on the network, the mDNS
   registrar follows the same process previously described, either
   reporting the error to the server or automatically choosing a new

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   The effect of this approach is that generally whichever server first
   registers a service under a particular name wins.  If a server comes
   along later and registers the same service with conflicting
   information, the newcomer's information is rejected.

   This works well for devices acting on their own behalf.  However, in
   the case of advertising proxies, it works poorly: typically an
   advertising proxy is proxying the contents of its proxy database
   using mDNS.  The source of truth for information in that database is
   some host that has registered with the proxy, for example using the
   Service Replication Protocol (SRP).

   In the case of an advertising proxy proxying an SRP database, what we
   want is not the oldest information, but the newest.  When the SRP
   client is able to continue registering with the same SRP server, this
   works well.  However, if SRP is being managed using anycast
   registration, there is no guarantee that an SRP client will register
   with the same server each time.

   When the SRP client registers with a different server, the behavior
   we expect with the current conflict resolution approach is that the
   SRP client will be given a new name, and both the old (stale)
   advertisement (A) and the new (more recent) advertisement (A') will
   be seen on the network, as separate services.

   This creates a new burden on consumers of that service: they need to
   parse through the whole list of services of that type, using metadata
   from the TXT record in the registration if needed to determine that
   service A and service A' are the same service.

   This document proposes an enhancement to the current conflict
   resolution algorithm for mDNS, which allows an mDNS proxy to report
   when it received the registration using a new Time Since Registered
   RR, which is attached to the name of the registration.

2.  Time Since Registered Resource Record

   The Time Since Registered (TSR) RR is attached to the name for which
   the TSR RR is asserting a registration time.  The TSR RR contains the
   time in seconds since the most recent registration that has been
   received.  This time is computed at the time that the mDNS message is
   transmitted, and can be treated by the receiver as relative to the
   current time.

   The resource record is formatted as described in Section 3.2.1 of
   [RFC1035].  The RDATA consists of the time offset in the form of a
   32-bit unsigned number in network byte order.

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3.  mDNS Registrar Behavior

   When probing, an mDNS registrar reports the TSR for the name for
   which it is probing.  When an mDNS Registrar receives a probe, it
   checks to see if it has any registration that conflicts with the
   probe announcement.  If it does, it compares its internal TSR with
   the TSR reported in the probe.  If the TSR in the probe is more
   recent than the internal TSR, the internal registration is marked as
   stale, and the registrar does not respond to the probe.  If the TSR
   in the probe is older than the internal TSR, the registrar reports a
   conflict as usual.

   Note that because TSR computations are affected by network latency,
   comparisons can't be considered accurate.  It is therefore necessary
   to tolerate some degree of error.  As a general rule, a probe
   containing a TSR that arrives at a registrar for which the timestamp
   comparison is close to zero should be assumed to be more recent than
   the registrar's copy: since the registrar already has a registration,
   that registration most likely arrived before the registration that
   triggered the probe.

   The Service Registration Protocol uses DNS update, and it takes a
   significant amount of time for a DNS update client to abandon one DNS
   server for another, so in the absence of significant congestion-
   related jitter in packet arrival times, it should never be the case
   that two SRP proxies receive an SRP update at the same time from the
   same client.  Given that SRP generally does not operate across
   network infrastructure operator boundaries, such delays are unlikely.
   Also, if such a situation does occur, the updates should contain the
   same data, and therefore should not be seen by the mDNS registrar as
   being in conflict.

   When a probe succeeds, the registrar that did the probe then
   announces the new service.  Registrars receiving this announcement
   that have internal registrations that conflict with it, which are
   marked stale, then remove the internal registration and report this
   event to the proxy that did the registration.

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4.  Internal Handling of TSR records

   The TSR record that is sent on the wire is expressed in seconds
   relative to the time of registration.  In order to derive a TSR
   record, the registrar must remember the time at which the
   registration occurred.  This time is recorded as an absolute time,
   not a relative time.  We will refer to it as the TSR timestamp.  When
   sending a TSR RR, the registrar computes the difference between the
   TSR timestamp, which must always be in the past, and the current
   system time.  This difference is converted to seconds, and that value
   is then sent as the TSR RR.

5.  Timeliness of Conflict Resolution

   It is expected that if a conflict exists, it will be recent, and will
   be resolved quickly.  Different systems may be able to record shorter
   or longer time differences, but because of this expectation of
   recentness, mDNS registrars should never report a TSR of longer than
   seven days.  It's reasonable to expect that every mDNS implementation
   should be able to remember time intervals of at least seven days.

6.  Legacy Registrars

   An mDNS registrar that does not support TSR will treat the TSR record
   as part of the registration.  Since the TSR record is only sent in
   probes, it will never be erroneously reported to any client that is
   browsing for services.  If a legacy mDNS registrar and an mDNS
   registrar that supports TSR both advertise the same service, the
   conflict resolution rules described in RFC6762 will be followed.

7.  When to Use TSR

   TSR is only relevant for mDNS proxies.  It SHOULD NOT be used by
   regular (non-proxy) mDNS registrants.  An mDNS registrant that is a
   proxy MUST explicitly request that a TSR be used for conflict
   resolution. mDNS registrars MUST NOT record a TSR timestamp unless
   the registrant has specifically requested it.

8.  Registrant API considerations

   When an mDNS proxy registers a service and requests the use of a TSR
   timestamp, the proxy MUST specify when it received the registration.
   In order to support this, the API is required not only to allow the
   registrant to specify that TSR is wanted, but must also provide a way
   for the proxy to specify an absolute time at which the registration
   was received.

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   This is important, for example, in the case of SRP Replication
   [I-D.lemon-srp-replication], where an SRP server may receive a
   registration from a peer during startup synchronization.  This
   registration will have occurred at some significant amount of time in
   the past, and so it would be incorrect for the mDNS proxy receiving
   the registration to use the time that the mDNS proxy registers the
   service as the TSR timestamp.

9.  Security Considerations

   The TSR RR is an optimization: it ameliorates an edge case for mDNS
   proxies.  A malicious host on the same link could use the TSR RR to
   win conflict resolution processes.  However, because TSR is only used
   by proxies, this technique will not work for normal mDNS service
   registrations: in that case, normal mDNS conflict resolution is done,
   and the attacker gains no benefit from using TSR.  In the case of
   proxied mDNS registrations, an attacker can in fact deny service by
   superseding existing registrations.

   However, such an attacker could achieve the same effect simply by
   responding to probes with conflict announcements.  Furthermore, such
   an attack would cause noticeable problems on the network which the
   network operator would then take steps to correct.

   Protocols that rely on mDNS MUST NOT assume that mDNS service is
   secure or private.  If security (authentication, authorization and/or
   secrecy) are needed, these must be provided at the application layer.
   The use of TSR provides a novel way of attacking some mDNS services,
   but the ground truth is that if security is not provided at the
   application layer, this novel attack actually provides no new
   advantage to the attacker.

10.  IANA Considerations

   IANA is requested to allocate a new RR Type from the DNS Resource
   Record (RR) TYPEs registry for the 'Time Since Registered' Resource
   Record.  The type shall be 'TSR'.  The value shall be allocated by
   IANA.  The Meaning shall be 'Multicast DNS Time Since Registered".
   Reference shall refer to this document, once published.  There is no
   template specified, and IANA shall determine the registration date.

11.  Informative References

12.  Normative References

   [RFC1035]  Mockapetris, P., "Domain names - implementation and
              specification", STD 13, RFC 1035, DOI 10.17487/RFC1035,
              November 1987, <>.

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   [RFC6762]  Cheshire, S. and M. Krochmal, "Multicast DNS", RFC 6762,
              DOI 10.17487/RFC6762, February 2013,

              Lemon, T., "Automatic Replication of DNS-SD Service
              Registration Protocol Zones", Work in Progress, Internet-
              Draft, draft-lemon-srp-replication-00, 26 July 2021,

Authors' Addresses

   Ted Lemon
   Apple Inc.
   One Apple Park Way
   Cupertino, California 95014
   United States of America


   Liang Qin
   Apple Inc.
   One Apple Park Way
   Cupertino, California 95014
   United States of America


   Additional contact information:

      Apple Inc.
      One Apple Park Way
      Cupertino, California 95014
      United States of America

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