Multicast DNS conflict resolution using the Time Since Received (TSR) EDNS option
draft-tllq-tsr-05

Internet Engineering Task Force                                 T. Lemon
Internet-Draft                                                Apple Inc.
Intended status: Standards Track                          秦 良 (L. Qin)
Expires: 24 April 2025                                   21 October 2024

 Multicast DNS conflict resolution using the Time Since Received (TSR)
                              EDNS option
                           draft-tllq-tsr-05

Abstract

   This document specifies a new conflict resolution mechanism for DNS,
   for use in cases where the advertisement is being proxied, rather
   than advertised directly, e.g. when using a combined DNS-SD
   Advertising Proxy and SRP registrar.  A new EDNS option is defined
   that communicates the time at which the set of resource records on a
   particular DNS owner name was most recently updated.

Status of This Memo

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   This Internet-Draft will expire on 24 April 2025.

Copyright Notice

   Copyright (c) 2024 IETF Trust and the persons identified as the
   document authors.  All rights reserved.

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   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
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Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   2
     1.1.  Current Behavior  . . . . . . . . . . . . . . . . . . . .   3
     1.2.  Problem Statement . . . . . . . . . . . . . . . . . . . .   4
   2.  Time Since Received EDNS Option . . . . . . . . . . . . . . .   5
   3.  mDNS Registrar Behavior . . . . . . . . . . . . . . . . . . .   6
     3.1.  Validating requested local RR registrations that include a
           TSR option  . . . . . . . . . . . . . . . . . . . . . . .   6
     3.2.  Probing resource records on names for which TSR data has
           been proposed . . . . . . . . . . . . . . . . . . . . . .   7
     3.3.  Processing questions for which TSR data exists  . . . . .   7
     3.4.  Processing messages containing TSR options  . . . . . . .   8
     3.5.  Constructing a mDNS message with TSR options  . . . . . .   9
   4.  The effect of network latency on time computations  . . . . .  10
   5.  Internal Handling of TSR data . . . . . . . . . . . . . . . .  10
   6.  Timeliness of Conflict Resolution . . . . . . . . . . . . . .  11
   7.  Legacy Behavior . . . . . . . . . . . . . . . . . . . . . . .  11
   8.  When to Use TSR . . . . . . . . . . . . . . . . . . . . . . .  11
   9.  Registrant API considerations . . . . . . . . . . . . . . . .  11
   10. Security Considerations . . . . . . . . . . . . . . . . . . .  12
   11. IANA Considerations . . . . . . . . . . . . . . . . . . . . .  12
   12. Informative References  . . . . . . . . . . . . . . . . . . .  12
   13. Normative References  . . . . . . . . . . . . . . . . . . . .  12
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  13

1.  Introduction

   Unlike the Domain Name System [RFC1034], 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 goal of mDNS conflict resolution is to prevent a newly
   advertised service from taking the place of an existing service with
   the same name that is already being advertised.  This goal, however,
   assumes that the entity advertising an mDNS service is in fact
   authoritative for that service.  In the case of an Advertising Proxy

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   [I-D.sctl-advertising-proxy], this is not the case: the source of
   truth for the service being advertised is an SRP [I-D.ietf-dnssd-srp]
   client.

   On a link with more than one SRP registrar, an SRP client may
   register with one SRP registrar, and then subsequently update its
   registration on a different SRP registrar.  Both SRP registrars may
   be acting as advertising proxies.  If so, the original server may
   still be advertising the old SRP registration using mDNS.  If the
   information in the new SRP registration is identical to that in the
   old registration, this is often not a problem.  However if some
   information has changed (e.g., a new IP address has been added, or a
   TXT record updated), then the new registration will be seen to be in
   conflict with the old registration.  In addition, the method used in
   mDNS to detect conflicts can sometimes produce apparent conflicts
   where no actual conflict exists because of the way records in mDNS
   packets are marshalled.

   In the case of such an apparent conflict, the current behavior of
   mDNS is for the older (stale) registration to win, and the newer
   (current) information to be discarded.  This behavior, which is
   entirely correct for services that are advertising on their own
   behalf, is exactly wrong when a service registration is being
   proxied.

1.1.  Current Behavior

   When a new service is to be advertised, the requestor (the entity
   requesting the registration) 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.
   This can be true whether the conflicting database entry is data for
   which the mDNS registrar is authoritative, or data it has received
   via mDNS and cached.

   In the case of such a conflict, no network transaction is required:
   the mDNS registrar detects it locally.  It addresses the conflict in
   one of two ways.  The first alternative is that the mDNS registrar
   will report the conflict to the requestor as an error, which it must
   fix.  Alternatively, the server requestor have indicated that the
   mDNS mDNS registrar should automatically choose a new name for it, in
   which case the mDNS registrar does so automatically, without
   notifying the requestor.

   Once any locally-detectable conflicts have been resolved, the mDNS
   registrar probes (see Section 8.1 of [RFC6762]) local network to see
   if any other host has already registered a service the conflicts with

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   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 requestor or
   automatically choosing a new name.

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

1.2.  Problem Statement

   The current behavior works well for requestors registering on their
   own behalf.  However, for example in the case of an SRP registrar, it
   works poorly: an SRP registrar acting as an advertising proxy
   publishes the contents of its registration dataset(s) using mDNS.
   The source of truth for information in such datasets is the SRP
   requestor not the SRP registrar (which is acting in proxy as the mDNS
   requestor) itself.

   In the case of an advertising proxy publishing an SRP dataset, what
   we want is not the oldest information, but the newest.  When the SRP
   requestor is able to continue registering with the same SRP
   registrar, this works well: stale data is automatically removed and
   replaced with current data.  However, if more than one SRP registrar
   is available, the requestor may wind up registering with a different
   SRP registrar.  This can happen as a result of a network partition,
   or in cases where the SRP server is advertised on a anycast address.

   When the SRP requestor registers with a different SRP registrar, the
   behavior we get 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 discoverable as separate services.

   This creates a new burden on consumers of such services: they need to
   parse through the whole list of services of their type, using
   metadata from the TXT record in the service instance data, if
   possible, to determine that service A and service A’ are the same
   service.  If no such information is present in the TXT record, the
   only way to determine that one of these two registrations is stale is
   to attempt to use the advertised service, which may no longer be
   reachable if, for example, the change that produced the conflict was
   an IP address change.  When the SRP lease for the stale service
   expires, that service's advertisement will be removed, and the
   service will no longer be discoverable under the original name, even
   if the IP address hasn't changed.

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   This document proposes an enhancement to the current conflict
   resolution algorithm for mDNS, which allows an mDNS proxy to report
   the time at which it received the registration it is newly
   advertising, and the source from which it was received.  This is done
   using a new Time Since Received EDNS option, for which there must be
   exactly one per name being registered by the proxy.

2.  Time Since Received EDNS Option

   Each Time Since Received (TSR) EDNS option is applicable to exactly
   one DNS owner name.  So all the records for that owner name that
   appear in the answer, authority and/or additional sections of an mDNS
   message would be covered by a single TSR option.

   The TSR EDNS option consists of three fields: the RR index (two byte
   integer in network byte order), a key checksum (four bytes), and a
   time of registration (four bytes).

   The RR index is the number of the RR in the mDNS packet.  Question
   RRs are not counted.  So if the message includes two answer RRs, one
   authority RR and two additional RRs, an index of 0 would refer to the
   first answer, an index of 1 to the second answer, and index of 2 to
   the single authority record, and so on.  Questions are excluded
   because they have no data associated with them, and so it makes no
   sense for them to have TSR records associated with them.

   If there is more than one record in the mDNS Message with the same
   owner name, only one TSR option is emitted for that name, and it
   applies to every RR in the mDNS Message with that owner name.  It is
   not possible in the SRP protocol for two updates at two different
   times to contain records that apply to the same name: in such a
   situation, the second update completely replaces the first, so all
   data in the first update is then rendered stale.

   The second field, the key checksum, is simple 32-bit checksum of the
   public key that the owner of the data (for example the SRP requestor)
   used to authenticate itself.  The key checksum is computed by
   treating the key as a series of 32-bit unsigned integers in network
   byte order, and adding these integers together to produce a 32-bit
   unsigned checksum.  Overflow is not considered.  This checksum need
   not be cryptographically secure: mDNS messages are not authenticated,
   so an attacker on the local link can always cause problems with mDNS
   by providing spurious responses.  The purpose of the checksum is
   simply to notice whether, for a specific owner name, two different
   authoritative sources have provided information.

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   The TSR time offset field contains the difference, in seconds,
   between the the time at which the TSR record is being generated and
   the time of receipt for recorded for that owner name.

   The time of registration is represented in the mDNS message as a time
   in seconds relative to the time when the mDNS message is sent.  If
   this difference is greater than seven days (7 * 24 * 60 * 60), the
   mDNS registrar MUST use a value of seven days rather than the larger
   value.  The relative time represented in the TSR option is converted
   to an absolute time when stored in a cache or authority database on
   an mDNS registrar, and is converted to a relative time whenever an
   mDNS message is generated from local data.

3.  mDNS Registrar Behavior

3.1.  Validating requested local RR registrations that include a TSR
      option

   When a local mDNS requestor asks an mDNS registrar to register one or
   more records on an owner name, and provides TSR data for that name,
   the mDNS requestor first checks to see if there are any records
   either in cache or from other local registrations on that owner name.
   If no such data exists, the mDNS registrar puts the record(s) in this
   registration in the probing state.

   When such data exists, the registrar MUST check to see if it has TSR
   data for that owner name.  If it does not, or if there is TSR data on
   that name but the key checksum does not match, the registrar MUST
   treat this registration as a conflict and return an appropriate error
   to the requestor.

   If such data exists and the key checksums match, there are three
   possibilities based on the known TSR time and the proposed TSR time:

   Known time is more recent  In this case, the registrar MUST treat the
      new registration as stale, and return an indication to the
      requestor that its registration is stale.  This indication must be
      different than the conflict indication.

   Both times are the same  In this case, the new record is added to the
      local registration database and put in the probing state.

   Proposed time is more recent  In this case, all cached data on the

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      name is discarded.  The requestor for any existing locally-
      registered data is notified that the data they have registered is
      stale is stale, and the data is removed from the local
      registration database.  The new data is added and put in the
      probing state, and the TSR data is updated with the proposed TSR
      data.

   It is in principle possible for two different mDNS requestors to ask
   the same mDNS registrar to publish different RRs on the same name,
   some of which are shared and some of which are unique (see Section 2
   of [RFC6762]).  If an mDNS requestor registers an RR on a name for
   which the registrar already has data, cached or authoritative, on the
   same name, whether of the same type or a different type, for which
   there is no TSR data, or for which the key checksum in the TSR data
   being registered does not match what is already known, the registrar
   MUST treat this as an immediate conflict, and MUST NOT probe.

   As with any local mDNS registration, the mDNS registrar treats all of
   the records in the registration as tentative (that is, in the probing
   state) until they have been probed and no conflicting answers have
   been received.

3.2.  Probing resource records on names for which TSR data has been
      proposed

   Section 8.1 of [RFC6762] describes how an mDNS registrar probes to
   ensure that there is no conflicting data for records in the probing
   state.  The behavior for records that are in the probing state on
   names to which no TSR data applies is unchanged.  When there is TSR
   data on a name for which records are being probed, the mDNS registar
   MUST include TSR options for each such name as described in
   Section 2.  Handling of responses is described in Section 3.4.

3.3.  Processing questions for which TSR data exists

   When processing a question for which local TSR data is present, the
   mDNS registrar MUST first check to see if there is corresponding data
   in the mDNS message being processed.  If there is, the question is
   part of a probe.  In this case, before constructing a response, the
   mDNS registrar MUST process the non-question records in the packet,
   since this could result in stale data being flushed.  Processing is
   performed as described in Section 3.4

   Once all non-question records have been processed, the responder MUST
   respond to any questions that match locally-registered resource
   records for which a known answer is not present in the query.
   Responses are constructed as described in Section 2.

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3.4.  Processing messages containing TSR options

   For each TSR option in an mDNS message, the mDNS registrar first
   determines the owner name of the TSR option by assigning an index to
   each non-question resource record in the mDNS message.  The index of
   each TSR option is then matched to the index of a resource record,
   and the owner name for that resource record is applied to the TSR
   option.  The time on the TSR option is then computed by taking the
   current local clock time and subtracting from it the time offset in
   the TSR record.

   If there is a TSR option in an mDNS message for which there is no
   matching resource record in the mDNS message, the mDNS registrar MUST
   ignore that TSR option.  The mDNS registrar MUST NOT use the index
   from the TSR option to search across the mDNS Packet since such an
   index can easily be out of bounds.

   Now, for each record in the mDNS message, the mDNS registrar first
   determines whether the record is an OPT record, is in the question
   section, or is a known answer (QD bit = 0 and it's a record in the
   answer section).  For all such records, no special processing is done
   for TSRs, since no TSR should exist in the mDNS message.

   For each remaining resource record in the mDNS message, the mDNS
   registrar MUST check to see if there is a TSR option in the mDNS
   message for that owner name.  If there is not, the mDNS registrar
   MUST check to see if there is TSR data with that owner name locally.
   If there is not, the record is processed normally.

   If there is local TSR data for the record's owner name, the mDNS
   registrar checks to see if there are any resource records in the
   local registration database (that is, not just in the cache) on that
   name.  If there are, the record is treated as a conflict.  This
   conflict exists even if the locally registered records are all shared
   records.  In cases where there are records on the name in the cache,
   those records are all discarded, because they are in conflict with
   the new data.

   In the case that there is TSR data for the record in the mDNS packet,
   and no local TSR record, this always means that any data is in
   conflict.  How that conflict is addressed depends on the data.
   First, note that resource records in the answer section of an mDNS
   Query (QR bit in the header is 0) are "known answers" and therefore
   are not relevant when adding data to the mDNSResponder cache.  Such
   records can never have TSR options associated with them.  However,
   resource records in the authority and additional sections of a query
   do need to be processed (but in the case of authority records, are
   not added to the cache).

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   In cases where the TSR data for a particular name is present both
   locally and in the mDNS message, the mDNS responder MUST compare the
   key checksums.  If they are different, then the records are always in
   conflict, and are handled according to the context of the conflict,
   as described in Section 9 of [RFC6762]

   In cases where the key checksums match, the mDNS registrar MUST
   compare the times.  When the TSR time from the mDNS Message is more
   recent than the local TSR time, local data in the cache is flushed
   and registered data is removed and reported to the requestor that
   registered it as stale.

   When the TSR times are the same, any resource records on that name in
   the answer section and additional section are added to the cache.

   When the local TSR time is more recent, the data in the message is
   not added to the cache, and no action is taken with respect to any
   locally-registered data.

3.5.  Constructing a mDNS message with TSR options

   For each non-question record that is added to the mDNS message, one
   of three things must be true:

   *  The mDNS server is has resource records locally registered on that
      owner name, which may or may not be in the probing state.

   *  It is sending an answer which is either an announcement or a
      response containing data it has already validated and for which it
      is authoritative

   *  The message is a query (QD=0) and the record is in the answer
      section, and is therefore a "known answer."

   As described in Section 7.1 of [RFC6762], an mDNS registrar asking a
   question about one or more RRs on a particular name populates the
   answer section of its mDNS message with the answers it already knows,
   to avoid unnecessary responses.  However, in this case it can't also
   be probing for records on the same name, because probes are only done
   for unique (non-shared) records.

   The requirements in Section 3.1 mean that there can never be an mDNS
   probe that contains known answers on an owner name for which any RR
   is being probed to which a TSR option applies.

   This means that for any particular owner name that might be
   represented in an mDNS packet, it must be the case either that it is
   not a known answer, or that it is a known answer and no other records

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   exist in the mDNS packet with the same owner name to which a TSR
   record would apply.  That is, one of two things must be true about
   the set of all records with a particular owner name being added to
   the mDNS packet: either a TSR option applies to all of the records,
   or it applies to none of the records.  Furthermore, either a record
   is a known answer from cache, or it is a locally-registred record.

   When constructing an mDNS message, the registrar maintains a set of
   names and associated TSR data.  Initially this set is empty.  When
   the registrar adds a record to the mDNS message, if that record is
   locally registered, and if the registrar has TSR data for that name,
   it first checks to see if it has already added TSR data for that name
   to the set.  If not, then it adds a new entry to the set containing
   the TSR data for the owner name of the RR.  The data added consists
   of the owner name, the index of the record being added (since it is
   the first), the key checksum, and the time of receipt.

   Once the mDNS responder has added all of the resource records it
   intends to to the mDNS message that is being constructed, it emits an
   OPT record in the additional section.  To this OPT record it adds a
   TSR record for every name in the set that was generated when adding
   resource records to the message.  The time of receipt is subtracted
   from the current time to prodiuce the time difference, and this is
   clamped to a maximum of seven days.

4.  The effect of network latency on time computations

   Because TSR computations are affected by network latency, comparisons
   can’t be considered accurate.  It is therefore necessary to tolerate
   some amount of error.  In practice, however, it should generally not
   be the case that two advertising proxies receive SRP updates from the
   same SRP client at nearly the same time.  So it should always be the
   case either that there is a clear ordering to the timestamps, or that
   there is no conflict in the data.  For example with anycast, a
   retransmission could go to a different SRP registrar, but in this
   case both servers would simultaneously receive identical data, so the
   close ordering or even equality of the timestamps should not affect
   the outcome.

5.  Internal Handling of TSR data

   The TSR option that is sent on the wire is expressed in seconds
   relative to the time of receipt of the registration.  In order to
   derive the time to send in a TSR option, the registrar must remember
   the time at which the registration occurred.  This time is recorded
   as an absolute time, not a relative time.  We refer to this as the
   time of receipt.  When constructing a TSR option, the registrar
   computes the difference between the current time and the time of

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   receipt, which must always be in the past.  This difference, which
   should be a positive integer, is converted to seconds, and that
   unsigned value is then used to synthesize the TSR RR.

6.  Timeliness of Conflict Resolution

   It is expected that if a conflict exists, it will be recent, and will
   be resolved quickly.  Different hosts may be able to record shorter
   or longer time differences.  However, because of this expectation of
   recentness, mDNS registrars should never need to 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.

7.  Legacy Behavior

   y mDNS registrars and queriers that do not support the TSR option are
   expected to ignore the option, so they will behave as if no TSR
   option was sent.  This may result in such registrars temporarily
   caching stale data.  However, in the normal course of processing,
   more recent data will win.  In cases where it does not, the Reconfirm
   process which is part of [RFC6762] already works to clear stale data:
   since we expect SRP servers to implement TSR, by the time a Reconfirm
   is attempted, all authoritative stale data should have been cleared.

8.  When to Use TSR

   TSR is only relevant for mDNS proxies.  Regular (non-proxy) mDNS
   registrants are not expected to use it, since it will produce the
   wrong behavior for this use case.  An mDNS registrant that is a proxy
   MUST explicitly request that a TSR be used for conflict resolution.
   mDNS registrars MUST NOT record a time of receipt unless the
   registrant has specifically requested it.

9.  Registrant API considerations

   When an mDNS proxy registers a service and requests the use of a time
   of receipt, 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 conflict resolution is wanted, but
   must also provide a way for the proxy to specify an absolute time at
   which the registration was received, and the key checksum used to
   identify the entity that's actually authoritative for the data.

   This is important, for example, in the case of SRP Replication
   [I-D.lemon-srp-replication], where an SRP registrar may receive a
   registration from a peer during startup synchronization.  This
   registration will have occurred at some significant amount of time in

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   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 time of receipt.

10.  Security Considerations

   The TSR option is an optimization: it ameliorates an edge case for
   mDNS proxies.  A malicious host on the same link could use the TSR
   option 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.

   Whether or not an mDNS registration has a recorded time of receipt,
   an attacker can deny service by announcing its own conflicting data
   and then answering the subsequent probe as described in Section 9 of
   [RFC6762].  Because it does not include a TSR record in its authority
   section, it can win the simultaneous conflict resolution process that
   follows its bogus announcement.

   So the TSR-based conflict resolution process creates no new
   vulnerability.  Addressing the existing vulnerability is out of scope
   for this document.  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, or by using DNSSEC rather than mDNS for
   service discovery.

11.  IANA Considerations

   IANA is requested to allocate a new OPT RR option code from the DNS
   EDNS0 Option Codes (OPT) registry for the 'Time Since Received'
   Option.  The Name shall be 'mDNS-TSR'.  The value shall be allocated
   by IANA.  The meaning shall be 'Multicast DNS Time Since Received".
   Reference shall refer to this document, once published.  IANA shall
   determine the registration date.

12.  Informative References

13.  Normative References

   [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>.

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

Lemon & Qin               Expires 24 April 2025                [Page 12]
Internet-Draft          TSR EDNS option for mDNS            October 2024

   [I-D.lemon-srp-replication]
              Lemon, T., Keshavarzian, A., and J. Hui, "Automatic
              Replication of DNS-SD Service Registration Protocol
              Zones", Work in Progress, Internet-Draft, draft-lemon-srp-
              replication-03, 22 February 2023,
              <https://datatracker.ietf.org/doc/html/draft-lemon-srp-
              replication-03>.

   [I-D.sctl-advertising-proxy]
              Cheshire, S. and T. Lemon, "Advertising Proxy for DNS-SD
              Service Registration Protocol", Work in Progress,
              Internet-Draft, draft-sctl-advertising-proxy-02, 12 July
              2021, <https://datatracker.ietf.org/doc/html/draft-sctl-
              advertising-proxy-02>.

   [I-D.ietf-dnssd-srp]
              Lemon, T. and S. Cheshire, "Service Registration Protocol
              for DNS-Based Service Discovery", Work in Progress,
              Internet-Draft, draft-ietf-dnssd-srp-25, 4 March 2024,
              <https://datatracker.ietf.org/doc/html/draft-ietf-dnssd-
              srp-25>.

Authors' Addresses

   Ted Lemon
   Apple Inc.
   One Apple Park Way
   Cupertino, California 95014
   United States of America
   Email: mellon@fugue.com

   Liang Qin
   Email: Leonqin0101@gmail.com

   Additional contact information:

      秦良

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