Multicast DNS conflict resolution using the Time Since Received (TSR) EDNS option
draft-tllq-tsr-04

Internet Engineering Task Force                                 T. Lemon
Internet-Draft                                                Apple Inc.
Intended status: Standards Track                          秦 良 (L. Qin)
Expires: 5 September 2024                                   4 March 2024

 Multicast DNS conflict resolution using the Time Since Received (TSR)
                              EDNS option
                           draft-tllq-tsr-04

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 5 September 2024.

Copyright Notice

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

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   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
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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.  When sending an mDNS message  . . . . . . . . . . . . . .   6
     3.2.  When processing an mDNS message . . . . . . . . . . . . .   6
   4.  mDNS querier behavior when processing an mDNS message . . . .   6
   5.  The effect of network latency on time computations  . . . . .   7
   6.  Internal Handling of TSR data . . . . . . . . . . . . . . . .   7
   7.  Timeliness of Conflict Resolution . . . . . . . . . . . . . .   7
   8.  Legacy Behavior . . . . . . . . . . . . . . . . . . . . . . .   8
   9.  When to Use TSR . . . . . . . . . . . . . . . . . . . . . . .   8
   10. Registrant API considerations . . . . . . . . . . . . . . . .   8
   11. Security Considerations . . . . . . . . . . . . . . . . . . .   8
   12. IANA Considerations . . . . . . . . . . . . . . . . . . . . .   9
   13. Informative References  . . . . . . . . . . . . . . . . . . .   9
   14. Normative References  . . . . . . . . . . . . . . . . . . . .   9
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  10

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 new
   service being advertised 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 [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.

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   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 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 the case of such a supposed 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 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.
   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 server as an error, which the server
   must fix.  Alternatively, the server may 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 server.

   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
   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 name.

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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.

1.2.  Problem Statement

   The current behavior works well for services 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 proxies
   the contents of its registration dataset(s) using mDNS.  The source
   of truth for information in such datasets is whatever service has
   registered with the SRP registrar, not the SRP registrar itself.

   In the case of an advertising proxy proxying an SRP dataset, what we
   want is not the oldest information, but the newest.  When the SRP
   client 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, and for some reason the original SRP registrar with which
   the registration was completed is still operating but no longer
   reachable (e.g., in the case of a network partition), the SRP client
   will wind up registering with a different SRP registrar.  Similarly,
   if the SRP service is being advertised using an anycast address,
   there is no guarantee that the SRP renewal will be delivered to the
   same SRP registrar.

   When the SRP client 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.  This is done using a new Time Since Received RR, which
   is attached to the name of the registration.

2.  Time Since Received EDNS Option

   The Time Since Received (TSR) EDNS option references a specific RR in
   the answer, authority or additional sections of an mDNS message.
   This option specific to an owner name, not to an individual RRset.
   When a service registration is successful, the mDNS registrar records
   the wall clock time at which the registration request was received.
   This may be the current time, or a time specified by a proxy service
   that is doing the registration.  This time is only recorded if the
   service requesting the registration specifies it; otherwise, the time
   of receipt is not recorded.  The registrar also has the public key
   that was used in the registration.

   The TSR EDNS option consists of three fields: the RR offset (two byte
   integer in network byte order), a key hash (32 bytes), and a time
   offset (four bytes).

   The RR offset points to the first name in a given section of the mDNS
   Message for which the TSR is in effect.  The first answer is at
   offset 0, the second answer at offset 1 and so on.  Or if there is
   only one answer but also an authority record, the first authority
   record is at offset 2.  If following that record there is a record in
   the additional section (including the OPT RR itself), that record
   would be at offset 3.  Questions can't have TSRs associated with
   them, and hence the counting starts with the first answer, not the
   first question.

   If there are multiple records 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.

   The second field, the key hash, is an SHA-256 hash (TBD) of the
   public key that was used to authenticate the SRP update that most
   recently updated the RRs on the owner name to which the TSR option
   applies.

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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.  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.

3.  mDNS Registrar Behavior

3.1.  When sending an mDNS message

   When constructing an mDNS message, the mDNS registrar includes one
   TSR option for each unique owner name for any RR added to the mDNS
   message for which the mDNS registrar is authoritative and which was
   registered using SRP.

3.2.  When processing an mDNS message

   When an mDNS registrar receives an mDNS message query, it processes
   it as usual, unless it discovers a conflict.  When data in the mDNS
   message conflicts with data in the mDNS querier's authority database,
   as described in Section 8.1 of [RFC6762], this is considered to be a
   conflict.

   When a conflict detected for which a TSR record in the mDNS message
   is applicable, and the local record was also registered using SRP,
   the registrar checks to see if the key hashes match.  If they do not,
   then the conflict is processed as usual.  If either the mDNS record
   or the local authoritative record does not contain TSR information,
   the conflict is processed as usual.

   When both versions of the data have TSR records, and the key hashes
   match, if the record in the mDNS response is more recent, the local
   authoritative data is discarded from the authority dtheatabase and
   the SRP registrar function is notified that that data is stale.

   If, however, the version of the data received in the mDNS response is
   less recent than the authoritative data, the registrar ignores the
   data in the mDNS response.

4.  mDNS querier behavior when processing an mDNS message

   As with with the mDNS registrar case, when an mDNS querier receives
   an mDNS Message, it compares TSR information in the local cache with
   the TSR information in the message.  If TSR information is present in
   both locations, and the data in the mDNS message is more recent, the
   information in the cache is replaced with the information in the mDNS
   message.  Otherwise, the information in the mDNS message is ignored.

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   Note that some mDNS registrars only answer internal queries out of
   cache, and populate their cache using information they receive both
   from their own mDNS announcements and answers and from information
   received in responses sent by other mDNS registrars.  In addition to
   the behavior described for mDNS registrars, such mDNS registrars MUST
   also implement the behavior described here for queriers with respect
   to cache updates.

5.  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.

6.  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
   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.

7.  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.

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8.  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.

9.  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.

10.  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.

   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
   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.

11.  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.

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   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.

12.  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.

13.  Informative References

14.  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>.

   [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>.

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   [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/api/v1/doc/document/draft-
              ietf-dnssd-srp/>.

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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