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Versions: 00 01 02 03 04 05 draft-dnsop-update-timeout

Internet Engineering Task Force                            T.J. Pusateri
Internet-Draft                                             T. Wattenberg
Intended status: Standards Track                            Unaffiliated
Expires: 7 May 2020                                      4 November 2019


                      DNS TIMEOUT Resource Record
                 draft-pusateri-dnsop-update-timeout-04

Abstract

   This specification defines a new DNS TIMEOUT resource record (RR)
   that associates a lifetime with one or more zone resource records.
   It is intended to be used to transfer resource record lifetime state
   between a zone's primary and secondary servers and to store lifetime
   state during server software restarts.

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 https://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 7 May 2020.

Copyright Notice

   Copyright (c) 2019 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 (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
   and restrictions with respect 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.




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Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   2
   2.  Requirements Language . . . . . . . . . . . . . . . . . . . .   3
   3.  Sources of TIMEOUT Expiry Time  . . . . . . . . . . . . . . .   3
   4.  Common Usage Patterns . . . . . . . . . . . . . . . . . . . .   4
     4.1.  TIMEOUT records vs. Update Leases . . . . . . . . . . . .   5
     4.2.  Testing for TIMEOUT . . . . . . . . . . . . . . . . . . .   6
   5.  Resource Record Composition . . . . . . . . . . . . . . . . .   6
     5.1.  Represented Record Type . . . . . . . . . . . . . . . . .   6
     5.2.  Represented Record Count  . . . . . . . . . . . . . . . .   7
     5.3.  Method Identifiers  . . . . . . . . . . . . . . . . . . .   7
       5.3.1.  Method Identifier 0: NO METHOD  . . . . . . . . . . .   8
       5.3.2.  Method Identifier 1: MD-SHA256-128  . . . . . . . . .   8
     5.4.  Expiry Time . . . . . . . . . . . . . . . . . . . . . . .   8
   6.  TIMEOUT RDATA Wire Format . . . . . . . . . . . . . . . . . .   8
   7.  Server Behavior . . . . . . . . . . . . . . . . . . . . . . .  10
     7.1.  Primary Server Behavior . . . . . . . . . . . . . . . . .  10
     7.2.  Secondary Server Behavior . . . . . . . . . . . . . . . .  10
   8.  TIMEOUT RDATA Presentation Format . . . . . . . . . . . . . .  10
   9.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .  11
   10. Security Considerations . . . . . . . . . . . . . . . . . . .  12
   11. Acknowledgments . . . . . . . . . . . . . . . . . . . . . . .  13
   12. References  . . . . . . . . . . . . . . . . . . . . . . . . .  13
     12.1.  Normative References . . . . . . . . . . . . . . . . . .  13
     12.2.  Informative References . . . . . . . . . . . . . . . . .  14
   Appendix A.  Example TIMEOUT resource records . . . . . . . . . .  14
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  17

1.  Introduction

   DNS Update [RFC2136] provides a mechanism to dynamically add/remove
   DNS resource records to/from a zone.  When a resource record is
   dynamically added, it remains in the zone until it is removed
   manually or via a subsequent DNS Update.  The context of a dynamic
   update may provide lifetime hints for the updated records (such as
   the EDNS(0) Update Lease option [I-D.sekar-dns-ul]), however, this
   lifetime is not communicated to secondary servers and will not
   necessarily endure through server software restarts.  This
   specification defines a new DNS TIMEOUT resource record that
   associates lifetimes with one or more resource records with the same
   owner name, type, and class that can be transferred to secondary
   servers through normal AXFR [RFC5936], IXFR [RFC1995] transfer
   mechanisms.

   An UPDATE lifetime could be stored in a proprietary database on an
   authoritative primary server but there is an advantage to saving it
   as a resource record: redundant master servers and secondary servers



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   capable of taking over as the primary server for a zone automatically
   can benefit from the existing database synchronization of resource
   records.  In addition, primary and secondary servers from multiple
   vendors can synchronize the lifetimes through the open format
   provided by a resource record.

   TIMEOUT records can be installed via policy by a primary server,
   manually, or via an external UPDATE from a client.  If TIMEOUT
   records are being managed by an UPDATE client, the client should be
   aware of server software policy with respect to TIMEOUT records to
   prevent the TIMEOUT records from being rejected.  The primary server
   has ultimate responsibility for the records in the database and the
   client must work within the restrictions of the policy of the primary
   server.

   TIMEOUT records can be thought of as a universal method for removing
   stale dynamic DNS records.  Clients such as DHCP servers who best
   know the lease lifetimes can include individual TIMEOUT records in
   the dynamic UPDATE messages specific for each lease lifetime.  These
   TIMEOUT records can be refreshed when the lease is refreshed and will
   timeout the A, AAAA, and PTR records if they are not refreshed by the
   DHCP server.  Additional use cases include service discovery resource
   records installed in unicast DNS servers via UPDATE described in
   [RFC6763], Active Directory Controllers publishing SRV records, DNS
   TXT resource records supporting ACME certificate management
   challenges as described in [RFC8555], Section 8.4, and the limited
   lifetime certificate representations produced by ACME that are stored
   in DANE TLSA resource records [RFC6698].

2.  Requirements Language

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   "SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and
   "OPTIONAL" in this document are to be interpreted as described in BCP
   14 [RFC2119] [RFC8174] when, and only when, they appear in all
   capitals, as shown here.  These words may also appear in this
   document in lower case as plain English words, absent their normative
   meanings.

3.  Sources of TIMEOUT Expiry Time

   The expire time may come from many different sources.  A few are
   listed here however, this list is not considered complete.  TIMEOUT
   records may be included along side the records they represent in the
   UPDATE message or they be synthesized by the primary server receiving
   the UPDATE.

   *  Via DHCP Lease Lifetimes.



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   *  Via EDNS(0) Update Lease option [I-D.sekar-dns-ul] communicated in
      DNS Update.

   *  Via an administrative default value such as one day (86400
      seconds).

4.  Common Usage Patterns

   TIMEOUT resource records are just one tool in the toolbox for
   cleaning up stale resource records.  They provide a failsafe in case
   other mechansims meant to clean up records fail.  It might be useful
   to think of them similar to Garbage Collection (GC) or Automatic
   Reference Counting (ARC) used by programming languages for memory
   management.  The model in which the TIMEOUT resource records are used
   depends on the support provided for them by the primary DNS server.

   As it cannot be presumed that all primary authoritative servers will
   manage TIMEOUT resource records internally, an external management of
   the TIMEOUT records and the resource records they represent might be
   necessary.  The client may perform external management of TIMEOUT
   records it creates through an UPDATE or a third party with
   appropriate permission may manage the records.

   If the primary server understands TIMEOUT records and manages them
   based on resource record updates, it will likely know when to remove
   the resource records referenced by the TIMEOUT records.  This is
   similar to ARC.


       ┌─────────────┐
       │Client UPDATE│
       │ with EDNS0  │───┐
       └─────────────┘   │   ┌───────────────┐
                         │   │    Primary    │     Zone     ┌─────────┐
    ┌────────────────┐   ├──▶│(Authoritative)│───Transfer──▶│Secondary│
    │  UPDATE with   │   │   └───────────────┘              └─────────┘
    │TIMEOUT included│───┘
    └────────────────┘


   If the primary server does not understand TIMEOUT records, then an
   external manager (client) will need to use DNS UPDATE to manage
   TIMEOUT records and the resource records they reference.  Garbage
   Collectors run periodically looking for memory no longer being used
   to reclaim.  In a similar way, external TIMEOUT record managers need
   to periodically scan the TIMEOUT records and send DNS UPDATE messages
   to add/remove records when the server doesn't manage them
   automatically.



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   ┌────────────────┐        ┌───────────────┐
   │  UPDATE with   │        │    Primary    │     Zone     ┌─────────┐
   │TIMEOUT included│───────▶│(Authoritative)│───Transfer──▶│Secondary│
   └────────────────┘        └───────────────┘              └─────────┘
                             │               ▲
                           Zone              │
                         Transfer         UPDATE
                             │               │
      ┌─────────────┐        ▼               │
      │Client UPDATE│        ┌───────────────┐
      │ with EDNS0  │───────▶│   timeoutd    │
      └─────────────┘        └───────────────┘


   It should be noted that similar to many instances of Garbage
   Collection, the precision with which TIMEOUT records and the resource
   records they reference are removed is not critical.  Gross timers
   and/or scanning mechanisms are perfectly appropriate and should not
   consume additional resources for the purpose of being precise.  As
   described in Section 5.4 below, expiry times use one second
   resolution.

4.1.  TIMEOUT records vs. Update Leases

   Each application will have to determine when it is better to use
   TIMEOUT resource records, EDNS(0) Update Lease options, or a
   combination of the two.  In some cases, either will serve the same
   purpose.  A differentiating factor is that TIMEOUT resource records
   use absolute time so that the records may be more easily synchronized
   across secondary servers whereas Update Leases are specified in
   relative time offsets.

   If your primary DNS server supports TIMEOUT records directly, it may
   be simpler to just provide an Update Lease lifetime in the DNS UPDATE
   message that the server will use to create the TIMEOUT records
   internally.  If your primary DNS server does not support TIMEOUT
   records and your application uses sources that have real-time clocks
   that are synchronized with standard time sources, TIMEOUT records are
   an available option to the client.  However, if your clients are
   using low-cost hardware without real-time clocks, they should send
   Update Leases to the primary server or an intermediate proxy with a
   synchronized real-time clock.









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4.2.  Testing for TIMEOUT

   There is no more reliable mechanism to determine if the primary DNS
   server supports the management of TIMEOUT records than explicitly
   trying it.  Before relying on a server to expire TIMEOUT records, the
   application should send test records and test if they are handled as
   expected.  If the preferred mode of operation is not supported,
   another mode can be attempted.  For example, if sending a DNS UPDATE
   with a EDNS(0) Update Lease of 1 second doesn't cause the record to
   be expired within 6 seconds (1 + 5 fuzz), then the application can
   try including a TIMEOUT record in the DNS UPDATE.  If that doesn't
   automatically expire, TIMEOUT records will need to be managed
   externally.

5.  Resource Record Composition

   TIMEOUT resource records provide expiry times for a mixed variety of
   resource record types with the same owner name, type, and class.
   Since there could exist multiple records of the same record type with
   the same owner name and class, the TIMEOUT resource record must be
   able to identify each of these records individually with only
   different RDATA.  As an example, PTR records for service discovery
   [RFC6763] provide a level of indirection to SRV and TXT records by
   instance name.  The instance name is stored in the PTR RDATA and
   multiple PTR records with the same owner name and only differing
   RDATA often exist.

   In order to distinguish each individual record with potentially
   different expiry times, the TIMEOUT resource record contains an
   expiry time, the record type, a method to identify the actual records
   for which the expiry time applies, and a count of the number of
   records represented.  Multiple TIMEOUT records with the same owner
   name and class are created for each expiry time, record type, and
   resource record representation.  If the expiry time is the same,
   multiple records can be combined into a single TIMEOUT record with
   the same owner name, class, and record type but this is not required.

   The fields and their values in a TIMEOUT record are defined as:

5.1.  Represented Record Type

   A 16-bit field containing the resource record type to which the
   TIMEOUT record applies.  Multiple TIMEOUT records for the same owner
   name, class, and represented type can exist.  Any resource record
   type can be specified in the Represented Record Type including
   another TIMEOUT record.  This specification does not put any
   restrictions on the record type but implementations in authoritative
   servers will likely do so for policy and security reasons.



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   QTYPEs and Meta-TYPEs MUST NOT be used as the represented record
   type.  For more information, refer to [RFC6895], Section 3.1.

5.2.  Represented Record Count

   The Represented Record Count is a 8-bit value that specifies the
   number of records of the specified record type with this expiry time.

   A count of zero indicates that it is not necessary to represent any
   records in the list.  This is a shortcut notation meaning all
   resource records with the same owner name, class, and record type use
   the same Expiry Time.  When the Represented Record Count is 0, the
   Method Identifer is set to NO METHOD (0) on transmission and ignored
   on reception.  A primary server MUST NOT install a TIMEOUT record
   with No Method/No Count at the same time that a TIMEOUT record exists
   for the same owner name, class, and type with a non-zero record
   count.  Either all records MUST match the No Method/No Count
   shorthand syntax or they MUST all be included in the list of matching
   records.

   In the unlikely event that the Represented Record Count exceeds 255
   which is the largest number representable in 8 bits, multiple
   instances of the same Expiry Time can exist.

5.3.  Method Identifiers

   The Method Identifier is a 8-bit value that specifies an identifier
   for the algorithm used to distinguish between resource records.  The
   identifiers are declared in a registry maintained by IANA for the
   purpose of listing acceptable methods for this purpose.  In addition
   to the method and the index, the registry MAY contain a fixed output
   length in bits of the method to be used or the term "variable" to
   denote a variable length output per record.  It is conceivable,
   though not likely, that the same method could be used with different
   fixed output lengths.  In this case, each fixed output length would
   require a different identifier in the registry.  Additions to this
   registry will be approved with additional documentation under expert
   review.  At the time that the registry is created by IANA, a group of
   expert reviewers will be established.

   Additional methods of representing records may be defined in the
   future.  If such methods are defined, a primary server could create
   TIMEOUT record using a new method that is not understood by a
   secondary server that could take over as the primary in the event of
   an outage or administrative change.  In this case, the new primary
   would not be able to identify the records it is supposed to TIMEOUT.
   This is a misconfiguration and it is the responsibility of the
   administrator to ensure that secondary servers in a position to



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   become primary understand the TIMEOUT record methods of the primary
   server.

5.3.1.  Method Identifier 0: NO METHOD

   The method identifier of 0 is defined as "NO METHOD" and MUST NOT be
   used if the represented record count is greater than 0.  The value of
   0 is to be included in the IANA registry of method identifier values.

5.3.2.  Method Identifier 1: MD-SHA256-128

   The method identifier of 1 is defined as "MD-SHA256-128".  Following
   the expiry time is a list of 128-bit values.  Each of these values is
   the first 128-bits of a message digest of the RDATA of a represented
   record in canonical DNSSEC form calculated using the 256-bit SHA-256
   hash algorithm defined in [FIPS180-4].  The canonical DNSSEC form is
   described in [RFC4034], Section 6.  The input length of RDATA for the
   message digest is the RDLEN of the represented record.

5.4.  Expiry Time

   The expiry time is a 64-bit number expressed as the number of seconds
   since the UNIX epoch (00:00:00 UTC on January 1, 1970).  This value
   is an absolute time at which the record will expire.  An absolute
   time is necessary so the TIMEOUT records do not have to change during
   zone transfers.

   There are circumstances when a relative expiry time would be
   convenient due to limited resources for clock synchronization in
   constrained devices.  In this case, DNS UPDATE messages should not
   contain precomputed TIMEOUT records but convey the relative expiry
   time using the EDNS(0) Update Lease Option defined in
   [I-D.sekar-dns-ul].  The relative time is then converted to an
   absolute expiry time when received by the primary server which will
   create the TIMEOUT resource records.

6.  TIMEOUT RDATA Wire Format

   The TIMEOUT resource record follows the same pattern as other DNS
   resource records including owner name, type, class, TTL, RDATA
   length, and RDATA as defined in [RFC1035], Section 3.2.1.

   The RDATA section of the resource record with method identifier NO
   METHOD (0) is illustrated in Figure 1:







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      0                   1                   2                   3
      0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |      Represented RR Type      |   Count (0)   |   Method (0)  |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |                       Expiry Time (64-bit)                    |
     |                                                               |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+


                   Figure 1: Method (0) RDATA Wire Format

   Figure 1 represents the TIMEOUT RDATA field of all matching records
   of the represented type for the same owner name and class.

   The RDATA section of the resource record with method identifier MD-
   SHA256-128 (1) is illustrated in Figure 2:


      0                   1                   2                   3
      0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |      Represented RR Type      |   Count (n)   |   Method (1)  |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |                      Expiry Time (64-bit)                     |
     |                                                               |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |                                                               |
     |                 First 128 bits of SHA256 hash                 |
     |                 of Represented Record 1 RDATA                 |
     |                                                               |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     .                                                               .
     .                                                               .
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |                                                               |
     |                 First 128 bits of SHA256 hash                 |
     |                 of Represented Record n RDATA                 |
     |                                                               |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+


               Figure 2: Method (1) MD-SHA256-128 Wire Format

   Figure 2 represents an arbitrary number of represented records with
   the same owner name, class, and represented type.  For each expiry
   time, a list of the first 128-bits of a SHA256 hash are appended.




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7.  Server Behavior

   A server may or may not understand TIMEOUT resource records.  If a
   server does not understand them, they are treated like any other
   resource record that the server may not understand.  See [RFC3597]
   for more information.

7.1.  Primary Server Behavior

   The primary server is the ultimate source of the database and
   policies established by the server may overrule the actions of
   external clients.  The primary server is ultimately responsible for
   ensuring the database is consistent but until TIMEOUT record
   management is built-in to authoritative server software, external
   UPDATE clients will likely manage the records.

   Upon receiving any DNS UPDATE deleting resource records that might
   have been covered by a TIMEOUT RR, a primary server MUST remove all
   represented records in all of the TIMEOUT records with the same owner
   name, class, and represented type.

   A TIMEOUT resource record MUST be removed when the last resource
   record it covers has been removed.  This may be due to the record
   expiring (reaching the expiry time) or due to a subsequent DNS Update
   or administrative action.

   The TIMEOUT record TTL should use the default TTL for the zone like
   any other record.  The TTL values of the records covered by a TIMEOUT
   are not affected by the TIMEOUT expiry time and may be longer than
   the expiry time.  The TIMEOUT RR is mostly for the benefit of the
   authoritative server to know when to remove the records.  The fact
   that some records might live longer in the cache of a resolver is no
   different than other records that might get removed while still in a
   remote resolver cache.

7.2.  Secondary Server Behavior

   A secondary server MUST NOT expire the records in a zone it maintains
   covered by the TIMEOUT resource record and it MUST NOT expire the
   TIMEOUT resource record itself when the last record it covers has
   expired.  The secondary server MUST always wait for the records to be
   removed or updated by the primary server.

8.  TIMEOUT RDATA Presentation Format

   Record Type:
         resource record type mnemonics.  When the mnemonic is unknown,
         the TYPE is represented by the word "TYPE" immediately followed



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         by the decimal RR type number, with no intervening whitespace
         as described in [RFC3597], Section 5

   Represented Record Count:
         unsigned decimal integer (0-255)

   Method Identifier:
         unsigned decimal integer (0-255)

   Expiry Time:
         The Expiry Time is displayed as a compact numeric-only
         representation of ISO 8601.  All punctuation is removed.  This
         form is slightly different than the recommendation in [RFC3339]
         but is common for DNS protocols.  It is defined in [RFC4034],
         Section 3.2 as YYYYMMDDHHmmSS in UTC.  This form will always be
         exactly 14 digits since no component is optional.

         YYYY is the year;

         MM is the month number (01-12);

         DD is the day of the month (01-31);

         HH is the hour, in 24 hour notation (00-23);

         mm is the minute (00-59); and

         SS is the second (00-60) where 60 is only possible as a leap
         second.

   List of 0 or more hashes depending on Method Identifier:
         ( hash-1 hash2 ... )

         hash values shown as upper case hexadecimal string;

         some type of white space MUST exist between hash values but
         MUST NOT exist within hash value;

         MUST only display parentheses for one or more hash values;

9.  IANA Considerations

   This document defines a new DNS Resource Record Type named TIMEOUT to
   be exchanged between authoritative primary and secondary DNS servers.
   It is assigned out of the DNS Parameters Resource Record (RR) Type
   registry.  The value for the TIMEOUT resource record type is TBA.





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       +---------+-------+----------------------------+------------+
       |   Type  | Value | Meaning                    | Definition |
       +=========+=======+============================+============+
       | TIMEOUT | TBA   | expire represented records | Section 5  |
       +---------+-------+----------------------------+------------+

              Table 1: DNS Parameters Resource Record Registry

   This document establishes a new registry of DNS TIMEOUT Resource
   Record Method Identifier values.  The registry shall include a
   numeric identifier, a method name, a description of the method, and
   the length of the output function in bits or the keyword "variable".
   The identifier is to be used in the RDATA section of the TIMEOUT
   resource record.

   Initially, there are two values defined in the registry.  Values from
   240 (0xF0) through 255 (0xFF) are reserved for experimental use.

   +---------+---------------+----------------+----------+------------+
   |    ID   | Method Name   | Description    |  Length  | Definition |
   |         |               |                |  (bits)  |            |
   +=========+===============+================+==========+============+
   |    0    | NO METHOD     | All records    |    0     | Section    |
   |         |               | match          |          | 5.3.1      |
   +---------+---------------+----------------+----------+------------+
   |    1    | MD-SHA256-128 | List of        | 128 bits | Section    |
   |         |               | 128-bit hashes |          | 5.3.2      |
   |         |               | of represented |          |            |
   |         |               | records RDATA  |          |            |
   +---------+---------------+----------------+----------+------------+
   | 240-255 | EXPERIMENTAL  | Reserved for   | variable | Section 9  |
   |         |               | Experimental   |          |            |
   |         |               | Use            |          |            |
   +---------+---------------+----------------+----------+------------+

               Table 2: TIMEOUT RR Method Identifier values

10.  Security Considerations

   There is no secure relationship between a TIMEOUT resource record and
   the represented resource records it applies to.  TIMEOUT records
   should typically only apply to resource records created through the
   UPDATE mechanism.  Protection for permanent resource records in a
   zone is advisable.

   Authenticated UPDATE operations MUST be REQUIRED at authoritative
   name servers supporting TIMEOUT resource records.




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11.  Acknowledgments

   This idea was motivated through conversations with Mark Andrews.
   Thanks to Mark as well as Paul Vixie, Joe Abley, Ted Lemon, Tony
   Finch, Robert Story, Paul Wouters, Dick Franks, JINMEI, Tatuya,
   Timothe Litt, and Stuart Cheshire for their suggestions, review, and
   comments.

12.  References

12.1.  Normative References

   [FIPS180-4]
              National Institute of Standards and Technology, U.S.
              Department of Commerce, "Secure Hash Standard (SHS) FIPS
              180-4", August 2015,
              <http://nvlpubs.nist.gov/nistpubs/FIPS/
              NIST.FIPS.180-4.pdf>.

   [RFC1035]  Mockapetris, P., "Domain names - implementation and
              specification", STD 13, RFC 1035, DOI 10.17487/RFC1035,
              November 1987, <https://www.rfc-editor.org/info/rfc1035>.

   [RFC2119]  Bradner, S., "Key words for use in RFCs to Indicate
              Requirement Levels", BCP 14, RFC 2119,
              DOI 10.17487/RFC2119, March 1997,
              <https://www.rfc-editor.org/info/rfc2119>.

   [RFC3339]  Klyne, G. and C. Newman, "Date and Time on the Internet:
              Timestamps", RFC 3339, DOI 10.17487/RFC3339, July 2002,
              <https://www.rfc-editor.org/info/rfc3339>.

   [RFC3597]  Gustafsson, A., "Handling of Unknown DNS Resource Record
              (RR) Types", RFC 3597, DOI 10.17487/RFC3597, September
              2003, <https://www.rfc-editor.org/info/rfc3597>.

   [RFC4034]  Arends, R., Austein, R., Larson, M., Massey, D., and S.
              Rose, "Resource Records for the DNS Security Extensions",
              RFC 4034, DOI 10.17487/RFC4034, March 2005,
              <https://www.rfc-editor.org/info/rfc4034>.

   [RFC6895]  Eastlake 3rd, D., "Domain Name System (DNS) IANA
              Considerations", BCP 42, RFC 6895, DOI 10.17487/RFC6895,
              April 2013, <https://www.rfc-editor.org/info/rfc6895>.

   [RFC8174]  Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC
              2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174,
              May 2017, <https://www.rfc-editor.org/info/rfc8174>.



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12.2.  Informative References

   [I-D.sekar-dns-ul]
              Cheshire, S. and T. Lemon, "Dynamic DNS Update Leases",
              Work in Progress, Internet-Draft, draft-sekar-dns-ul-02, 2
              August 2018,
              <https://tools.ietf.org/html/draft-sekar-dns-ul-02>.

   [RFC1995]  Ohta, M., "Incremental Zone Transfer in DNS", RFC 1995,
              DOI 10.17487/RFC1995, August 1996,
              <https://www.rfc-editor.org/info/rfc1995>.

   [RFC2136]  Vixie, P., Ed., Thomson, S., Rekhter, Y., and J. Bound,
              "Dynamic Updates in the Domain Name System (DNS UPDATE)",
              RFC 2136, DOI 10.17487/RFC2136, April 1997,
              <https://www.rfc-editor.org/info/rfc2136>.

   [RFC5936]  Lewis, E. and A. Hoenes, Ed., "DNS Zone Transfer Protocol
              (AXFR)", RFC 5936, DOI 10.17487/RFC5936, June 2010,
              <https://www.rfc-editor.org/info/rfc5936>.

   [RFC6698]  Hoffman, P. and J. Schlyter, "The DNS-Based Authentication
              of Named Entities (DANE) Transport Layer Security (TLS)
              Protocol: TLSA", RFC 6698, DOI 10.17487/RFC6698, August
              2012, <https://www.rfc-editor.org/info/rfc6698>.

   [RFC6763]  Cheshire, S. and M. Krochmal, "DNS-Based Service
              Discovery", RFC 6763, DOI 10.17487/RFC6763, February 2013,
              <https://www.rfc-editor.org/info/rfc6763>.

   [RFC8555]  Barnes, R., Hoffman-Andrews, J., McCarney, D., and J.
              Kasten, "Automatic Certificate Management Environment
              (ACME)", RFC 8555, DOI 10.17487/RFC8555, March 2019,
              <https://www.rfc-editor.org/info/rfc8555>.

Appendix A.  Example TIMEOUT resource records

   The following example shows sample TIMEOUT resource records based on
   DNS UPDATEs containing A and AAAA address records plus the
   corresponding PTR records.

   A host sending a name registration at time Tn for "A" and "AAAA"
   records with lease lifetime Ln would have a series of UPDATEs (one
   for each zone) that contain:







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   +---------------------------------------------+----+----------------+
   | Name                                        | RR | Value          |
   |                                             |Type|                |
   +=============================================+====+================+
   | s.example.com.                              | A  | 192.0.2.5      |
   +---------------------------------------------+----+----------------+
   | s.example.com.                              |AAAA| 2001:db8::5    |
   +---------------------------------------------+----+----------------+
   | 5.2.0.192.in-addr.arpa.                     |PTR | s.example.com. |
   +---------------------------------------------+----+----------------+
   | 5.0.0.0.0.0.0.0.0.0.0.0.b8.d.1.20.ip6.arpa. |PTR | s.example.com. |
   +---------------------------------------------+----+----------------+

                  Table 3: Example Address Records Update

   Next, consider the TIMEOUT resource records that would be generated
   for the records in Table 3.

   +---------------------------------------------+----+---+---+--------+
   | Owner Name                                  |Type|Cnt|Mth| Expire |
   +=============================================+====+===+===+========+
   | s.example.com.                              | A  | 0 | 0 | Tn+Ln  |
   +---------------------------------------------+----+---+---+--------+
   | s.example.com.                              |AAAA| 0 | 0 | Tn+Ln  |
   +---------------------------------------------+----+---+---+--------+
   | 5.2.0.192.in-addr.arpa.                     |PTR | 0 | 0 | Tn+Ln  |
   +---------------------------------------------+----+---+---+--------+
   | 5.0.0.0.0.0.0.0.0.0.0.0.b8.d.1.20.ip6.arpa. |PTR | 0 | 0 | Tn+Ln  |
   +---------------------------------------------+----+---+---+--------+

                      Table 4: Address TIMEOUT records

   Next, assume there are two hosts advertising the same service type
   (different service types will have different owner names).  We will
   use _ipp._tcp.example.com as an example.

   Host A sends an UPDATE at time Ta with lease life La for PTR, SRV, A,
   AAAA, and TXT records.  Host B sends an UPDATE at time Tb with lease
   life Lb for PTR, SRV, A, and TXT records.












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    +---------------------------+---------+---------------------------+
    | Owner name                | RR Type | Value                     |
    +===========================+=========+===========================+
    | _ipp._tcp.example.com.    | PTR     | p1._ipp._tcp.example.com. |
    +---------------------------+---------+---------------------------+
    | p1._ipp._tcp.example.com. | SRV     | 0 0 631 p1.example.com.   |
    +---------------------------+---------+---------------------------+
    | p1._ipp._tcp.example.com. | TXT     | paper=A4                  |
    +---------------------------+---------+---------------------------+
    | p1.example.com.           | A       | 192.0.2.1                 |
    +---------------------------+---------+---------------------------+
    | p1.example.com.           | AAAA    | 2001:db8::1               |
    +---------------------------+---------+---------------------------+

                      Table 5: DNS UPDATE from Host A

    +---------------------------+---------+---------------------------+
    | Owner name                | RR Type | Value                     |
    +===========================+=========+===========================+
    | _ipp._tcp.example.com.    | PTR     | p2._ipp._tcp.example.com. |
    +---------------------------+---------+---------------------------+
    | p2._ipp._tcp.example.com. | SRV     | 0 0 631 p2.example.com.   |
    +---------------------------+---------+---------------------------+
    | p2._ipp._tcp.example.com. | TXT     | paper=B4                  |
    +---------------------------+---------+---------------------------+
    | p2.example.com.           | A       | 192.0.2.2                 |
    +---------------------------+---------+---------------------------+

                      Table 6: DNS UPDATE from Host B

   For these printer registrations, the TIMEOUT records on the server
   would look like the following:



















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   +-------------------------+----+-+-+--------------------------------+
   | Owner Name              |Type|C|M| Expire / Hash                  |
   +=========================+====+=+=+================================+
   | _ipp.tcp.example.com.   |PTR |1|1| Ta+La                          |
   |                         |    | | |69D67BCB98E8809702B9DFCA6B865558|
   +-------------------------+----+-+-+--------------------------------+
   | _ipp.tcp.example.com.   |PTR |1|1| Tb+Lb                          |
   |                         |    | | |7EBE34BC8B3E7306F8FCF1D6805331E1|
   +-------------------------+----+-+-+--------------------------------+
   |p1._ipp._tcp.example.com.|SRV |0|0| Ta + La                        |
   +-------------------------+----+-+-+--------------------------------+
   |p1._ipp._tcp.example.com.|TXT |0|0| Ta + La                        |
   +-------------------------+----+-+-+--------------------------------+
   |p2._ipp._tcp.example.com.|SRV |0|0| Tb + Lb                        |
   +-------------------------+----+-+-+--------------------------------+
   |p2._ipp._tcp.example.com.|TXT |0|0| Tb + Lb                        |
   +-------------------------+----+-+-+--------------------------------+
   | p1.example.com.         | A  |0|0| Ta + La                        |
   +-------------------------+----+-+-+--------------------------------+
   | p1.example.com.         |AAAA|0|0| Ta + La                        |
   +-------------------------+----+-+-+--------------------------------+
   | p2.example.com.         | A  |0|0| Tb + Lb                        |
   +-------------------------+----+-+-+--------------------------------+

                      Table 7: Service TIMEOUT records

Authors' Addresses

   Tom Pusateri
   Unaffiliated
   Raleigh, NC
   United States of America

   Email: pusateri@bangj.com


   Tim Wattenberg
   Unaffiliated
   Cologne
   Germany

   Email: mail@timwattenberg.de









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