ntp

Internet Engineering Task Force (IETF)                           R. Salz
Internet-Draft
Request for Comments: 9748                           Akamai Technologies
Updates: 5905, 5906, 8573, 7821, 7822, 7821 (if                 20 August 2024
         approved)
Intended status: Standards Track
Expires: 21 8573                      February 2025
Category: Standards Track
ISSN: 2070-1721

                      Updating the NTP Registries
                  draft-ietf-ntp-update-registries-16

Abstract

   The Network Time Protocol (NTP) and Network Time Security (NTS)
   documents define a number of assigned number registries, collectively called the NTP
   registries.

   Some registries have wrong values, are correct, but some registries do not follow
   current common practice, include incorrect assignments
   and some are just right. don’t follow common practice.  For the sake of completeness,
   this document reviews all NTP and NTS registries, and
   makes updates corrects the
   registries where necessary.

   This document updates RFC RFCs 5905, RFC 5906, RFC 8573, RFC 7821, 7822, and RFC
   7821.

Notes 8573.

Status of This Memo

   This note is to be removed before publishing as an RFC. Internet Standards Track document.

   This document is a product of the NTP Working Group
   (https://dt.ietf.org/wg/ntp).  Source for this draft and an issue
   tracker can be found at https://github.com/richsalz/draft-rsalz-
   update-registries.

   RFC Editor: Please update 'this RFC' to refer to this document, once
   its RFC number is known, through Internet Engineering Task Force
   (IETF).  It represents the document.

Status consensus of This Memo

   This Internet-Draft is submitted in full conformance with the
   provisions of BCP 78 IETF community.  It has
   received public review and BCP 79.

   Internet-Drafts are working documents of has been approved for publication by the
   Internet Engineering
   Task Force (IETF).  Note that other groups may also distribute
   working documents as Internet-Drafts.  The list Steering Group (IESG).  Further information on
   Internet Standards is available in Section 2 of RFC 7841.

   Information about the current Internet-
   Drafts is at https://datatracker.ietf.org/drafts/current/.

   Internet-Drafts are draft documents valid for a maximum status of six months this document, any errata,
   and how to provide feedback on it may be updated, replaced, or obsoleted by other documents obtained 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 21 February 2025.
   https://www.rfc-editor.org/info/rfc9748.

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   Copyright (c) 2024 2025 IETF Trust and the persons identified as the
   document authors.  All rights reserved.

   This document is subject to BCP 78 and the IETF Trust's Legal
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   in the Revised BSD License.

Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   2
   2.  Existing Registries . . . . . . . . . . . . . . . . . . . . .   3
     2.1.  Reference ID, ID and Kiss-o'-Death . . . . . . . . . . . . . . .   3 Registries
     2.2.  Extension Field Types . . . . . . . . . . . . . . . . . .   3
     2.3.  Network Time Security Registries  . . . . . . . . . . . .   4
   3.  Updated Registries  . . . . . . . . . . . . . . . . . . . . .   5  NTP Registry Updates
     3.1.  Guidance to  Designated Experts  . . . . . . . . . . . . .   5
   4.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .   5
     4.1.  NTP Reference Identifier Codes  . . . . . . . . . . . . .   5
     4.2.  NTP Kiss-o'-Death Codes . . . . . . . . . . . . . . . . .   6
     4.3.  NTP Extension Field Types . . . . . . . . . . . . . . . .   6
   5.  Security Considerations . . . . . . . . . . . . . . . . . . .  10
   6.  Acknowledgements  . . . . . . . . . . . . . . . . . . . . . .  10
   7.  Normative References  . . . . . . . . . . . . . . . . . . . .  10
   Acknowledgements
   Author's Address  . . . . . . . . . . . . . . . . . . . . . . . .  11

1.  Introduction

   The Network Time Protocol (NTP) and Network Time Security (NTS)
   documents define a number of assigned number registries, collectively called the NTP
   registries.  The NTP registries can all be found at
   https://www.iana.org/assignments/ntp-parameters/ntp-parameters.xhtml
   (https://www.iana.org/assignments/ntp-parameters/ntp-
   parameters.xhtml)
   <https://www.iana.org/assignments/ntp-parameters> and the NTS
   registries can all be found at
   https://www.iana.org/assignments/nts/nts.xhtml
   (https://www.iana.org/assignments/nts/nts.xhtml). <https://www.iana.org/assignments/
   nts>.

   Some registries have wrong values, are correct, but some registries do not follow
   current common practice, include incorrect assignments
   and some are just right. don’t follow common practice.  For the sake of completeness,
   this document reviews all NTP and NTS registries, and
   makes updates corrects the
   registries where necessary.

   The bulk of this document can be divided into two parts:

   *  First, each registry, its defining document, and  a summary of its the relevant registries, including syntax is defined.

   *  Second,
      requirements, registration procedures, and the defining documents.

   *  a revised format and entries for each registry that is being modified is specified. modified.

2.  Existing Registries

   This section describes the registries and the rules for them.  It is
   intended to be a short summary of the syntax and registration
   requirements for each registry.  The semantics and protocol
   processing rules for each registry -- that is, how an implementation
   acts when sending or receiving any of the fields -- are not described
   here.

2.1.  Reference ID, ID and Kiss-o'-Death Registries

   [RFC5905] defined defines two registries; the registries: "NTP Reference ID Identifier Codes" in
   Section 7.3, 7.3 and the "NTP Kiss-o'-Death Codes" in Section 7.4.  Both of these are allowed to
   Reference identifiers and kiss codes can be up to four ASCII characters;
   characters, padded on the right with all-bits-zero if necessary.
   Entries that start with 0x58, the ASCII letter uppercase X, are
   reserved for Private or Experimental Use. Both registries are
   first-come first-served. First
   Come First Served.  The formal request to define the registries
   is in [RFC5905], were created per Section 16. 16 of
   [RFC5905].

2.2.  Extension Field Types

   [RFC5905],

   Section 7.5 defined of [RFC5905] defines the on-the-wire format of extension
   fields but did does not create a registry for them.

   [RFC5906],

   Section 13 mentioned of [RFC5906] mentions the "NTP Extension Field Types Types"
   registry, and defined defines it indirectly by defining 30 extensions (10
   each for request, response, and error response).  It did does not provide
   a formal definition of the columns in the registry.  [RFC5906],  Section 10 of
   [RFC5906] splits the Field Type into four subfields, only for use
   within the Autokey extensions.

   [RFC7821] added adds a new entry, Checksum Complement, to the "NTP
   Extension Field Types Types" registry.

   [RFC7822] clarified clarifies the processing rules for Extension Field Types,
   particularly around the interaction with the Message Authentication
   Code (MAC) field.  NTPv4 packets may contain a MAC that appears where
   one would expect the next extension field header.

   [RFC8573] changed changes the cryptography used in the MAC field.

   [RFC8915] added adds four new entries to the "NTP Extension Field Types Types"
   registry.

   The following problems exist with the current registry:

   *  Many of the entries in the "NTP Extension Field Types Types" registry
      have swapped some of the nibbles; 0x1234 is listed as 0x1432 for
      example.  This example, 0x0302 was listed
      for Cookie Message Request instead of 0x0203.  The errors are due
      to documentation errors with the original implementation of
      Autokey.  This document marks the erroneous values as reserved, in
      case there is an implementation that used using the registered values
      instead of what the original implementation used.  Applications
      that might have used those values would have realized that they did not
      interoperate with the dominant (if not only) implementation at the
      time.  Marking the values as reserved ensures that any such
      applications would still be able continue to work as- as is.

   *  Some values were mistakenly re-used. reused.

2.3.  Network Time Security Registries

   [RFC8915] defines the NTS protocol.  Its  The related registries are
   listed here for completeness, but there are no changes to them are specified in
   this document.

   In [RFC8915]:

   Sections 7.1 through 7.5 (inclusive) added entries to existing
   registries.

   Section 7.6 created a new registry, NTS the "Network Time Security Key Establishment
   Record
   Types, Types" registry that partitions the assigned numbers range into three different
   registration policies: IETF Review, Specification Required, and
   Private or Experimental Use.

   Section 7.7 created a new registry, NTS the "Network Time Security Next Protocols, Protocols"
   registry that similarly partitions the assigned numbers. range.

   Section 7.8 created two new registries, NTS the "Network Time Security Error Codes Codes" and NTS
   "Network Time Security Warning Codes. Codes" registries.  Both registries
   are also partitioned the same way.

3.  Updated Registries  NTP Registry Updates

   The following general guidelines apply to all registries updated
   here: the NTP registries:

   *  Every registry reserves a  A partition of the "NTP Extension Field Types" registry is
      reserved for Private or Experimental Use.

   *  Entries  In the "NTP Reference Identifier Codes" and "NTP Kiss-o'-Death
      Codes" registries, entries with ASCII fields are now limited to
      uppercase letters or
      digits; fields digits.  Fields starting with 0x58, the
      uppercase letter "X", are reserved for Private or Experimental
      Use.

   *  The policy for every each registry is now Specification Required, as
      defined in [RFC8126], Section 4.6.

3.1.  Designated Experts

   The IESG is requested to choose three designated experts, experts (DEs), with
   approvals from two being required to approve implement a registry change.  Guidance
   for such the experts is given below.

   Each entry described in the sub-sections below is intended to
   completely replace the existing entry with the same name.

3.1.  Guidance to Designated Experts

   The designated experts (DE) DEs should be familiar with [RFC8126], particularly Section 5.
   As that reference suggests, the DE should ascertain the existence of
   a suitable specification, specification and verify that it is publicly available.
   The DE is also expected to check the clarity of purpose and use of
   the requested code points.

   In addition, the DE is expected to be familiar with this document,
   specifically the history documented here.

4.  IANA Considerations

   Each entry described in the subsections below is intended to
   completely replace the existing entry with the same name.

4.1.  NTP Reference Identifier Codes

   The registration procedure is has been changed to Specification Required. Required
   and this document has been added as a reference.

   The Note is has been changed to read as follows:

   *

   |  Codes beginning with the character "X" are reserved for
   |  experimentation and development.  IANA cannot assign them.

   The columns are defined as follows:

   *

   ID (required):  a four-byte value padded on the right with all-
      bits-zero. all-bits-
      zero.  Each byte other than padding must be an ASCII uppercase letter
      letters or digits.

   *
   Clock source (required): A  a brief text description of the ID.

   *
   Reference (required):  the publication defining the ID.

   The existing entries are left unchanged.

4.2.  NTP Kiss-o'-Death Codes

   The registration procedure is changed to Specification Required. Required and
   this document has been added as a reference.

   The Note is has been changed to read as follows:

   *

   |  Codes beginning with the character "X" are reserved for
   |  experimentation and development.  IANA cannot assign them.

   The columns are defined as follows:

   *

   ID (required):  a four-byte value padded on the right with all-
      bits-zero. all-bits-
      zero.  Each byte other than padding must be an ASCII uppercase letter
      letters or digits.

   *
   Meaning source (required): A  a brief text description of the ID.

   *
   Reference (required):  the publication defining the ID.

   The existing entries are left unchanged.

4.3.  NTP Extension Field Types

   The registration procedure is has been changed to Specification Required.

   The reference Required
   and [RFC5906] should be added, if possible. and this document have been added as references.

   The following two Notes are have been added:

   *

   |  Field Types in the range 0xF000 through 0xFFFF, inclusive, are
   |  reserved for experimentation and development.  IANA cannot assign
   |  them.  Both NTS Cookie and Autokey Message Request have the same
   |  Field Type; in practice this is not a problem as the field
   |  semantics will be determined by other parts of the message.

   *

   |  The "Reserved for historic reasons" is for differences between the
   |  original documentation and implementation of Autokey and marks the
   |  erroneous values as reserved, in case there is an implementation
   |  that used the registered values instead of what the original
   |  implementation used.

   The columns are defined as follows:

   *

   Field Type (required): A  a two-byte value in hexadecimal.

   *
   Meaning (required): A  a brief text description of the field type.

   *
   Reference (required):  the publication defining the field type.

   The table is replaced with the following entries.

   IANA is requested
   to replace "This RFC" with has updated the actual RFC number once assigned.

     +============+====================================+=============+ registry as shown in Table 1.

   +===============+====================================+=============+
   | Field Type    | Meaning                            | Reference   |
     +============+====================================+=============+
   +===============+====================================+=============+
   | 0x0000        | Crypto-NAK; authentication failure | RFC 5905 [RFC5905]   |
     +------------+------------------------------------+-------------+
   +---------------+------------------------------------+-------------+
   | 0x0002        | Reserved for historic reasons      | This RFC 9748    |
     +------------+------------------------------------+-------------+
   +---------------+------------------------------------+-------------+
   | 0x0102        | Reserved for historic reasons      | This RFC 9748    |
     +------------+------------------------------------+-------------+
   +---------------+------------------------------------+-------------+
   | 0x0104        | Unique Identifier                  | RFC 8915, [RFC8915],  |
   |               |                                    | Section 5.3 |
     +------------+------------------------------------+-------------+
   +---------------+------------------------------------+-------------+
   | 0x0200        | No-Operation Request               | RFC 5906 [RFC5906]   |
     +------------+------------------------------------+-------------+
   +---------------+------------------------------------+-------------+
   | 0x0201        | Association Message Request        | RFC 5906 [RFC5906]   |
     +------------+------------------------------------+-------------+
   +---------------+------------------------------------+-------------+
   | 0x0202        | Certificate Message Request        | RFC 5906 [RFC5906]   |
     +------------+------------------------------------+-------------+
   +---------------+------------------------------------+-------------+
   | 0x0203        | Cookie Message Request             | RFC 5906 [RFC5906]   |
     +------------+------------------------------------+-------------+
   +---------------+------------------------------------+-------------+
   | 0x0204        | Autokey Message Request            | RFC 5906 [RFC5906]   |
     +------------+------------------------------------+-------------+
   +---------------+------------------------------------+-------------+
   | 0x0204        | NTS Cookie                         | RFC 8915, [RFC8915],  |
   |               |                                    | Section 5.4 |
     +------------+------------------------------------+-------------+
   +---------------+------------------------------------+-------------+
   | 0x0205        | Leapseconds Message Request        | RFC 5906 [RFC5906]   |
     +------------+------------------------------------+-------------+
   +---------------+------------------------------------+-------------+
   | 0x0206        | Sign Message Request               | RFC 5906 [RFC5906]   |
     +------------+------------------------------------+-------------+
   +---------------+------------------------------------+-------------+
   | 0x0207        | IFF Identity Message Request       | RFC 5906 [RFC5906]   |
     +------------+------------------------------------+-------------+
   +---------------+------------------------------------+-------------+
   | 0x0208        | GQ Identity Message Request        | RFC 5906 [RFC5906]   |
     +------------+------------------------------------+-------------+
   +---------------+------------------------------------+-------------+
   | 0x0209        | MV Identity Message Request        | RFC 5906 [RFC5906]   |
     +------------+------------------------------------+-------------+
   +---------------+------------------------------------+-------------+
   | 0x0302        | Reserved for historic reasons      | This RFC 9748    |
     +------------+------------------------------------+-------------+
   +---------------+------------------------------------+-------------+
   | 0x0304        | NTS Cookie Placeholder             | RFC 8915, [RFC8915],  |
   |               |                                    | Section 5.5 |
     +------------+------------------------------------+-------------+
   +---------------+------------------------------------+-------------+
   | 0x0402        | Reserved for historic reasons      | This RFC 9748    |
     +------------+------------------------------------+-------------+
   +---------------+------------------------------------+-------------+
   | 0x0404        | NTS Authenticator and Encrypted    | RFC 8915, [RFC8915],  |
   |               | Extension Fields                   | Section 5.6 |
     +------------+------------------------------------+-------------+
   +---------------+------------------------------------+-------------+
   | 0x0502        | Reserved for historic reasons      | This RFC 9748    |
     +------------+------------------------------------+-------------+
   +---------------+------------------------------------+-------------+
   | 0x0602        | Reserved for historic reasons      | This RFC 9748    |
     +------------+------------------------------------+-------------+
   +---------------+------------------------------------+-------------+
   | 0x0702        | Reserved for historic reasons      | This RFC 9748    |
   +---------------+------------------------------------+-------------+
   | 0x0802        | Reserved for historic reasons      | RFC 9748    |
     +------------+------------------------------------+-------------+
   +---------------+------------------------------------+-------------+
   | 0x0902        | Reserved for historic reasons      | This RFC 9748    |
     +------------+------------------------------------+-------------+
   +---------------+------------------------------------+-------------+
   | 0x2005        | UDP Checksum Complement            | RFC 7821 [RFC7821]   |
     +------------+------------------------------------+-------------+
   +---------------+------------------------------------+-------------+
   | 0x8002        | Reserved for historic reasons      | This RFC 9748    |
     +------------+------------------------------------+-------------+
   +---------------+------------------------------------+-------------+
   | 0x8102        | Reserved for historic reasons      | This RFC 9748    |
     +------------+------------------------------------+-------------+
   +---------------+------------------------------------+-------------+
   | 0x8200        | No-Operation Response              | RFC 5906 [RFC5906]   |
     +------------+------------------------------------+-------------+
   +---------------+------------------------------------+-------------+
   | 0x8201        | Association Message Response       | RFC 5906 [RFC5906]   |
     +------------+------------------------------------+-------------+
   +---------------+------------------------------------+-------------+
   | 0x8202        | Certificate Message Response       | RFC 5906 [RFC5906]   |
     +------------+------------------------------------+-------------+
   +---------------+------------------------------------+-------------+
   | 0x8203        | Cookie Message Response            | RFC 5906 [RFC5906]   |
     +------------+------------------------------------+-------------+
   +---------------+------------------------------------+-------------+
   | 0x8204        | Autokey Message Response           | RFC 5906 [RFC5906]   |
     +------------+------------------------------------+-------------+
   +---------------+------------------------------------+-------------+
   | 0x8205        | Leapseconds Message Response       | RFC 5906 [RFC5906]   |
     +------------+------------------------------------+-------------+
   +---------------+------------------------------------+-------------+
   | 0x8206        | Sign Message Response              | RFC 5906 [RFC5906]   |
     +------------+------------------------------------+-------------+
   +---------------+------------------------------------+-------------+
   | 0x8207        | IFF Identity Message Response      | RFC 5906 [RFC5906]   |
     +------------+------------------------------------+-------------+
   +---------------+------------------------------------+-------------+
   | 0x8208        | GQ Identity Message Response       | RFC 5906 [RFC5906]   |
     +------------+------------------------------------+-------------+
   +---------------+------------------------------------+-------------+
   | 0x8209        | MV Identity Message Response       | RFC 5906 [RFC5906]   |
     +------------+------------------------------------+-------------+
   +---------------+------------------------------------+-------------+
   | 0x8302        | Reserved for historic reasons      | This RFC 9748    |
     +------------+------------------------------------+-------------+
   +---------------+------------------------------------+-------------+
   | 0x8402        | Reserved for historic reasons      | This RFC 9748    |
     +------------+------------------------------------+-------------+
   +---------------+------------------------------------+-------------+
   | 0x8502        | Reserved for historic reasons      | This RFC 9748    |
     +------------+------------------------------------+-------------+
   +---------------+------------------------------------+-------------+
   | 0x8602        | Reserved for historic reasons      | This RFC 9748    |
     +------------+------------------------------------+-------------+
   +---------------+------------------------------------+-------------+
   | 0x8702        | Reserved for historic reasons      | This RFC 9748    |
     +------------+------------------------------------+-------------+
   +---------------+------------------------------------+-------------+
   | 0x8802        | Reserved for historic reasons      | This RFC 9748    |
     +------------+------------------------------------+-------------+
   +---------------+------------------------------------+-------------+
   | 0x8902        | Reserved for historic reasons      | This RFC 9748    |
     +------------+------------------------------------+-------------+
   +---------------+------------------------------------+-------------+
   | 0xC002        | Reserved for historic reasons      | This RFC 9748    |
     +------------+------------------------------------+-------------+
   +---------------+------------------------------------+-------------+
   | 0xC102        | Reserved for historic reasons      | This RFC 9748    |
     +------------+------------------------------------+-------------+
   +---------------+------------------------------------+-------------+
   | 0xC200        | No-Operation Error Response        | RFC 5906 [RFC5906]   |
     +------------+------------------------------------+-------------+
   +---------------+------------------------------------+-------------+
   | 0xC201        | Association Message Error Response | RFC 5906 [RFC5906]   |
     +------------+------------------------------------+-------------+
   +---------------+------------------------------------+-------------+
   | 0xC202        | Certificate Message Error Response | RFC 5906 [RFC5906]   |
     +------------+------------------------------------+-------------+
   +---------------+------------------------------------+-------------+
   | 0xC203        | Cookie Message Error Response      | RFC 5906 [RFC5906]   |
     +------------+------------------------------------+-------------+
   +---------------+------------------------------------+-------------+
   | 0xC204        | Autokey Message Error Response     | RFC 5906 [RFC5906]   |
     +------------+------------------------------------+-------------+
   +---------------+------------------------------------+-------------+
   | 0xC205        | Leapseconds Message Error Response | RFC 5906 [RFC5906]   |
     +------------+------------------------------------+-------------+
   +---------------+------------------------------------+-------------+
   | 0xC206        | Sign Message Error Response        | RFC 5906 [RFC5906]   |
     +------------+------------------------------------+-------------+
   +---------------+------------------------------------+-------------+
   | 0xC207        | IFF Identity Message Error         | RFC 5906 [RFC5906]   |
   |               | Response                           |             |
     +------------+------------------------------------+-------------+
   +---------------+------------------------------------+-------------+
   | 0xC208        | GQ Identity Message Error Response | RFC 5906 [RFC5906]   |
     +------------+------------------------------------+-------------+
   +---------------+------------------------------------+-------------+
   | 0xC209        | MV Identity Message Error Response | RFC 5906 [RFC5906]   |
     +------------+------------------------------------+-------------+
   +---------------+------------------------------------+-------------+
   | 0xC302        | Reserved for historic reasons      | This RFC 9748    |
     +------------+------------------------------------+-------------+
   +---------------+------------------------------------+-------------+
   | 0xC402        | Reserved for historic reasons      | This RFC 9748    |
     +------------+------------------------------------+-------------+
   +---------------+------------------------------------+-------------+
   | 0xC502        | Reserved for historic reasons      | This RFC 9748    |
     +------------+------------------------------------+-------------+
   +---------------+------------------------------------+-------------+
   | 0xC602        | Reserved for historic reasons      | This RFC 9748    |
     +------------+------------------------------------+-------------+
   +---------------+------------------------------------+-------------+
   | 0xC702        | Reserved for historic reasons      | This RFC 9748    |
     +------------+------------------------------------+-------------+
   +---------------+------------------------------------+-------------+
   | 0xC802        | Reserved for historic reasons      | This RFC 9748    |
     +------------+------------------------------------+-------------+
   +---------------+------------------------------------+-------------+
   | 0xC902        | Reserved for historic reasons      | This RFC 9748    |
     +------------+------------------------------------+-------------+
   +---------------+------------------------------------+-------------+
   | 0xF000- 0xF000-0xFFFF | Reserved for Experimental Use      | This RFC 9748    |
     | 0xFFFF     |                                    |             |
     +------------+------------------------------------+-------------+
   +---------------+------------------------------------+-------------+

                                 Table 1

5.  Security Considerations

   This document adds no new security considerations, as they are
   defined in the document that defines the extension.  See the
   References column of the appropriate table.

6.  Acknowledgements

   The members of the NTP Working Group helped a great deal.  Notable
   contributors include:

   *  Miroslav Lichvar, Red Hat

   *  Daniel Franke, formerly at Akamai Technologies

   *  Danny Mayer, Network Time Foundation

   *  Michelle Cotton, formerly at IANA

   *  Tamme Dittrich, Tweede Golf

7. registry.

6.  Normative References

   [RFC5905]  Mills, D., Martin, J., Ed., Burbank, J., and W. Kasch,
              "Network Time Protocol Version 4: Protocol and Algorithms
              Specification", RFC 5905, DOI 10.17487/RFC5905, June 2010,
              <https://www.rfc-editor.org/rfc/rfc5905>.
              <https://www.rfc-editor.org/info/rfc5905>.

   [RFC5906]  Haberman, B., Ed. and D. Mills, "Network Time Protocol
              Version 4: Autokey Specification", RFC 5906,
              DOI 10.17487/RFC5906, June 2010,
              <https://www.rfc-editor.org/rfc/rfc5906>.
              <https://www.rfc-editor.org/info/rfc5906>.

   [RFC7821]  Mizrahi, T., "UDP Checksum Complement in the Network Time
              Protocol (NTP)", RFC 7821, DOI 10.17487/RFC7821, March
              2016, <https://www.rfc-editor.org/rfc/rfc7821>. <https://www.rfc-editor.org/info/rfc7821>.

   [RFC7822]  Mizrahi, T. and D. Mayer, "Network Time Protocol Version 4
              (NTPv4) Extension Fields", RFC 7822, DOI 10.17487/RFC7822,
              March 2016, <https://www.rfc-editor.org/rfc/rfc7822>. <https://www.rfc-editor.org/info/rfc7822>.

   [RFC8126]  Cotton, M., Leiba, B., and T. Narten, "Guidelines for
              Writing an IANA Considerations Section in RFCs", BCP 26,
              RFC 8126, DOI 10.17487/RFC8126, June 2017,
              <https://www.rfc-editor.org/rfc/rfc8126>.
              <https://www.rfc-editor.org/info/rfc8126>.

   [RFC8573]  Malhotra, A. and S. Goldberg, "Message Authentication Code
              for the Network Time Protocol", RFC 8573,
              DOI 10.17487/RFC8573, June 2019,
              <https://www.rfc-editor.org/rfc/rfc8573>.
              <https://www.rfc-editor.org/info/rfc8573>.

   [RFC8915]  Franke, D., Sibold, D., Teichel, K., Dansarie, M., and R.
              Sundblad, "Network Time Security for the Network Time
              Protocol", RFC 8915, DOI 10.17487/RFC8915, September 2020,
              <https://www.rfc-editor.org/rfc/rfc8915>.
              <https://www.rfc-editor.org/info/rfc8915>.

Acknowledgements

   The members of the NTP Working Group helped a great deal.  Notable
   contributors include:

   *  Miroslav Lichvar, Red Hat

   *  Daniel Franke, formerly at Akamai Technologies

   *  Danny Mayer, Network Time Foundation

   *  Michelle Cotton, formerly at IANA

   *  Tamme Dittrich, Tweede Golf

Author's Address

   Rich Salz
   Akamai Technologies
   Email: rsalz@akamai.com