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The Network Time Protocol Version 4 (NTPv4) Extension Fields
draft-ietf-ntp-extension-field-03

The information below is for an old version of the document.
Document Type
This is an older version of an Internet-Draft that was ultimately published as RFC 7822.
Authors Tal Mizrahi , Danny Mayer
Last updated 2015-08-14 (Latest revision 2015-06-28)
RFC stream Internet Engineering Task Force (IETF)
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Stream WG state Submitted to IESG for Publication
Document shepherd Karen O'Donoghue
Shepherd write-up Show Last changed 2015-08-13
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Responsible AD Brian Haberman
Send notices to ntp-chairs@ietf.org, draft-ietf-ntp-extension-field.shepherd@ietf.org, kodonog@pobox.com, draft-ietf-ntp-extension-field.ad@ietf.org, draft-ietf-ntp-extension-field@ietf.org
draft-ietf-ntp-extension-field-03
NTP Working Group                                             T. Mizrahi
Internet Draft                                                   Marvell
Intended status: Standards Track                                D. Mayer
Updates: 5905                                    Network Time Foundation
Expires: December 2015                                     June 29, 2015

        The Network Time Protocol Version 4 (NTPv4) Extension Fields
                   draft-ietf-ntp-extension-field-03.txt

Abstract

   The Network Time Protocol Version 4 (NTPv4) defines the optional
   usage of extension fields. An extension field, defined in RFC5905, is
   an optional field that resides at the end of the NTP header, and can
   be used to add optional capabilities or additional information that
   is not conveyed in the standard NTP header. This document updates
   RFC5905 by clarifying some points regarding NTP extension fields and
   their usage with Message Authentication Codes (MAC).

Status of this Memo

   This Internet-Draft is submitted to IETF in full conformance with the
   provisions of BCP 78 and BCP 79.

   Internet-Drafts are working documents of the Internet Engineering
   Task Force (IETF), its areas, and its working groups.  Note that
   other groups may also distribute working documents as Internet-
   Drafts.

   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."

   The list of current Internet-Drafts can be accessed at
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   The list of Internet-Draft Shadow Directories can be accessed at
   http://www.ietf.org/shadow.html.

   This Internet-Draft will expire on December 29, 2015.

Copyright Notice

   Copyright (c) 2015 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
   (http://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.

Table of Contents

   1. Introduction ................................................. 2
   2. Conventions Used in this Document ............................ 3
      2.1. Terminology ............................................. 3
      2.2. Terms & Abbreviations ................................... 3
   3. NTP Extension Fields - RFC 5905 Update ....................... 3
   4. Security Considerations ...................................... 6
   5. IANA Considerations .......................................... 6
   6. Acknowledgments .............................................. 6
   7. References ................................................... 7
      7.1. Normative References .................................... 7
      7.2. Informative References .................................. 7
   Appendix A. Requirements from NTPv4 and Autokey ................. 7
      A.1. NTP Extension Field for Future Extensions ............... 7
      A.2. NTP Extension Field with or without a MAC ............... 7
      A.3. The NTP Extension Field Format .......................... 8
      A.4. NTP Extension Field in Autokey .......................... 8

1. Introduction

   The NTP header format consists of a set of fixed fields that may be
   followed by some optional fields. Two types of optional fields are
   defined, Message Authentication Codes (MAC), and extension fields
   (Appendix A.3.).

   If a MAC is used, it resides at the end of the packet. This field can
   be either 24 octets long, 20 octets long, or a 4-octet crypto-NAK.

   NTP extension fields were defined in [RFC5905] as a generic mechanism
   that allows to add future extensions and features without modifying
   the NTP header format (Appendix A.1.).

   The only currently defined extension field is the one used by the
   AutoKey protocol [RFC5906]. This extension field is always followed
   by a MAC, and [RFC5906] specifies the parsing rules that allow a host
   to distinguish between an extension field and a MAC (Appendix A.4.).

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   However, a MAC is not mandatory after an extension field; an NTPv4
   packet can include one or more extension fields without including a
   MAC (Appendix A.2.).

   This document updates [RFC5905] by clarifying some points regarding
   the usage of extension fields. Specifically, this document updates
   Section 7.5 of [RFC5905], clarifying the relationship between
   extension fields and MACs, and defining the behavior of a host that
   receives an unknown extension field.

2. Conventions Used in this Document

2.1. Terminology

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
   document are to be interpreted as described in [KEYWORDS].

2.2. Terms & Abbreviations

   NTPv4        Network Time Protocol Version 4 [RFC5905]

   MAC          Message Authentication Code

3. NTP Extension Fields - RFC 5905 Update

   This document updates Section 7.5 of [RFC5905] and [RFC5905Err] as
   follows:

   OLD:

   7.5. NTP Extension Field Format

   In NTPv4, one or more extension fields can be inserted after the
   header and before the MAC, if a MAC is present. If a MAC is not
   present, one or more extension fields can be inserted after the
   header, according to the following rules:

   o If the packet includes a single extension field, the length of the
      extension field MUST be at least 7 words, i.e., at least 28
      octets.

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   o If the packet includes more than one extension field, the length
      of the last extension field MUST be at least 28 octets. The length
      of the other extension fields in this case MUST be at least 16
      octets each.

   Other than defining the field format, this document makes no use of
   the field contents.  An extension field contains a request or
   response message in the format shown in Figure 14.

       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
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |          Field Type           |            Length             |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      .                                                               .
      .                            Value                              .
      .                                                               .
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |                       Padding (as needed)                     |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
                     Figure 14: Extension Field Format

   All extension fields are zero-padded to a word (four octets)
   boundary.  The Field Type field is specific to the defined function
   and is not elaborated here.  While the minimum field length
   containing required fields is four words (16 octets), a maximum field
   length remains to be established.

   The Length field is a 16-bit unsigned integer that indicates the
   length of the entire extension field in octets, including the Padding
   field.

   NEW:

   7.5. NTP Extension Field Format

   In NTPv4, one or more extension fields can be inserted after the
   header and before the MAC, if a MAC is present.

   Other than defining the field format, this document makes no use of
   the field contents.  An extension field contains a request or
   response message in the format shown in Figure 14.

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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
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |          Field Type           |            Length             |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      .                                                               .
      .                            Value                              .
      .                                                               .
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |                       Padding (as needed)                     |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
                     Figure 14: Extension Field Format

   All extension fields are zero-padded to a word (four octets)
   boundary.

   The Field Type field is specific to the defined function and is not
   elaborated here. If a host receives an extension field with an
   unknown Field Type value, the host SHOULD ignore the extension field
   and MAY drop the packet altogether if policy requires it. Note that
   in the presence of an unknown extension field any MAC that may be
   present may be misinterpreted as an unknown extension though in this
   case the apparent extension length will be inconsistent with the
   total length of the rest of the packet.

   While the minimum field length containing required fields is four
   words (16 octets), the maximum field length cannot be longer than
   65532 octets due to the maximum size of the length field.

   The Length field is a 16-bit unsigned integer that indicates the
   length of the entire extension field in octets, including the Padding
   field.

   7.5.1 Extension Fields and MACs

   7.5.1.1 Extension Fields in the Presence of a MAC

   An extension field can be used in an NTP packet that includes a MAC,
   for example, as defined in [RFC5906]. A specification that defines a
   new extension field MUST specify whether the extension field requires
   a MAC or not. If the extension field requires a MAC, the extension
   field specification MUST define the algorithm to be used to create
   the MAC and the length of the MAC thus created. An extension field
   MAY allow for more than one algorithm to be used in which case the

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   information about which one was used MUST be included in the
   extension field itself.

   7.5.1.2 Multiple Extension Fields with a MAC

   If there are multiple extension fields that require a MAC they MUST
   all require use of the same algorithm and MAC length. Extension
   fields that do not require a MAC can be included with extension
   fields that do require a MAC.

   7.5.1.3 MAC in the absence of an Extension field

   A MAC MUST NOT be longer than 24 octets if there is no extension
   field present unless through a previous exchange of packets with an
   extension field which defines the size and algorithm of the MAC
   transmitted in the packet and is agreed upon by both client and
   server.

   7.5.1.4 Extension Fields in the Absence of a MAC

   If a MAC is not present, one or more extension fields can be inserted
   after the header, according to the following rules:

   o If the packet includes a single extension field, the length of the
      extension field MUST be at least 7 words, i.e., at least 28
      octets.

   o If the packet includes more than one extension field, the length
      of the last extension field MUST be at least 28 octets. The length
      of the other extension fields in this case MUST be at least 16
      octets each.

4. Security Considerations

   The security considerations of the network time protocol are
   discussed in [RFC5905]. This document clarifies some ambiguity with
   regards to the usage of the NTP extension field, and thus the
   behavior described in this document does not introduce new security
   considerations.

5. IANA Considerations

   There are no new IANA considerations implied by this document.

6. Acknowledgments

   The authors thank Dave Mills for his insightful comments.

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   This document was prepared using 2-Word-v2.0.template.dot.

7. References

7.1. Normative References

   [KEYWORDS]    Bradner, S., "Key words for use in RFCs to Indicate
                 Requirement Levels", BCP 14, RFC 2119, March 1997.

   [RFC5905]     Mills, D., Martin, J., Burbank, J., Kasch, W.,
                 "Network Time Protocol Version 4: Protocol and
                 Algorithms Specification", RFC 5905, June 2010.

   [RFC5905Err]  RFC 5905 Technical Erratum 3627, May 2014.

7.2. Informative References

   [RFC5906]     Haberman, B., Mills, D., "Network Time Protocol
                 Version 4: Autokey Specification", RFC 5906, June
                 2010.

   [RFC5906ERR]  RFC 5906 Technical Erratum 4026, July 2014..

Appendix A.                 Requirements from NTPv4 and Autokey

A.1. NTP Extension Field for Future Extensions

   The following paragraph is quoted from Section 16 of [RFC5905].

   This document introduces NTP extension fields allowing for the
   development of future extensions to the protocol, where a particular
   extension is to be identified by the Field Type sub-field within the
   extension field.

A.2. NTP Extension Field with or without a MAC

   The following paragraph is quoted from Section 7.5 of [RFC5905], as
   updated by [RFC5905Err].

   In NTPv4, one or more extension fields can be inserted after the
   header and before the MAC, if a MAC is present. If a MAC is not
   present, one or more extension fields can be inserted after the
   header, according to the following rules:

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   o If the packet includes a single extension field, the length of the
      extension field MUST be at least 7 words, i.e., at least 28
      octets.

   o If the packet includes more than one extension field, the length
      of the last extension field MUST be at least 28 octets. The length
      of the other extension fields in this case MUST be at least 16
      octets each.

A.3. The NTP Extension Field Format

   The NTP extension field format, presented below, is quoted from
   [RFC5905]. For further details refer to [RFC5905].

       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
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |          Field Type           |            Length             |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      .                                                               .
      .                            Value                              .
      .                                                               .
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |                       Padding (as needed)                     |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
                     Figure 14: Extension Field Format

A.4. NTP Extension Field in Autokey

   The following paragraph is quoted from Section 10 of [RFC5906], as
   updated by [RFC5906ERR].

   One or more extension fields follow the NTP packet header and the
   last followed by the MAC.  The extension field parser initializes a
   pointer to the first octet beyond the NTP packet header and
   calculates the number of octets remaining to the end of the packet If
   the remaining length is 20 (128-bit digest plus 4-octet key ID) or 24
   (160-bit digest plus 4-octet key ID), the remaining data are the MAC
   and parsing is complete.  If the remaining length is greater than 24,
   an extension field is present.  If the remaining length is less than
   8 or not a multiple of 4, a format error has occurred and the packet
   is discarded; otherwise, the parser increments the pointer by the
   extension field length and then uses the same rules as above to
   determine whether a MAC is present or another extension field.

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Authors' Addresses

   Tal Mizrahi
   Marvell
   6 Hamada St.
   Yokneam, 20692 Israel

   Email: talmi@marvell.com

   Danny Mayer
   Network Time Foundation
   PO Box 918
   Talent OR 97540

   Email: mayer@ntp.org

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