NTPv4 Short Extension Fields

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Internet Engineering Task Force                               M. Lichvar
Internet-Draft                                                   Red Hat
Updates: RFC7822 (if approved)                            April 25, 2019
Intended status: Standards Track
Expires: October 27, 2019

                      NTPv4 Short Extension Fields


   This document specifies a new packet format for the Network Time
   Protocol version 4 (NTPv4) which is compatible with RFC 7822, but
   allows NTPv4 packets to contain shorter extension fields.

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   This Internet-Draft will expire on October 27, 2019.

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Lichvar                 Expires October 27, 2019                [Page 1]
Internet-Draft        NTPv4 Short Extension Fields            April 2019

1.  Introduction

   RFC 7822 [RFC7822] specifies a minimum length of extension fields in
   NTPv4 packets in order to prevent ambiguities in their parsing.
   Without these rules, an extension field in a valid NTPv4 packet could
   be parsed as a Message Authentication Code (MAC), or a MAC could be
   parsed as an extension field.

   The minimum length of 28 octets forces extension fields that do not
   contain enough data to reach the minimum length to waste space.  With
   multiple extension fields in a packet the wasted space accumulates.

   A different issue with extension fields in NTPv4 packets is that
   servers/clients cannot pad a response/request to a specific length,
   e.g. to make their length symmetric when they contain different
   extension fields, or the sums of their lengths are different, unless
   one of the extension fields included in the request/response supports

   This document specifies a new NTPv4 format using three new extension

   1.  An extension field which contains other extension fields with no
       requirements on minimum length

   2.  An extension field which does not contain any information and can
       always be used for padding

   3.  An extension field which contains MAC

   Together, these extension fields allow NTPv4 packets to contain short
   extension fields, minimize the wasted space, and allow the packets to
   be padded to any length that meets the requirements of RFC 7822.

   Older NTP implementations which follow RFC 7822 will parse a packet
   in the new format as a valid packet which contains a single unknown
   extension field, skipping all extension fields and/or MAC, and can
   respond as appropriate.

   An implementation which supports the new format will parse all
   extension fields and/or MAC contained in the packet.

1.1.  Requirements Language

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   document are to be interpreted as described in RFC 2119 [RFC2119].

Lichvar                 Expires October 27, 2019                [Page 2]
Internet-Draft        NTPv4 Short Extension Fields            April 2019

2.  New extension fields

2.1.  Packing Field

   The Packing Field is an NTP extension field following RFC 7822
   [RFC7822], which contains one or more other extension fields.  The
   format of the extension field is shown below.

    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             |
   |       Subfield 1 Type         |        Subfield 1 Length      |
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