NTP Interleaved Modes

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Document Type Active Internet-Draft (ntp WG)
Authors Miroslav Lichvar  , Aanchal Malhotra 
Last updated 2018-11-06 (latest revision 2018-06-28)
Replaces draft-mlichvar-ntp-interleaved-modes
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Internet Engineering Task Force                               M. Lichvar
Internet-Draft                                                   Red Hat
Intended status: Standards Track                             A. Malhotra
Expires: December 30, 2018                             Boston University
                                                           June 28, 2018

                         NTP Interleaved Modes


   This document extends the specification of Network Time Protocol
   (NTP) version 4 in RFC 5905 with special modes called the NTP
   interleaved modes, that enable NTP servers to provide their clients
   and peers with more accurate transmit timestamps that are available
   only after transmitting NTP packets.  More specifically, this
   document describes three modes: interleaved client/server,
   interleaved symmetric, and interleaved broadcast.

Status of This Memo

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   This Internet-Draft will expire on December 30, 2018.

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   Copyright (c) 2018 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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   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.

1.  Introduction

   RFC 5905 [RFC5905] describes the operations of NTPv4 in basic client/
   server, symmetric, and broadcast mode.  The transmit timestamp is one
   of the four timestamps included in every NTP packet used for time
   synchronization.  A packet that strictly follows RFC 5905, i.e. it
   contains a transmit timestamp corresponding to the packet itself, is
   said to be in basic mode.

   There are, at least, four options where a transmit timestamp can be
   captured i.e. by NTP daemon, by network drivers, or at the MAC or
   physical layer of the OSI model.  A typical transmit timestamp in a
   software NTP implementation in the basic mode is the one captured by
   the NTP daemon using the system clock, before the computation of
   message digest and before the packet is passed to the operating
   system, and does not include any processing and queuing delays in the
   system, network drivers, and hardware.  These delays may add a
   significant error to the offset and network delay measured by clients
   and peers of the server.

   For best accuracy, the transmit timestamp should be captured as close
   to the wire as possible, but that is difficult to implement in the
   current packet since this timestamp is available only after the
   packet transmission.  The protocol described in RFC 5905 does not
   specify any mechanism for the server to provide its clients and peers
   with this more accurate timestamp.

   Different mechanisms could be used to exchange this more accurate
   timestamp.  This document describes interleaved modes, in which an
   NTP packet contains a transmit timestamp corresponding to the
   previous packet that was sent to the client or peer.  This transmit
   timestamp could be captured at one of the any four places mentioned
   above.  More specifically, this document:

   1.  Introduces and specifies a new interleaved client/server mode.

   2.  Specifies the interleaved symmetric mode based on the NTP
       reference implementation with some modifications.

   3.  Specifies the interleaved broadcast mode based purely on the NTP
       reference implementation.

   The protocol does not change the NTP packet header format.  Only the
   semantics of some timestamp fields is different.  NTPv4 that supports

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   client/server and broadcast interleaved modes is compatible with
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