NTP Interleaved Modes

Document Type Active Internet-Draft (ntp WG)
Last updated 2020-02-03
Replaces draft-mlichvar-ntp-interleaved-modes
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Internet Engineering Task Force                               M. Lichvar
Internet-Draft                                                   Red Hat
Updates: RFC5905 (if approved)                               A. Malhotra
Intended status: Standards Track                       Boston University
Expires: August 6, 2020                                      Feb 3, 2020

                         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 August 6, 2020.

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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 a client/
   server, symmetric, and broadcast mode.  The transmit and receive
   timestamps are two of the four timestamps included in every NTPv4
   packet used for time synchronization.

   For a highly accurate and stable synchronization, the transmit and
   receive timestamp should be captured close to the beginning of the
   actual transmission and the end of the reception respectively.  An
   asymmetry in the timestamping causes the offset measured by NTP to
   have an error.

   There are at least four options where a timestamp of an NTP packet
   may be captured with a software NTP implementation running on an
   operating system:

   1.  User space (software)

   2.  Network device driver or kernel (software)

   3.  Data link layer (hardware - MAC chip)

   4.  Physical layer (hardware - PHY chip)

   Software timestamps captured in the user space in the NTP
   implementation itself are least accurate.  They do not include system
   calls used for sending and receiving packets, processing and queuing
   delays in the system, network device drivers, and hardware.  Hardware
   timestamps captured at the physical layer are most accurate.

   A transmit timestamp captured in the driver or hardware is more
   accurate than the user-space timestamp, but it is available to the
   NTP implementation only after it sent the packet using a system call.
   The timestamp cannot be included in the packet itself unless the
   driver or hardware supports NTP and can modify the packet before or
   during the actual transmission.

   The protocol described in RFC 5905 does not specify any mechanism for
   a server to provide its clients and peers with a more accurate
   transmit timestamp that is known only after the transmission.  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

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   Different mechanisms could be used to exchange timestamps known after
   the transmission.  The server could respond to each request with two
   packets.  The second packet would contain the transmit timestamp
   corresponding to the first packet.  However, such a protocol would
   enable a traffic amplification, or it would use packets with an
   asymmetric length, which would cause an asymmetry in the network
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