NTP Interleaved Modes
draft-ietf-ntp-interleaved-modes-00
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
draft-ietf-ntp-interleaved-modes-00
Abstract
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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provisions of BCP 78 and BCP 79.
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This Internet-Draft will expire on December 30, 2018.
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Internet-Draft NTP Interleaved Modes June 2018
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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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