NTP Interleaved Modes
draft-ietf-ntp-interleaved-modes-04
Internet Engineering Task Force M. Lichvar
Internet-Draft Red Hat
Updates: 5905 (if approved) A. Malhotra
Intended status: Standards Track Boston University
Expires: March 21, 2021 Sep 17, 2020
NTP Interleaved Modes
draft-ietf-ntp-interleaved-modes-04
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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This Internet-Draft will expire on March 21, 2021.
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 2
1.1. Requirements Language . . . . . . . . . . . . . . . . . . 3
2. Interleaved Client/server mode . . . . . . . . . . . . . . . 4
3. Interleaved Symmetric mode . . . . . . . . . . . . . . . . . 7
4. Interleaved Broadcast mode . . . . . . . . . . . . . . . . . 9
5. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . 10
6. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 11
7. Security Considerations . . . . . . . . . . . . . . . . . . . 11
8. References . . . . . . . . . . . . . . . . . . . . . . . . . 11
8.1. Normative References . . . . . . . . . . . . . . . . . . 12
8.2. Informative References . . . . . . . . . . . . . . . . . 12
8.3. URIs . . . . . . . . . . . . . . . . . . . . . . . . . . 12
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 12
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
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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
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