NTP Interleaved Modes
draft-ietf-ntp-interleaved-modes-02
Internet Engineering Task Force M. Lichvar
Internet-Draft Red Hat
Updates: RFC5905 (if approved) A. Malhotra
Intended status: Standards Track Boston University
Expires: November 24, 2019 May 23, 2019
NTP Interleaved Modes
draft-ietf-ntp-interleaved-modes-02
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 November 24, 2019.
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Lichvar & Malhotra Expires November 24, 2019 [Page 1]
Internet-Draft NTP Interleaved Modes May 2019
include Simplified BSD License text as described in Section 4.e of
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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
mode.
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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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