Internet DRAFT - draft-stenn-ntp-leap-smear-refid
draft-stenn-ntp-leap-smear-refid
Internet Engineering Task Force H. Stenn
Internet-Draft Network Time Foundation
Intended status: Standards Track March 25, 2019
Expires: September 26, 2019
Network Time Protocol Leap Smear REFID
draft-stenn-ntp-leap-smear-refid-02
Abstract
Leap Seconds are part of UTC. NTP timestamps are based on POSIX
timestamps, which require each day to have exactly 86,400 seconds per
day. Some applications and environments choose to "smear" leap
second corrections over a period that can last up to 24 hours' time,
and implement NTP servers that offer smeared time to clients asking
them for the time.
Both NTP clients and operators have no current way to tell if an NTP
server is offering leap-smeared time or not. This is a problem.
Similarly, an NTP server may choose to offer leap-smeared time to
clients that do not appear to know that a leap event is in-process.
This is a problem.
This proposal offers a mechanism that provides a simple and clean
solution to problems, by giving a way that clients (and operators)
can trivially ask for leap-smeared time and detect a server that is
offering leap-smeared time.
Status of This Memo
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This Internet-Draft will expire on September 26, 2019.
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Copyright Notice
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 2
1.1. Requirements Language . . . . . . . . . . . . . . . . . . 3
2. Leap Smear REFID . . . . . . . . . . . . . . . . . . . . . . 3
3. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . 5
4. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 5
5. Security Considerations . . . . . . . . . . . . . . . . . . . 5
6. Normative References . . . . . . . . . . . . . . . . . . . . 5
Author's Address . . . . . . . . . . . . . . . . . . . . . . . . 5
1. Introduction
Leap Seconds are applied as needed to UTC in order to keep its time
of day close to UT1's mean solar time.
RFC 5905 [RFC5905] and earlier versions of NTP are the overwhelming
method of distributing time on networks. The timescale used by NTP
is based on POSIX which, for better or worse, ignores any instances
where there are not the ordinary 86,400 seconds per day.
Leap Seconds will continue to exist for the foreseeable future, and
similarly, POSIX can be expected to ignore leap seconds for the
foreseeable future.
Different applications have different requirements for the stability
of time during the application of a leap second. Some applications
are tolerant of a fast application of the correction, while other
applications prefer to "smear" the leap second over a longer period,
where the time reported by leap-second aware servers is gradually
applied so there is no abrupt change to time during the processing of
a leap second.
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While leap second processing can be expected to be properly handled
by up-to-date software and by time servers, there are large numbers
of out-of-date software installations and client systems that are
just not able to properly handle a leap second correction.
Additionally, some use-cases for calculating elapsed time (a
"difference clock") that use POSIX timestamps are greatly complicated
in the possible presence of a leap-second corrections. If the
presence of leap-smeared time is of greater value than legally-
correct time, leap smearing is the choice some administrators will
take.
This proposal offers a way for a system to generate a REFID that
indicates that the time being supplied in the NTP packet already
contains an amount of leap smear correction, and what that amount is.
It also provides part of a solution whereby a client can receive
leap-smeared time in the case where part of the leap smear occurs
before the actual leap second, and the remainder of the leap smear
occurs after the actual leap second.
1.1. Requirements Language
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
document are to be interpreted as described in RFC 2119 [RFC2119].
2. Leap Smear REFID
RFC 5905 [RFC5905] defines the data type of NTP time values in
Section 6, "Data Types":
All NTP time values are represented in twos-complement format,
with bits numbered in big-endian (as described in Appendix A of
[RFC0791]) fashion from zero starting at the left, or high-order,
position. ...
The 32 bit signed integer seconds portion and the 32 bit unsigned
fractional seconds portion, or 32:32 format is:
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Seconds |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Fraction |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
NTP Timestamp Format (32:32)
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This format provides coverage for 136 years' time to a precision of
232 picoseconds. If a leap-second addition is being completely
smeared just before before the stroke of the next POSIX second then
the smear correction will be (0,1). If this was the only way to
apply a leap smear correction then we could simply use an unsigned
value to represent the correction. But while the first popular leap
smear implementation applied the correction over an appropriate
number of hours' time before the actual leap second so the system
time was corrected at the stroke of 00:00, that meant that the
difference between system time and UTC spent half of the duration of
the smear application at [.5,1) "off" of correct time. The second
popular implementation of the leap smear applied the first half-
second correction before the stroke of 00:00 for a correction range
of (0,.5] and the last half-second correction starting at the stroke
of 00:00 for a [-.5,0) correction range. This also means we need a
signed value to represent the amount of correction.
If a system implements the leap-smear REFID, the REFID of a system
that is supplying smeared time to client requests while leap-smear
correction is active would be 254.b1.b2.b3, where the three octets
(b1, b2, and b3) are a 2:22 formatted value, yielding precision to
238 nanoseconds, or about a quarter of a microsecond.
Note that if an NTP server decides to offer smeared time corrections
to clients, it SHOULD only offer this time in response to CLIENT time
requests. There is something to be said for further only offering
smeared time to CLIENT time requests that show an LI value of 0, and
perhaps 3. The reason for this is that if a client knows a leap
second is pending, it can be expected to know how to process that
leap second. An NTP server that is offering smeared time SHOULD NOT
send smeared time in any peer exchanges. Also, CLIENT machines
SHOULD NOT be distributing time (smeared or otherwise) to other
systems.
We also note that during the application of a leap smear, the REFID
from a system offering smeared time cannot provide detection of a
timing loop. This is not expected to be a problem because time
server systems are not expected to make CLIENT connections with each
other, so they should not be receiving smeared time. Moreso, if a
time server is configured to make CLIENT connections to a server that
offers smeared time, with the mechanism described here it can detect
when it is getting smeared time, and either ignore time from that
source, or "undo" the leap smear correction and use the corrected
time for that sample.
This proposal is not an attempt to justify servers offering leap
smeared time. Its purpose is to make it easy to identify when a
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client is receiving smeared time, and provide the client a way to
know the amount of smear correction as of the latest successful poll.
3. Acknowledgements
The author wishes to acknowledge the contributions of Juergen
Perlinger.
4. IANA Considerations
This memo requests that IANA allocate a pseudo Extension Field Type
of 0xFEFF so the proposed "I-Do" exchange can report whether or not
this server can offer leap smeared time in response to CLIENT time
requests, identifying the amount of correction using the above REFID.
5. Security Considerations
No special or unusual security issues have been identified that are
directly related to this proposal.
Additional information TBD.
6. Normative References
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119,
DOI 10.17487/RFC2119, March 1997,
<https://www.rfc-editor.org/info/rfc2119>.
[RFC5905] Mills, D., Martin, J., Ed., Burbank, J., and W. Kasch,
"Network Time Protocol Version 4: Protocol and Algorithms
Specification", RFC 5905, DOI 10.17487/RFC5905, June 2010,
<https://www.rfc-editor.org/info/rfc5905>.
Author's Address
Harlan Stenn
Network Time Foundation
P.O. Box 918
Talent, OR 97540
US
Email: stenn@nwtime.org
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