Internet Engineering Task Force                                 H. Stenn
Internet-Draft                                   Network Time Foundation
Intended status: Standards Track                         October 4, 2018
Expires: April 7, 2019

                 Network Time Protocol Leap Smear REFID


   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.

   This proposal offers a mechanism by which clients (and operators) can
   trivially detect a server that is offering leap-smeared time.

Status of This Memo

   This Internet-Draft is submitted in full conformance with the
   provisions of BCP 78 and BCP 79.

   Internet-Drafts are working documents of the Internet Engineering
   Task Force (IETF).  Note that other groups may also distribute
   working documents as Internet-Drafts.  The list of current Internet-
   Drafts is at

   Internet-Drafts are draft documents valid for a maximum of six months
   and may be updated, replaced, or obsoleted by other documents at any
   time.  It is inappropriate to use Internet-Drafts as reference
   material or to cite them other than as "work in progress."

   This Internet-Draft will expire on April 7, 2019.

Copyright Notice

   Copyright (c) 2018 IETF Trust and the persons identified as the
   document authors.  All rights reserved.

   This document is subject to BCP 78 and the IETF Trust's Legal
   Provisions Relating to IETF Documents

Stenn                     Expires April 7, 2019                 [Page 1]

Internet-Draft   Network Time Protocol Leap Smear REFID     October 2018

   ( in effect on the date of
   publication of this document.  Please review these documents
   carefully, as they describe your rights and restrictions with respect
   to this document.  Code Components extracted from this document must
   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.

Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   2
     1.1.  Requirements Language . . . . . . . . . . . . . . . . . .   3
   2.  Leap Smear REFID  . . . . . . . . . . . . . . . . . . . . . .   3
   3.  Acknowledgements  . . . . . . . . . . . . . . . . . . . . . .   4
   4.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .   4
   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 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.

   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.

Stenn                     Expires April 7, 2019                 [Page 2]

Internet-Draft   Network Time Protocol Leap Smear REFID     October 2018

   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.

1.1.  Requirements Language

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   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)

   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

Stenn                     Expires April 7, 2019                 [Page 3]

Internet-Draft   Network Time Protocol Leap Smear REFID     October 2018

   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

   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
   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

4.  IANA Considerations

   This memo requests that IANA allocate a pseudo Extension Field Type
   of 0xFFFE so the proposed "I-Do" exchange can report whether or not
   this server will offer leap smeared time in response to CLIENT time
   requests, identifying the amount of correction using the above REFID.

Stenn                     Expires April 7, 2019                 [Page 4]

Internet-Draft   Network Time Protocol Leap Smear REFID     October 2018

5.  Security Considerations

   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,

   [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,

Author's Address

   Harlan Stenn
   Network Time Foundation
   P.O. Box 918
   Talent, OR  97540


Stenn                     Expires April 7, 2019                 [Page 5]