On Implementing Time

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Internet Engineering Task Force                              A. Malhotra
Internet-Draft                                         Boston University
Intended status: Informational                                K. Teichel
Expires: April 25, 2019                                              PTB
                                                             M. Hoffmann
                                                               W. Toorop
                                                              NLnet Labs
                                                        October 22, 2018

                          On Implementing Time


   This document describes the properties of different types of clocks
   available on digital systems.  It provides implementors of
   applications with guidance on choices they have to make when working
   with time to provide basic functionality and security guarantees.

Status of This Memo

   This Internet-Draft is submitted in full conformance with the
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   This Internet-Draft will expire on April 25, 2019.

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   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
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   (https://trustee.ietf.org/license-info) in effect on the date of
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   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.

1.  Introduction

   It is hard to understate the importance of time in modern digital
   systems.  The functionality and security of applications (distributed
   or local to one system) and that of network protocols generally hinge
   on some notion of time.  For implementation, these applications and
   protocols have to choose one of the types of clocks available on
   their system, each of which has its own specific properties.
   However, currently many of these applications seem to be oblivious to
   the implications of choosing one or the other clock for
   implementation.  This behavior can be attributed to: a) the lack of
   clear understanding of the distinct properties of these clocks, b)
   trade-offs of using one or the other for an application, and c)
   availability and compatibility of these clocks on different systems.
   This document discusses a) and b).

   More specifically, in this document we first define different methods
   used by protocols and applications to express time.  We then define
   properties of clocks maintained by modern digital systems.  Next we
   describe how systems obtain these values from these clocks and the
   security considerations of using these values to implement protocols
   and applications that use time.  Finally we discuss trade-offs
   between security and precision of choosing a clock.  The document
   aims to provide guidance to the implementors make an informed choice
   with an example of POSIX system.

2.  Scope of the document

   This document aims to provide software developers implementing
   protocols and applications that have to deal with time with the
   knowledge and understanding to make informed decisions regarding the
   available clocks and their respective trade-offs.

   It does not describe functionality that is specific to the
   architecture of a PC, or other devices such as phones, IoT devices,
   switches, routers, base stations, or synchrophasors.  Nor is the
   document applicable to a specific operating system.  Throughout the
   document we assume that one or the other clock is available on most
   devices.  How these clocks are available on different PCs or other
   devices is out of scope of this document.

   We do not exactly recommend which clock should be used.  We discuss
   the available options and trade-offs.  The final decision would vary

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