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

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   include Simplified BSD License text as described in Section 4.e of
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   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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   depending on the availability of clocks and the security requirements
   of the specific application under implementation.

   Note: Since there is a lack of standards on terminology related to
   time, we define some terms in the following section.  Also,
   throughout the document, we define the terms as they become relevant.
   Different systems, depending on their OS, may use different terms for
   the same types of clocks.  A survey on this is not in the scope of
   this document.  We provide a discussion on how to access these values
   on POSIX and Windows systems.  On other systems, implementors will
   have to determine themselves which of these values are available.

3.  Expressing Time

   Protocols and applications can express time in several forms,
   depending on whether they need to express a point in time or a time

3.1.  Absolute Time

   Absolute time expresses a universally agreed upon reference to a
   specific point in time.  Such a reference can be expressed in
   different ways.  For instance, Unix Time refers to the number of
   seconds since midnight UTC, January 1st, 1970, while in everyday
   life, we reference such a point through year, month, day, and so on.

   Because absolute time expresses a shared view of time, a system needs
   to synchronize its clock with a common reference clock, for instance
   one based on UTC.

   Absolute time is often used to express the start or end of the
   validity of objects with a limited lifetime that are shared over the

3.2.  Relative Time

   Relative time measures the time interval that has elapsed from some
   well-defined reference point (e.g., 20 minutes from the time of your

   Since relative time does not express a point in time, it does not
   rely on synchronized clocks between systems but only on a shared rate
   of passage of this time.

   Relative time is commonly used in network protocols, for instance to
   determine when a packet should be considered dropped or to express
   Time To Live (TTL) values that govern the length of time for which an
   object is valid or usable.

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4.  Keeping Time: Different Clocks

   Because time is relative to an observer, there cannot be a
   universally agreed upon time.  At best we can achieve an
   approximation by constantly updating our own clocks against a common
   reference clock.  Remaining close to this reference clock is a
   complex process that comes with its own set of difficulties.

   In this section, we will have a look at the different clocks a system
   uses and how it maintains these clocks.

4.1.  Native Clock

   Each system has its own perception of time.  It gains access to it
   via its native clock.  Typcially, this clock counts cycles of an
   oscillator but some systems use process CPU times or thread CPU
   timers (via timers provided by the CPU).  The quality of the native
   clock therefore dependends on either the stability of the oscillator
   or the CPU timer.

   The timescale of the native clock is purely subjective -- no general
   meaning can be attached to any specific clock value.  One can only
   obtain relative time by comparing two values.  Because the value of
   the native clock always grows at a steady pace, never decreases,
   never make unexpected jumps, and never skips, the difference between
   two clock values provides the time interval between the two

   The independence of the native clock from any external time sources
   renders it resistant to any manipulation but in return there is no
   guarantee that its clock rate is similar to that of any other system.
   This difference in rate, especially when compared to a reference
   clock, is called clock drift.

   Clock drift depends on the quality of the clock itself but also on
   factors such as system load or ambient temperatur which makes it hard
   to predict.

4.2.  World Clock

   The native clock only provides means to measure relative time.  In
   order to be able to also process absolute time, the system needs to
   be synchronized with a global reference clock.  Since this clock
   strives to be the same on all systems, we call it the world clock.

   There are a number of ways to maintain the world clock based on the
   system's native clock.

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   The first is to manually maintain an offset between values of the
   native clock and the reference world clock.  Because of the clock
   drift of the native clock, this offset needs to be updated from time
   to time if a minimal divergence from the reference clock is to be

   Secondly, a hardware clock provided by the system and set to be
   equivalent to the reference time can be used, allowing the system to
   retain the offset across reboots.

   Finally, the reference clock can be obtained from an external time
   source.  Typically, the Internet is used through a variety of timing
   protocols including the Network Time Protocol (NTP) [RFC5905],
   Chrony, SNTP, OpenNTP and others.

   Each of these approaches has own problems attached to it.

   Manual configurations can be subject to errors and misconfiguration.
   Also, for mobile devices, when moving between time zones, the offset
   must be corrected manually.

   Accessing the hardware clock requires an I/O operation which is
   resource intensive, therefore many systems use the hardware clock
   only upon reboot, to initialize the clock offset; subsequent updates
   are made either manually or through timing protocols.

   Further, on many systems the quality of the hardware clock isn't very
   high, leading to a large clock drift if solely relying on it.  Worse,
   systems like microcontrollers that operate within embedded systems
   (e.g., Raspberry Pi, Arduino, etc.) often lack hardware clocks
   altogether.  These systems rely on external time sources upon reboot
   and have no means to process absolute time until synchronization with
   these sources has completed.

   Relying on Internet timing protocols opens up the system time to
   attack.  Recent papers show vulnerabilities in NTP [SECNTP], [MCBG]
   and SNTP that allow attackers to maliciously alter system's world
   clock -- pushing it into the past or even into the future.  Moreover,
   many of these time-shifting attacks can be performed by off-path
   attackers, who do not occupy a privileged position on the network
   between the victim system and its time sources on the Internet.
   Researchers have also demonstrated off-path denial of service attacks
   on timing protocols that prevent systems from synchronizing their

   In other words, the process of obtaining the offset necessary to
   provide a world clock creates dependencies that can be exploited.

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5.  Implementation Approaches

   Because absolute time relies on a shared interpretation of a value
   expressing time, the world clock is necessary when processing such

   For relative time, however, where only the rate of passage of time
   needs to be close enough to that of the other systems involved, there
   is no need to rely on the world clock when determining whether an
   interval has passed.

   Instead, by obtaining a value from the native clock when the interval
   has started only the native clock is necessary to determine when this
   interval ends.  As the native clock does not rely on any external
   time sources, the implementation becomes resistant to the
   difficulties of coordinating with these sources.

   However, using the native clock in this way comes with a caveat.
   Since the native clock is not subject to any adjustments by timing
   protocols, it is not adjusted for the error introduced by clock
   drift.  While this is likely of little consequence for short
   intervals, it may become significant for intervals that span long
   periods of time.

   Consequently, the choice of clock to be used is application-specific.
   If applications can tolerate a certain amount of clock drift or if
   the time intervals are short, implementers may prefer using the
   native clock.  If the application relies on precise timing over long
   periods one has no choice but to fall back to the world clock.

6.  Accessing the Native Clock on Selected Operating Systems

   In most operating systems, the standard functions to access time use
   the world clock since that is normally what users would expect.  This
   section provides an overview how the native clock can be accesses on
   some common operating systems.

6.1.  POSIX

   POSIX defines a system C API function which may provide native time:
   clock_gettime(), when used with a clock_id of CLOCK_MONOTONIC (when
   supported by the system).  POSIX does not make a distinction between
   raw time and adjusted raw time in the definition of this function.
   Beware that, with some systems, CLOCK_MONOTONIC deliveres adjusted
   raw time and that CLOCK_MONOTONIC_RAW needs to be used as clock_id to
   get unadjusted raw time.  Non-POSIX systems may provide different

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6.2.  Microsoft Windows

   In the Microsoft Windows operating system, native time is called
   'Windows Time' and can be accessed through the GetTickCount and
   GetTickCount64 API functions.  The returned value is normally the
   number of milliseconds since system start.  GetTickCount will return
   a 32 bit value while GetTickCount64 returns a value 64 bits wide that
   will wrap around less often.

7.  Acknowledgements

   We are thankful to Sharon Goldberg and Benno Overreinder for useful

8.  IANA Considerations

   This memo includes no request to IANA.

9.  Security Considerations

   Time is a fundamental component for the security guarantees claimed
   by various applications.  A system that uses a time distribution
   protocol may be affected by the security aspects of the time
   protocol.  The security considerations of time protocols in general
   are discussed in [RFC7384].  This document discusses the security
   considerations with respect to implementing time values in
   applications in various sections.

10.  Informative References

              Marouani, H. and M. Dagenais, "Internal clock drift
              estimation in computer clusters", 2008,

   [MCBG]     Malhotra, A., Cohen, I., Brakke, E., and S. Goldberg,
              "Attacking the Network Time Protocol", 2015,

   [RFC5280]  Cooper, D., Santesson, S., Farrell, S., Boeyen, S.,
              Housley, R., and W. Polk, "Internet X.509 Public Key
              Infrastructure Certificate and Certificate Revocation List
              (CRL) Profile", RFC 5280, DOI 10.17487/RFC5280, May 2008,

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   [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,

   [RFC7384]  Mizrahi, T., "Security Requirements of Time Protocols in
              Packet Switched Networks", RFC 7384, DOI 10.17487/RFC7384,
              October 2014, <>.

   [SECNTP]   Malhotra, A., Gundy, M., Varia, M., Kennedy, H., Gardner,
              J., and S. Goldberg, "The Security of NTP's Datagram
              Protocol", 2016, <>.

Authors' Addresses

   Aanchal Malhotra
   Boston University
   111 Cummington Mall
   Boston  02215


   Kristof Teichel
   Physikalisch-Technische Bundesanstalt
   Bundesallee 100
   Braunschweig  D-38116


   Martin Hoffmann
   NLnet Labs
   Science Park 400
   Amsterdam  1098 XH


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   Willem Toorop
   NLnet Labs
   Science Park 400
   Amsterdam  1098 XH


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