(12) INTERNATIONAL APPLICATION PUBLISHED UNDER THE PATENT COOPERATION TREATY (PCT)
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`(19) World Intellectual Property
`Organization
`International Bureau
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`(43) International Publication Date
`29 March 2018 (29.03.2018)
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`5/”
`WIPOI PCT
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`(10) International Publication Number
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`WO 2018/057322 A1
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`HR, HU, IE, IS, IT, LT, LU, Lv, MC, MK, MT, NL, No,
`PL, PT, RO, RS, SE, SI, SK, SM, TR).
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`Published:
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`— with international search report (Art. 21(3))
`— before the expiration of the time limit for amending the
`claims and to be republished in the event of receipt of
`amendments (Ra/e 48.2(h))
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`(51) International Patent Classification:
`H04] 3/06 (2006.01)
`H04L 9/12 (2006.01)
`G06F 21/64 (2013.01)
`H04L 29/06 (2006.01)
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`(21) International Application Number:
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`PCT/USZOl7/050817
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`(22) International Filing Date:
`08 September 2017 (08.09.2017)
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`(25) Filing Language:
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`(26) Publication Language:
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`English
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`English
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`(30) Priority Data:
`62/399,313
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`23 September 2016 (23.09.2016) US
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`(71) Applicant: KWOURZ RESEARCH LLC [US/US]; 1000
`N West Street, Suite 1200, Wilmington, Delaware 19801
`(US).
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`(74) Agent: SEEGERS, Paul T.; Meyertons, Hood, Kivlin,
`Kowert & Goetzel, P.C., PO. Box 398, Austin, Texas
`78767-0398 (US).
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`(81) Designated States (unless otherwise indicated, for every
`kind of national protection available): AU, BR, CA, CN,
`DE, GB, IN, JP, KR, MX, US.
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`(84) Designated States (unless otherwise indicated, for every
`kind of regional protection available): European (AL, AT,
`BE, BG, CH, CY, CZ, DE, DK, EE, ES, FI, FR, GB, GR,
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`(54) Title: NETWORK TIMING SYNCHRONIZATION
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`Network Clock
`142
`154 w:
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`77me Sync
`Network
`J 100
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`r
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`(x 152Sync Communication
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`PdetayExchange %
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`(57) Abstract: Techniques are disclosed relating to time synchroniAation in a network.
`In some embodiments, an apparatus includes a first circuit having a first clock config-
`ured to maintain a local time value for a node coupled to a network. The first circuit
`is configured to send a first message to a second circuit. The first message includes a
`first nonce. The second circuit has a second clock that maintains a reference time value
`for the network. The first circuit receives a second message from the second circuit,
`the second message including a second nonce and is associated with a timestamp iden-
`tifying the reference time value. The first circuit compares the first nonce to the sec-
`ond nonce to determine whether the timestamp is valid and, in response to determining
`that the timestamp is valid, uses the timestamp to synchronize the first clock with the
`second clock.
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`
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`Time Info.
`134
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`137A ‘3
`Local
`Clock 130A
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`Stock 1305
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`FIG. 1
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`TITLE: NETWORK TIMING SYNCHRONIZATION
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`BACKGROUND
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`Technical Field
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`[0001] This disclosure relates generally to computing devices, and, more specifically,
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`to
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`synchronizing time among multiple devices communicating over a network.
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`Description of the Related Art
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`[0002] Various protocols have been developed to synchronize computers clocks over a computer
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`network.
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`In many instances, a protocol may facilitate distributing timing information provided
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`from a source that maintains a master clock such as the atomic clock at
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`the US. Naval
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`Observatory. For example, one of the most notable protocols is the network time protocol (NTP)
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`used to synchronize time among computers connected to the Internet. The timing information
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`received via this protocol may be used to not only to display time on a user’s computer, but also
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`coordinate performance of various operations such as distributing software updates to computers,
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`displaying notifications,
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`invoking a backup utility,
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`time stamping emails, etc. As timing
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`demands have changed, other network protocols have been developed to enable more precise
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`time synchronization. For example, IEEE 802.1AS may be used in various audio and video
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`applications to synchronize time among devices that are consuming or producing audio and video
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`content.
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`W
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`[0003] The present disclosure describes embodiments in which time is synchronized between
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`nodes in a network.
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`In various embodiments, an apparatus includes a first circuit having a first
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`clock configured to maintain a local time value for a node coupled to a network. The first circuit
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`is configured to send a first message including a first nonce to a second circuit that maintains a
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`reference time value for the network. The first circuit receives, from the second circuit, a second
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`message including a second nonce and associated with a timestamp identifying the reference time
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`value. The first circuit compares the first nonce to the second nonce to determine whether the
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`timestamp is valid and, in response to determining that the timestamp is valid, uses the timestamp
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`to synchronize the first clock with the second clock. In some embodiments, the first circuit sends
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`the first message during an exchange with the second circuit to determine a propagation delay
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`between the first circuit and the second circuit, and uses the timestamp to synchronize the first
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`clock with the second clock by determining an offset between the first clock and the second clock
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`[J]
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`based on the propagation delay and the timestamp.
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`BRIEF DESCRIPTION OF THE DRAWINGS
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`[0004] Fig.
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`1
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`is a block diagram illustrating an example of a network that implements time
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`synchronization.
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`[0005] Fig. 2 is a block diagram illustrating an example of a hardware security module
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`configured to facilitate time synchronization.
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`[0006] Figs. 3A and 3B are diagrams illustrating examples of messages in a synchronization
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`communication.
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`[0007] Figs. 4A-4C are diagrams illustrating examples of messages in a propagation delay
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`exchange.
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`[0008] Fig. 5 is a diagram illustrating an example of an integrity check value field included in
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`one or more of the messages.
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`[0009] Fig. 6 is a diagram illustrating an example of a nonce field included in one or more of the
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`messages.
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`[0010] Figs. 7A and 7B are communication diagrams
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`illustrating examples of a time
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`synchronization that
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`includes
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`a propagation
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`delay
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`exchange
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`and
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`a
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`synchronization
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`communication.
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`[0011] Fig. 8 is a flow diagram illustrating an example of a bounds check for a synchronization
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`communication.
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`[0012] Figs. 9A and 9B are flow diagrams illustrating examples of other methods that may be
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`performed by network components.
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`[0013] Fig. 10 is a block diagram illustrating an exemplary computer system.
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`[0014] This disclosure includes references to “one embodiment” or “an embodiment.” The
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`appearances of the phrases “in one embodiment” or “in an embodiment” do not necessarily refer
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`to the same embodiment. Particular features, structures, or characteristics may be combined in
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`any suitable manner consistent with this disclosure.
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`[0015] Within this disclosure, different entities (which may variously be referred to as “units,”
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`“circuits,” other components, etc.) may be described or claimed as “configured” to perform one
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`or more tasks or operations. This formulation—[entity] configured to [perform one or more
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`tasks]—is used herein to refer to structure (i.e., something physical, such as an electronic circuit).
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`More specifically, this formulation is used to indicate that this structure is arranged to perform
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`the one or more tasks during operation. A structure can be said to be “configured to” perform
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`some task even if the structure is not currently being operated. A “node configured to
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`communicate traffic over a network” is intended to cover, for example, a device that has circuitry
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`that performs this function during operation, even if the device in question is not currently being
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`used (e.g., a power supply is not connected to it). Thus, an entity described or recited as
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`“configured to” perform some task refers to something physical, such as a device, circuit,
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`memory storing program instructions executable to implement the task, etc. This phrase is not
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`used herein to refer to something intangible. Thus, the “configured to” construct is not used
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`herein to refer to a software entity such as an application programming interface (API).
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`[0016] The term “configured to” is not intended to mean “configurable to.’ An unprogrammed
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`FPGA, for example, would not be considered to be “configured to” perform some specific
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`function, although it may be “configurable to” perform that function and may be “configured to”
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`perform the function after programming.
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`[0017] Reciting in the appended claims that a structure is “configured to” perform one or more
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`tasks is expressly intended n_ot to invoke 35 U.S.C. § 112(f) for that claim element. Accordingly,
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`none of the claims in this application as filed are intended to be interpreted as having means-plus-
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`function elements. Should Applicant wish to invoke Section 112(f) during prosecution, it will
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`recite claim elements using the “means for” [performing a function] construct.
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`[0018] As used herein, the terms “first,” “second,” etc. are used as labels for nouns that they
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`precede, and do not imply any type of ordering (e.g., spatial, temporal, logical, etc.) unless
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`specifically stated. For example, an exchange between a first circuit and a second circuit may
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`include the communication of multiple messages. The terms “first” and “second” can be used to
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`refer to any two of messages in the exchange.
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`In other words, the “first” and “second” messages
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`are not limited to the initial two messages in the exchange, for example.
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`[0019] As used herein, the term “based on” is used to describe one or more factors that affect a
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`determination. This term does not foreclose the possibility that additional factors may affect a
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`determination. That is, a determination may be solely based on specified factors or based on the
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`specified factors as well as other, unspecified factors. Consider the phrase “determine A based
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`on B.” This phrase specifies that B is a factor is used to determine A or that affects the
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`determination of A. This phrase does not foreclose that the determination of A may also be
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`based on some other factor, such as C. This phrase is also intended to cover an embodiment in
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`which A is determined based solely on B. As used herein,
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`the phrase “based on” is thus
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`synonymous with the phrase “based at least in part on.”
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`[0020] As used herein,
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`the term “synchronize” refers to calculating, for a first clock, an
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`adjustment to be made to the first clock to determine the value of a second clock. While this
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`term encompasses adjusting the value in the first clock to have the same time as the second clock,
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`this term, for the sake of this disclosure, also encompasses not adjusting the value in the first
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`clock. For example, “synchronizing” a first clock with a second clock may include calculating
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`an adjustment for the first clock and applying the adjustment to a time value output by the first
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`clock in order to determine a time value of the second clock.
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`DETAILED DESCRIPTION
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`[0021] A problem with traditional time synchronization schemes is that they typically do not take
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`security concerns into consideration. This design failure may allow a malicious person to
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`potentially interfere with a computing device’s perception of the current time. For example, the
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`person may attempt to spoof a trusted time source by providing a message announcing an
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`incorrect time. The person might also attempt to a replay message after a delay and have the
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`recipient consider the replayed messages as being fresh (e.g., replaying the message with a delay
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`that corresponds to the receiver’s offset clock).
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`In doing so, the person may be able to interfere
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`with scheduled operations performed by the device and potentially cause other problems.
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`[0022] The present disclosure describes various techniques for improving the security of a time
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`synchronization system.
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`In various embodiments described below, a computer network
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`implements a protocol for managing clock synchronization among network nodes having
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`respective local clocks. This protocol may be used to periodically announce the current time of a
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`network clock via communicating a first set of one or more messages from a master clock to a
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`slave clock as well as determine offsets between the clocks via communicating a second set of
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`messages in order to account for the propagation delays introduced by the network in distributing
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`the announced current time. To improve the security of these messages, in some embodiments,
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`an integrity check value (ICV) is added to each message. As used herein, the term “integrity
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`check value” is to be interpreted according to its understood meaning in the art, and includes a
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`value that is usable to verify the integrity of data and calculated by applying a keyed function
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`(i.e., a function that uses a cryptographic key) to the data.
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`Inclusion of this value may allow
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`messages to be authenticated and resistant
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`to tampering by a malicious entity.
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`In some
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`embodiments, one or more nonces are included in an exchange that is used to determine a
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`propagation delay and then repeated in a subsequent announcement of the current time. As used
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`herein, the term “nonce” is to be interpreted according to its understood meaning in the art, and
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`includes an arbitrary and unpredictable number that is only used once in an operation or a set of
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`operations. For example, a nonce may be determined using a random or pseudo random number
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`generator algorithm.
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`Inclusion of a nonce in this manner may prevent older announcements from
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`being replayed by a malicious entity attempting to interfere with a node’s local time.
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`In some
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`embodiments, a network node may collect information for determining a propagation delay that
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`is also usable to determine an expected offset between a local clock and the network clock. The
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`node may then compare this expected offset with an offset calculated from a timestamp
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`identifying the current time in a subsequent announcement.
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`If the expected offset differs
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`significantly from the calculated offset, the node may discard the announcement as it may be
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`illegitimate.
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`[0023] In some embodiments, messages associated with time synchronization may also be
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`processed by secure circuits coupled to network nodes using the timing information (as opposed
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`to the nodes themselves handling the processing). As used herein, the term “secure circuit”
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`refers to a circuit that protects an isolated, internal resource from being directly accessed by an
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`external entity. As will be described below,
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`in various embodiments, a secure circuit may
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`maintain the local clock for a network node and keys used to generate or verify ICVs included in
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`messages. In some instances, using secure circuits to process messages (as opposed to the nodes)
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`may provide additional security to the network.
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`[0024] The present disclosure begins with a description of components in a secure network that
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`implements time synchronization in conjunction with Figs.
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`1 and 2. Network communications
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`between nodes are described with respect to Figs. 3A-7B. Methods performed by network
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`components are described with respect to Figs. 8-9B. Lastly, an exemplary computer system that
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`may be used to implement one or more network components is discussed with Fig. 10.
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`[0025] Turning now to Fig. l, a block diagram of a network 100 configured to implement time
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`synchronization is depicted.
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`In the illustrated embodiment, network 100 includes a switch 110
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`coupled to multiple nodes 120A-C via links 112. Nodes 120A-B, in turn, are coupled via links
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`122 to respective hardware security modules (HSMs) 13OA-B, which include local clocks 132A-
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`B. As shown, node 120C is coupled to a grand master (GM) 140, which includes a network
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`clock 142. Switch 110 is also coupled to an HSM 130D, which includes a local clock 132D.
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`In
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`various embodiments. network 100 may be implemented differently than shown. Accordingly, in
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`some embodiments, more (or less) switches 110 and/or nodes 120 may be present.
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`In some
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`embodiments, GM 140 may be coupled to a switch 110 (as opposed to a node 120).
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`In some
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`embodiments, functionality described herein with respect to HSMs 130 may be implemented by
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`circuitry in nodes 120.
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`[0026] Network 100,
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`in some embodiments,
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`is a local area network (LAN) configured to
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`communicate network traffic among nodes 120.
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`In various embodiments, network traffic is
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`routed among nodes 120 by switches 110. Accordingly, switches 110 may be configured to
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`queue received messages (i.e., data frames) from nodes 120 and analyze the source and
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`destination addresses specified by the frames in order to appropriately send the messages on to
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`their specified destinations.
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`In some embodiments, switches 110 are configured to route data
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`frames in accordance with IEEE 802.3 (i.e., Ethernet Frames); however, in other embodiments,
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`other networking protocols may be supported.
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`[0027] Nodes 120 may correspond to any suitable devices configured to communicate over a
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`network.
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`In some embodiments, nodes 120 may be devices within a home or office network such
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`as desktop and laptop computers, mobile devices, smart television, smart appliances, etc.
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`In
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`some embodiments, nodes 120 are machines within a fabrication plant that are configured to
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`perform various operations.
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`In some embodiments, nodes 120 are electronic control units
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`(ECUs) in a vehicle such as an aircraft, boat, automobile, recreational vehicle, etc. As used
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`herein, the term “electronic control unit (ECU)” is to be interpreted according to its understood
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`meaning in the art, and includes an embedded system (e.g., microcontroller) that controls one or
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`more operations of a vehicle.
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`In some embodiments, nodes 120 may include, for example, a
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`motor ECU that communicates torque-control messages and wheel-speed messages in order to
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`control operation of a motor, a brake-system ECU that communicates brake control messages in
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`order to apply braking, a LIDAR ECU that processes data received from one or more LIDAR
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`sensors, a flight yoke ECU that communicates angles of the yoke controls, etc.
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`[0028] In various embodiments, nodes 120 (and switch I 10) are configured to use synchronized
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`time for various purposes, including coordinating communication of traffic over network 100.
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`For example, an ECU receiving streams from multiple LIDAR sensors may generate a holistic
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`View of a vehicle’s surroundings that is dependent on synchronized time among the ECU and its
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`sensors delivering the streams. That is, first and second LIDAR sensors may timestamp their
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`respective streams so that the ECU can determine what every sensor is viewing at a given time.
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`If these timestamps are generated by unsynchronized clocks, the ECU might have a problem
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`determining whether an object is present in front of the vehicle when one LIDAR sensor is
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`indicating the presence of the object while another sensor gives the appearance that nothing is
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`present in front of the vehicle because it included the wrong timestamp. As another example, in
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`some embodiments, traffic communicated over network 100 is coordinated in accordance with a
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`schedule that indicates when particular nodes 120 are supposed to communicate particular traffic.
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`If time is not synchronized across nodes 120, node 120A might begin communicating network
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`traffic during a time slot allocated to node 120B, for example. This collision could result in a
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`node getting the wrong information or significantly delayed information, which may be
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`associated with a time-critical task.
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`[0029] Hardware security modules (HSMs) 130,
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`in one embodiment, are secure circuits
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`configured to provide time information 134 to nodes 120 for various purposes.
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`In some
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`embodiments, each HSM 130 includes a local clock 132 configured to maintain a value of the
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`local time at that node 120, which is synchronized with network clock 142 by the HSM 130. As
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`will be described below in greater detail,
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`in various embodiments, time synchronization for
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`network 100 may be performed such that a given HSM 130 synchronizes its local clock 132 with
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`a neighboring node 120’s or switch 110’s HSM 130 that resides on the path between the given
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`HSM 130 and grand master 140, and is a hop closer to grand master 140. Accordingly, HSMS
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`130A and HSMs 130B may synchronize their local clocks 132A and 132B with local clock 132D
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`in HSM 130D being coupled to a node 120 one hop closer to node 120C and grand master 140.
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`HSM 130D, in turn, may synchronize with network clock 142 in grand master 140 as there are no
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`other closer nodes 120 in Fig. 1. Said differently, time synchronization of network 100 may be
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`viewed as a tree structure in which a given child node synchronizes with its parent node, and
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`grand master 140 functions as the root node in the tree structure.
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`[0030] When synchronization between clocks of two HSMs 130 is performed, the HSM 130
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`providing the timing information (e. g., HSM 130D) may be referred to as the master while HSM
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`130 receiving the timing information (e. g, HSMs 130A and 130B) may be referred to as the
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`slave. As part of performing a synchronization, in various embodiments, a slave may determine
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`a propagation delay between it and its master (e.g., the delay between HSM 130A and HSM
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`130D). The slave may also determine an offset between its local clock 132 and master’s clock
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`(e. g, the offset between clocks 132A and 132D). Once these values have been determined, the
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`HSM 130 may then provide information 134 to its node 120, where the timing information
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`includes a time value associated with the current value of network clock 142 and is calculated
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`based on the value of local clock 132, the propagation delay, and the determined offset.
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`In
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`various embodiments, HSMs 130 are relied on for maintaining local time and synchronizing time
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`(as opposed to nodes 120) as HSMs 130 may employ various security techniques making them
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`more secure than nodes 120 as will be described below with respect to Fig. 2.
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`[0031] Grand master (GM) 140, in one embodiment, is a circuit configured to maintain network
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`time for network 100—i.e., the reference time to which local clocks 132 are synchronized. That
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`is, while HSM 130D may act as a master to HSM 130A and HSM 130B, GM 140 is the ultimate
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`authority (i.e., “grand master”) in the illustrated embodiment. As noted above, in the illustrated
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`embodiment, GM 140 includes a network clock 142 configured to track the current value of
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`network time.
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`In some embodiments, GM 140 may be one of several master clocks and is
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`selected as the grand master through an election process. As noted above, in some embodiments,
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`GM 140 may be coupled to switch 110 (as opposed to node 120E as shown in Fig. 1) and/or may
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`implement functionality of an HSM 130 discussed below.
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`In various embodiments, GM 140 is
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`also configured to facilitate time synchronization with HSM 130D via synchronization (sync)
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`communications 152 and propagation delay (Pdelay) exchanges 154. (Although communications
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`152 and exchanges 154 are shown only between GM 140 and HSM 130D in Fig. 1 to simplify
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`the diagram, communications 152 and exchanges 154 are also provided from HSM 130D to
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`HSMs 130A and 130B as well.)
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`[0032] Sync communications 152, in the illustrated embodiment, are messages communicated by
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`a master (e.g., GM 140 0r HSM 130D) to announce the current value of network time as
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`identified by its synchronized clock 132 or network clock 142.
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`In various embodiments, a given
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`sync communication 152 includes a timestamp identifying the current value of network time
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`when a sync communication 152 is sent from the master and is usable by a given slave to
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`determine the offset (i.e., difference) between the slave’s local clock 132 and the master’s clock
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`132 or clock 142.
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`In various embodiments, the determined offset is both 1) the time difference
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`between the master’s clock and the slaves clock and 2) the frequency difference between
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`master’s clock and the slave’s clock as they may have slightly different frequencies in some
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`instances. Sync communications 152 may be sent on a periodic basis (e. g, eight times a second)
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`as the individual frequencies of clocks 132 and 142 may fluctuate with temperature changes.
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`In
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`some embodiments, a given sync communication 152 includes two messages: a sync message
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`(discussed below with Fig. 3A) that
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`is sent at a particular time and a follow up message
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`(discussed below with Fig. 3B) that includes a timestamp specifying the value of network time at
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`the particular time—i.e., the time at which the sync message was sent.
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`In some embodiments,
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`these messages include the content of IEEE 802.1AS frames.
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`[0033] Pdelay exchanges 154, in the illustrated embodiment, are an exchange between a master
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`and slave in order to determine a propagation delay between the master and slave. That is,
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`because it takes time for a message to traverse network 100, a timestamp specified in a sync
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`communication 152 is delayed by the time it arrives at an HSM 130. To account for this, the
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`HSM 130 determines the propagation delay and residence time to appropriately adjust the
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`timestamp in the sync communication 152 so that it accurately reflects the current network time.
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`In some embodiments, an exchange 154 includes a slave sending a message to a master (referred
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`to below and in Fig. 4A as a Pdelay request) and receiving a corresponding response from the
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`master (referred to below and in Fig. 4B as a Pdelay response). The propagation delay may then
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`be determined by 1) calculating the difference between the arrival time of the first message and
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`its departure time, 2) calculating the difference between the arrival time of the second message
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`and its departure time, and 3) determining an average of the differences. Notably, the arrival and
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`departure times of both messages are used in order to account for the fact that the arrival and
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`departure timestamps are supplied by the master’s clock and the slave’s clock, which may be
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`unsynchronized as noted above. For example, HSM 130D may record the departure time of the
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`first message and the arrival time of the second message based on local clock 132D, but rely on
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`GM 140 to provide timestamps for the arrival time of the first message and the departure time of
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`the second message, which are based on network clock 142.
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`In some embodiments, the second
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`message may include both of these timestamps.
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`In other embodiments, the second message
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`includes the timestamp for the arrival time of the first message, and a third message (referred to
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`below and in Fig. 4C as Pdelay follow-up) is sent that specifies the timestamp for the departure
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`time of the second message.
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`In some embodiments, Pdelay exchanges 154 are performed
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`periodically (e. g., once per second) as the propagation delays may change as the temperature of
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`links 112 and 122 change.
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`In some embodiments, messages communicated in an exchange 154
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`include the content of IEEE 802.1AS frames.
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`[0034] To improve the security of sync communications 152 and Pdelay exchanges 154,
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`in
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`various embodiments, HSMs 130 and GM 140 are configured to perform various techniques to
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`make them more resistant
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`to tampering by a malicious entity.
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`In some instances,
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`these
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`techniques may protect an HSM 130 if node 120 becomes compromised and begins spoofing that
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`HSM 130’s master or GM 140 by broadcasting its own sync communications 152, These
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`techniques may also protect against man-in-the-middle attacks such as switch 110 becoming
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`compromised and attempting to replay messages from an old sync communication 152 or Pdelay
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`exchange 154.
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`[0035] First, in some embodiments, an integrity check values (ICV) is added to one or more
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`messages sent between GM 140 and HSMs 130 during sync communication 152 and a Pdelay
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`exchange 154. This ICV may be calculated using any of various algorithms. For example, in
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`one embodiment, GM 140 and HSMs 130 are configured to calculate ICVs using a message
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`authentication code (MAC) algorithm such as the Hash—based MAC (HMAC) algorithm.
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`another embodiment,
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`ICVs are digital
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`signatures calculated using the Digital Signature
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`Algorithm (DSA).
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`In still another embodiment, ICVs include a MAC encrypted using the
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`Advanced Encrypted Standard (AES)
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`in cipher block chaining (CBC) mode.
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`In some
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`embodiments, the key used to determine the ICV is unique to a given HSM 130 participating in
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`the communication and known to that HSM 130 and its master. For example, if HSM 130D is
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`sending a first message to HSM 130A and a second message to HSM 130B, the ICV in the first
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`message is calculated using a first key, and the ICV in the second message is calculated using a
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`second key different from the first key. As a result, HSM 130A and 130B may be able to verify
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`not only the integrity of a received message, but also authenticate the message because HSM
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`130D is the only other entity with the key unique to that HSM 130. Accordingly, if an HSM 130
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`receives a message including an ICV and is unable to determine that the ICV is valid, the
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`message is suspect and may be disregarded by the HSM 130.
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`In doing so, HSMs 130 and GM
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`140 may protect against an entity attempting to spoof messages from GM 140 or a particular
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`HSM 130. Examples of messages including an ICV are discussed below with respect to Figs.
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`3A-5.
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`[0036] Second, in some embodiments, an HSM 130 is configured to provide a nonce during a
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`Pdelay exchange 154, which is repeated by its master during one or more subsequent sync
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`communications 152.
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`(In some embodiments, the master may also include an additional nonce in
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`the exchange as will be discussed with Fig. 7B.)
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`In doing so, HSM 130 and GM 140 may
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`prevent a malicious entity from replaying sync communications 152 sent prior to providing the
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`nonce as
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`they would lack the nonce. Accordingly,
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`if the HSM 130 receives a sync
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`communication 152 including a nonce that is different from the earlier provided one, the HSM
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`130 may disregard the communication 152 as suspect. Examples of messages including a nonce
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`are discussed below with respect to Figs. 3A, 3B, 4A, and 6.
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`[0037] Third, in some embodiments, HSMs 130 are configured to determine an expected offset
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`during a Pdelay exchange 154 and use the expected offset to perform a bounds check on an offset
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`calculated from a given sync communication 152 in order to determine whether that sync
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`communication 152 is valid. Although a Pdelay exchange 154 is performed primarily to
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`determine a propagation delay as noted above, it is possible to determine an offset between a
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`master’s clock and a slave’s clock from information obtained in determining the delay.
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`In
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`particular, this offset may be determined by 1) calculating an average of the first message’s
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`departure time and the second message’s arrival time, 2) calculating an average of the first
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`message’s arrival time and the second message’s departure time, 3) determining the difference
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`between the two averages such that the difference is the offset.
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`In such an embodiment, HSM
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`130 is configured to determine this offset and compare it against an offset calculated based on a
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`timestamp included in a subsequent sync communicate 152.
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`If the calculated offset differs from
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`what was expected by more than a particular threshold,
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`the timestamp included in the sync
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`communication 152 is suspect. As a result, an HSM 130 may disregard the sync communication
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`152 and not attempt to provide time information 134 based on the suspect timestamp.
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`In doing
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`so, HSMs 130 may protect against spoofing and/or a man-in-the-middle attack. An example of a
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`method for performing a bounds check is described in greater detail with respect to Fig. 8.
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`[0038] Turning now to Fig. 2, a block diagram of an HSM 130 is depicted. As noted above, in
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`some embodiments, HSMs 130 are used for timing keeping and synchronization in lieu of nodes
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`120 because HSMs 130 may be more secure for the reasons discussed below. As also noted, in
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`some embodiments, GM 140 may implement functionality described with respect to HSM 130 in
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