ThePrecision Time Protocol (PTP) is aprotocol forclock synchronization throughout acomputer network with relatively highprecision and thereforepotentially high accuracy. In alocal area network (LAN), accuracy can be sub-microsecond – making it suitable for measurement and control systems.^[1] PTP is used to synchronizefinancial transactions,mobile phone tower transmissions, sub-seaacoustic arrays, and networks that require precise timing but lack access tosatellite navigation signals.^{[citation needed]}

The first version of PTP,IEEE 1588-2002, was published in 2002.IEEE 1588-2008, also known as PTP Version 2, is notbackward compatible with the 2002 version.IEEE 1588-2019 was published in November 2019 and includes backward-compatible improvements to the 2008 publication. IEEE 1588-2008 includes aprofile concept defining PTP operating parameters and options. Several profiles have been defined for applications includingtelecommunications,electric power distribution andaudiovisual uses.IEEE 802.1AS is an adaptation of PTP, called gPTP, for use withAudio Video Bridging (AVB) andTime-Sensitive Networking (TSN).

According to John Eidson, who led the IEEE 1588-2002 standardization effort, "IEEE 1588 is designed to fill a niche not well served by either of the two dominant protocols,NTP andGPS. IEEE 1588 is designed for local systems requiring accuracies beyond those attainable using NTP. It is also designed for applications that cannot bear the cost of aGPS receiver at each node, or for which GPS signals are inaccessible."^[2]

PTP was originally defined in the IEEE 1588-2002 standard, officially titledStandard for a Precision Clock Synchronization Protocol for Networked Measurement and Control Systems, and published in 2002. In 2008, IEEE 1588-2008 was released as a revised standard; also known as PTP version 2 (PTPv2), it improves accuracy, precision and robustness but is notbackward compatible with the original 2002 version.^[3] IEEE 1588-2019 was published in November 2019,^[4] is informally known asPTPv2.1 and includes backwards-compatible improvements to the 2008 publication.^[5]

The IEEE 1588 standards describe ahierarchicalmaster–slave architecture for clockdistribution consisting of one or morenetwork segments and one or more clocks. Anordinary clock is a device with a single network connection that is either the source of or the destination for a synchronization reference. A source is called amaster (alternatelytimeTransmitter^[6]), and a destination is called aslave (alternatelytimeReceiver^[6]). Aboundary clock has multiple network connections and synchronizes one network segment to another. A single, synchronization leader is selected, a.k.a. elected, for each network segment. The root timing reference is called thegrandmaster.^[7]

A relatively simple PTP architecture consists of ordinary clocks on a single-segment network with no boundary clocks. A grandmaster is elected and all other clocks synchronize to it.

IEEE 1588-2008 introduces a clock associated with network equipment used to convey PTP messages. Thetransparent clock modifies PTP messages as they pass through the device.^[8]Timestamps in the messages are corrected for time spent traversing the network equipment. This scheme improves distribution accuracy by compensating fordelivery variability across the network.

PTP typically uses the sameepoch asUnix time (start of 1 January 1970).^[a] While the Unix time is based onCoordinated Universal Time (UTC) and is subject toleap seconds, PTP is based onInternational Atomic Time (TAI). The PTP grandmaster communicates the current offset between UTC and TAI, so that UTC can be computed from the received PTP time.

Synchronization and management of a PTP system is achieved through the exchange of messages across the communications medium. To this end, PTP uses the following message types.

• Sync,Follow_Up,Delay_Req andDelay_Resp messages are used byordinary andboundary clocks and communicate time-related information used to synchronize clocks across the network.
• Pdelay_Req,Pdelay_Resp andPdelay_Resp_Follow_Up are used bytransparent clocks to measure delays across the communications medium so that they can be compensated for by the system.Transparent clocks and these messages associated with them are not available in original IEEE 1588-2002 PTPv1 standard, and were added in PTPv2.
• Announce messages are used by thebest master clock algorithm in IEEE 1588-2008 to build a clock hierarchy and select thegrandmaster.^[b]
• Management messages are used bynetwork management to monitor, configure and maintain a PTP system.
• Signaling messages are used for non-time-critical communications between clocks. Signaling messages were introduced in IEEE 1588-2008.

Messages are categorized asevent andgeneral messages.Event messages are time-critical in that accuracy in transmission and receipt timestamp accuracy directly affects clock distribution accuracy.Sync,Delay_Req,Pdelay_Req andPdelay_resp areevent messages.General messages are more conventionalprotocol data units in that the data in these messages is of importance to PTP, but their transmission and receipt timestamps are not.Announce,Follow_Up,Delay_Resp,Pdelay_Resp_Follow_Up,Management andSignaling messages are members of thegeneral message class.^{[9]: Clause 6.4}

PTP messages may use theUser Datagram Protocol overInternet Protocol (UDP/IP) for transport. IEEE 1588-2002 uses onlyIPv4 transports,^{[10]: Annex D} but this has been extended to includeIPv6 in IEEE 1588-2008.^{[9]: Annex F} In IEEE 1588-2002, all PTP messages are sent usingmulticast messaging, while IEEE 1588-2008 introduced an option for devices to negotiateunicast transmission on a port-by-port basis.^{[9]: Clause 16.1} Multicast transmissions useIP multicast addressing, for which multicast group addresses are defined for IPv4 and IPv6 (see table).^{[9]: Annex D and E} Time-criticalevent messages (Sync, Delay_req, Pdelay_Req and Pdelay_Resp) are sent toport number 319.General messages (Announce, Follow_Up, Delay_Resp, Pdelay_Resp_Follow_Up, management and signaling) use port number 320.^{[9]: Clause 6.4}

In IEEE 1588-2008, encapsulation is also defined forDeviceNet,^{[9]: Annex G}ControlNet^{[9]: Annex H} andPROFINET.^{[9]: Annex I}

A domain^[i] is an interacting set of clocks that synchronize to one another using PTP. Clocks are assigned to a domain by virtue of the contents of theSubdomain name (IEEE 1588-2002) or thedomainNumber (IEEE 1588-2008) fields in PTP messages they receive or generate. Domains allow multiple clock distribution systems to share the same communications medium.

Thebest master clock algorithm (BMCA) performs a distributed selection of the best clock to act as leader based on the following clock properties:

• Identifier – A universally unique numeric identifier for the clock. This is typically constructed based on a device'sMAC address.
• Quality – Both versions of IEEE 1588 attempt to quantify clock quality based on expected timing deviation, technology used to implement the clock or location in aclock stratum schema, although only V1 (IEEE 1588-2002) knows a data fieldstratum. PTP V2 (IEEE 1588-2008) defines the overall quality of a clock by using the data fieldsclockAccuracy andclockClass.
• Priority – An administratively assigned precedence hint used by the BMCA to help select agrandmaster for the PTP domain. IEEE 1588-2002 used a singleBoolean variable to indicate precedence. IEEE 1588-2008 features two 8-bit priority fields.
• Variance – A clock's estimate of its stability based on observation of its performance against the PTP reference.

IEEE 1588-2008 uses a hierarchical selection algorithm based on the following properties, in the indicated order:^{[9]: Figure 27}

1. Priority 1 – the user can assign a specific static-designed priority to each clock, preemptively defining a priority among them. Smaller numeric values indicate higher priority.
2. Class – each clock is a member of a given class, each class getting its own priority.
3. Accuracy – precision between clock and UTC, in nanoseconds (ns)
4. Variance – variability of the clock
5. Priority 2 – final-defined priority, defining backup order in case the other criteria were not sufficient. Smaller numeric values indicate higher priority.
6. Unique identifier – MAC address-based selection is used as a tiebreaker when all other properties are equal.

IEEE 1588-2002 uses a selection algorithm based on similar properties.

Clock properties are advertised in IEEE 1588-2002Sync messages and in IEEE 1588-2008Announce messages. The current leader transmits this information at regular interval. A clock that considers itself a better leader will transmit this information in order to invoke a change of leader. Once the current leader recognizes the better clock, the current leader stops transmittingSync messages and associated clock properties (Announce messages in the case of IEEE 1588-2008) and the better clock takes over as leader.^[11] The BMCA only considers the self-declared quality of clocks and does not take network link quality into consideration.^[12]

Via BMCA, PTP selects a source of time for an IEEE 1588 domain and for each network segment in the domain.

Clocks determine the offset between themselves and their leader.^[13] Let the variable${\displaystyle t}$ represent physical time. For a given follower device, the offset${\displaystyle o(t)}$ at time${\displaystyle t}$ is defined by:

${\displaystyle o(t)=s(t)-m(t)}$

where${\displaystyle s(t)}$ represents the time measured by the follower clock at physical time${\displaystyle t}$, and${\displaystyle m(t)}$ represents the time measured by the leader clock at physical time${\displaystyle t}$.

The leader periodically broadcasts the current time as a message to the other clocks. Under IEEE 1588-2002 broadcasts are up to once per second. Under IEEE 1588-2008, up to 10 per second are permitted.

Each broadcast begins at time${\displaystyle T_1}$ with aSync message sent by the leader to all the clocks in the domain. A clock receiving this message takes note of the local time${\displaystyle T_1^{\prime }}$ when this message is received.

The leader may subsequently send a multicastFollow_Up with accurate${\displaystyle T_1}$ timestamp. Not all leaders have the ability to present an accurate timestamp in theSync message. It is only after the transmission is complete that they are able to retrieve an accurate timestamp for theSync transmission from their network hardware. Leaders with this limitation use theFollow_Up message to convey${\displaystyle T_1}$. Leaders with PTP capabilities built into their network hardware are able to present an accurate timestamp in theSync message and do not need to send Follow_Up messages.

In order to accurately synchronize to their leader, clocks must individually determine the network transit time of theSync messages. The transit time is determined indirectly by measuring round-trip time from each clock to its leader. The clocks initiate an exchange with their leader designed to measure the transit time${\displaystyle d}$. The exchange begins with a clock sending aDelay_Req message at time${\displaystyle T_2}$ to the leader. The leader receives and timestamps theDelay_Req at time${\displaystyle T_2^{\prime }}$ and responds with aDelay_Resp message. The leader includes the timestamp${\displaystyle T_2^{\prime }}$ in theDelay_Resp message.

Through these exchanges a clock learns${\displaystyle T_1}$,${\displaystyle T_1^{\prime }}$,${\displaystyle T_2}$ and${\displaystyle T_2^{\prime }}$.

If${\displaystyle d}$ is the transit time for theSync message, and${\displaystyle \tilde{o}}$ is the constant offset between leader and follower clocks, then

${\displaystyle T_1^{\prime }-T_1=\tilde{o}+d\text{ and }{T}_2^{\prime }-T_2=-\tilde{o}+d}$

Combining the above two equations, we find that

${\displaystyle \tilde{o}=\frac{1}{2}(T_1^{\prime }-T_1-T_2^{\prime }+T_2)}$

The clock now knows the offset${\displaystyle \tilde{o}}$ during this transaction and can correct itself by this amount to bring it into agreement with their leader.

One assumption is that this exchange of messages happens over a period of time so small that this offset can safely be considered constant over that period. Another assumption is that the transit time of a message going from the leader to a follower is equal to the transit time of a message going from the follower to the leader. Finally, it is assumed that both the leader and follower can accurately measure the time they send or receive a message. The degree to which these assumptions hold true determines the accuracy of the clock at the follower device.^{[9]: Clause 6.2}

IEEE 1588-2008 standard lists the following set of features that implementations may choose to support:

• Alternate Time-Scale
• Grand Master Cluster
• Unicast Masters
• Alternate Master
• Path Trace

IEEE 1588-2019 adds additional optional and backward-compatible features:^[5]

• Modular transparent clocks
• Special PTP ports to interface with transports with built-in time distribution
• UnicastDelay_Req andDelay_Resp messages
• Manual port configuration overriding BMCA
• Asymmetry calibration
• Ability to utilize a physical layer frequency reference (e.g.Synchronous Ethernet)
• Profile isolation
• Inter-domain interactions
• Security TLV for integrity checking
• Standard performance reporting metrics
• Slave port monitoring

• TheInternational IEEE Symposium on Precision Clock Synchronization for Measurement, Control and Communication (ISPCS) is an IEEE-organized annual event that includes aplugtest and a conference program with paper and poster presentations, tutorials and discussions covering several aspects of PTP.^[14]
• The Institute of Embedded Systems (InES) of theZurich University of Applied Sciences/ZHAW is addressing the practical implementation and application of PTP.
• IEEE 1588 is a key technology in theLXI Standard for Test and Measurement communication and control.
• IEEE 802.1AS-2011 is part of the IEEEAudio Video Bridging (AVB) group of standards.^[k] It specifies a profile for use of IEEE 1588-2008 for time synchronization over a virtual bridged local area network as defined byIEEE 802.1Q. In particular, 802.1AS defines howIEEE 802.3 (Ethernet),IEEE 802.11 (Wi-Fi), andMoCA can all be parts of the same PTP timing domain.^[15]
• SMPTE 2059-2 is a PTP profile for use in synchronization of broadcast media systems.^[16]
• TheAES67 audio networking interoperability standard includes a PTPv2 profile compatible with SMPTE ST2059-2.^[17]
• Dante uses PTPv1 for synchronization.^[18]
• Q-LAN^[19] andRAVENNA^[18] use PTPv2 for time synchronization.
• The White Rabbit Project combinesSynchronous Ethernet and PTP.
• Precision Time Protocol Industry Profile PTP profiles (L2P2P and L3E2E) for industrial automation in IEC 62439-3
• IEC/IEEE 61850-9-3 PTP profile for substation automation adopted by IEC 61850
• Parallel Redundancy Protocol use of PTP profiles (L2P2P and L3E2E) for industrial automation in parallel networks
• PTP is being studied to be applied as a secure time synchronization protocol in power systems' Wide Area Monitoring^[20]

• List of PTP implementations – Systems supporting Precision Time Protocol
• Real-time communication – Protocols and communication hardware that give real-time guarantees

• NIST IEEE 1588 site
• PTP documentation at InES
• PTP and Synchronization of LTE mobile networks
• PTP explained under the installation / maintenance point of view
• Hirschmann PTP Whitepaper
• PTP overview in Cisco CGS 2520 Switch Software Configuration Guide
• Perspectives and priorities on RuggedCom Smart Grid Research IEC 61850 Technologies
• Projects with Smart Substation Solution
• McGhee, Jim; Goraj, Maciej (2010), "Smart High Voltage Substation Based on IEC 61850 Process Bus and IEEE 1588 Time Synchronization",2010 First IEEE International Conference on Smart Grid Communications, pp. 489–494,doi:10.1109/SMARTGRID.2010.5622092,ISBN 978-1-4244-6510-1,S2CID 30638718
• Ingram, D.M.E; Campbell, D.A.; Schaub, P.; Ledwich, G.F. (2011)."Test and evaluation system for multi-protocol sampled value protection schemes".2011 IEEE Trondheim PowerTech. Trondheim, Norway: IEEE. pp. 1–7.doi:10.1109/PTC.2011.6019243.ISBN 978-1-4244-8419-5.S2CID 42991214.
• The White Rabbit Project PTP
• IEC&IEEE Precision Time Protocol, Pacworld, September 2016
• IEC 62439-3 Annexes A-E Redundant attachment of clocks and network management
• PTPv2 Timing protocol in AV networks
• FSMLabs: Single source IEEE PTP 1588 cannot meet financial regulatory standards
• A PTP Testing Guide for Reliable and Accurate Verification by Fran Hens (ALBEDO 2019)

• Synchronization
• IEEE standards
• Network time-related software
• Network protocols
• Application layer protocols

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