The Precision Time Protocol (PTP) is a protocol for clock synchronization throughout a computer network with relatively high precision as compared to using the earlier developed Network Time Protocol (NTP) and therefore potentially higher accuracy depending on the configuration. In a local area network (LAN), accuracy can be sub-microsecond making it suitable for measurement and control systems applications. PTP can be used to synchronize financial transactions, mobile phone tower transmissions, sub-sea acoustic arrays, and networks that require precise timing as an alternative to using the timestamp of satellite navigation signals or where sub-nanosecond accuracy as provided by the White Rabbit Project is unnecessary.
The first version of PTP, IEEE 1588-2002, was published in 2002. IEEE 1588-2008, also known as PTP Version 2, is not backward 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 a profile concept defining PTP operating parameters and options. Several profiles have been defined for applications including telecommunications, electric power distribution and audiovisual uses. is an adaptation of PTP, called gPTP, for use with Audio Video Bridging (AVB) and Time-Sensitive Networking (TSN).
History 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 and GPS. 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 a GPS receiver at each node, or for which GPS signals are inaccessible."
PTP was originally defined in the IEEE 1588-2002 standard, officially titled Standard 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 not backward compatible with the original 2002 version. IEEE 1588-2019 was published in November 2019, is informally known as PTPv2.1 and includes backwards-compatible improvements to the 2008 publication.
Architecture The IEEE 1588 standards describe a hierarchical master–slave architecture for clock distribution consisting of one or more network segments and one or more clocks. An ordinary 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 a master (alternately timeTransmitter), and a destination is called a slave (alternately timeReceiver). A boundary 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 the grandmaster.
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. The transparent clock modifies PTP messages as they pass through the device. Timestamps in the messages are corrected for time spent traversing the network equipment. This scheme improves distribution accuracy by compensating for delivery variability across the network.
PTP typically uses the same epoch as Unix time (start of 1 January 1970). While the Unix time is based on Coordinated Universal Time (UTC) and is subject to leap seconds, PTP is based on International 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.
Protocol details 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 and Delay_Resp messages are used by ordinary and boundary clocks and communicate time-related information used to synchronize clocks across the network. Pdelay_Req, Pdelay_Resp and Pdelay_Resp_Follow_Up are used by transparent 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 the original IEEE 1588-2002 PTPv1 standard, and were added in PTPv2. Announce messages are used by the best master clock algorithm in IEEE 1588-2008 to build a clock hierarchy and select the grandmaster. Management messages are used by network 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 as event and general 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 and Pdelay_resp are event messages. General messages are more conventional protocol 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 and Signaling messages are members of the general message class.
Message transport PTP messages may use the User Datagram Protocol over Internet Protocol (UDP/IP) for transport. IEEE 1588-2002 uses only IPv4 transports, but this has been extended to include IPv6 in IEEE 1588-2008. In IEEE 1588-2002, all PTP messages are sent using multicast messaging, while IEEE 1588-2008 introduced an option for devices to negotiate unicast transmission on a port-by-port basis. Multicast transmissions use IP multicast addressing, for which multicast group addresses are defined for IPv4 and IPv6 (see table). Time-critical event messages (Sync, Delay_req, Pdelay_Req and Pdelay_Resp) are sent to port number 319. General messages (Announce, Follow_Up, Delay_Resp, Pdelay_Resp_Follow_Up, management and signaling) use port number 320.
+ Multicast group addresses Messages IPv4 IPv6 IEEE 802.3 Ethernet Type All except peer delay messages 224.0.1.129 FF0x::181 01-1B-19-00-00-00 Forwardable Peer delay messages: Pdelay_Req, Pdelay_Resp and Pdelay_Resp_Follow_Up 224.0.0.107 FF02::6B 01-80-C2-00-00-0E Non-forwardable
In IEEE 1588-2008, encapsulation is also defined for DeviceNet, ControlNet and PROFINET.
Domains A domain 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 the Subdomain name (IEEE 1588-2002) or the domainNumber (IEEE 1588-2008) fields in PTP messages they receive or generate. Domains allow multiple clock distribution systems to share the same communications medium.
Subdomain name field contents () IPv4 multicast address() domainNumber() Notes _DFLT 224.0.1.129 0 Default domain _ALT1 224.0.1.130 1 Alternate domain 1 _ALT2 224.0.1.131 2 Alternate domain 2 _ALT3 224.0.1.132 3 Alternate domain 3 Application specific up to 15 octets 224.0.1.130, 131 or 132 as per hash function on Subdomain name 4 through 127 User-defined domains
Best master clock algorithm The best master clock algorithm (BMCA) performs a distributed selection of the best clock to act as leader based on the following clock properties: IdentifierA universally unique numeric identifier for the clock. This is typically constructed based on a device's MAC address. QualityBoth versions of IEEE 1588 attempt to quantify clock quality based on expected timing deviation, technology used to implement the clock or location in a clock stratum schema, although only V1 (IEEE 1588-2002) knows a data field stratum. PTP V2 (IEEE 1588-2008) defines the overall quality of a clock by using the data fields clockAccuracy and clockClass. PriorityAn administratively assigned precedence hint used by the BMCA to help select a grandmaster for the PTP domain. IEEE 1588-2002 used a single Boolean variable to indicate precedence. IEEE 1588-2008 features two 8-bit priority fields. VarianceA 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: Priority 1the user can assign a specific static-designed priority to each clock, preemptively defining a priority among them. Smaller numeric values indicate higher priority. Classeach clock is a member of a given class, each class getting its own priority. Accuracyprecision between clock and UTC, in nanoseconds (ns) Variancevariability of the clock Priority 2final-defined priority, defining backup order in case the other criteria were not sufficient. Smaller numeric values indicate higher priority. Unique identifierMAC 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 1588-2002 Sync messages and in IEEE 1588-2008 Announce 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 transmitting Sync messages and associated clock properties (Announce messages in the case of IEEE 1588-2008) and the better clock takes over as leader. The BMCA only considers the self-declared quality of clocks and does not take network link quality into consideration.
Synchronization 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.International standard IEC 61588: Precision clock synchronization protocol for networked measurement and control systems. 2004. Let the variable represent physical time. For a given follower device, the offset at time is defined by:
where represents the time measured by the follower clock at physical time , and represents the time measured by the leader clock at physical time .
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 with a Sync message sent by the leader to all the clocks in the domain. A clock receiving this message takes note of the local time when this message is received.
The leader may subsequently send a multicast Follow_Up with accurate timestamp. Not all leaders have the ability to present an accurate timestamp in the Sync message. It is only after the transmission is complete that they are able to retrieve an accurate timestamp for the Sync transmission from their network hardware. Leaders with this limitation use the Follow_Up message to convey . Leaders with PTP capabilities built into their network hardware are able to present an accurate timestamp in the Sync 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 the Sync 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 . The exchange begins with a clock sending a Delay_Req message at time to the leader. The leader receives and timestamps the Delay_Req at time and responds with a Delay_Resp message. The leader includes the timestamp in the Delay_Resp message.