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The original 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 original 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. IEEE 802.1AS is an adaptation of PTP for use with Audio Video Bridging and Time-Sensitive Networking.
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 entitled "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.
The IEEE 1588 standards describe a hierarchical master-slave architecture for clock distribution. Under this architecture, a time distribution system consists of one or more communication media (network segments), and one or more clocks. An ordinary clock is a device with a single network connection and is either the source of (master or leader) or destination for (slave or follower) a synchronization reference. A boundary clock has multiple network connections and can accurately synchronize one network segment to another. A synchronization master is selected for each of the network segments in the system. The root timing reference is called the grandmaster. The grandmaster transmits synchronization information to the clocks residing on its network segment. The boundary clocks with a presence on that segment then relay accurate time to the other segments to which they are also connected.
A simplified PTP system frequently consists of ordinary clocks connected to a single network, and no boundary clocks are used. A grandmaster is elected and all other clocks synchronize directly 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.
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 original IEEE 1588-2002 PTPv1 standard, and were added in PTPv2.
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.
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.
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 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.
The best master clock (BMC) algorithm performs a distributed selection of the best candidate clock based on the following clock properties:
Identifier – A universally unique numeric identifier for the clock. This is typically constructed based on a device's MAC 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 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.
Priority – An administratively assigned precedence hint used by the BMC 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.
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:
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.
Class – each clock is a member of a given class, each class getting its own priority.
Accuracy – precision between clock and UTC, in nanoseconds (ns)
Variance – variability of the clock
Priority 2 – final-defined priority, defining backup order in case the other criteria were not sufficient. Smaller numeric values indicate higher priority.
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 1588-2002 Sync messages and in IEEE 1588-2008 Announce messages. The current clock master transmits this information at regular interval. A clock which considers itself a better master clock will transmit this information in order to invoke a change of master clock. Once the current master recognises the better clock, the current master stops transmitting Sync messages and associated clock properties (Announce messages in the case of IEEE 1588-2008) and the better clock takes over as master. The BMC algorithm only considers the self-declared quality of clocks and does not take network link quality into consideration.
Through use of the BMC algorithm, PTP selects a master source of time for an IEEE 1588 domain and for each network segment in the domain.
Clocks determine the offset between themselves and their master. 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 master clock at physical time .
The master 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.
IEEE 1588 synchronisation mechanism and delay calculation
Each broadcast begins at time with a Sync message sent by the master to all the clocks in the domain. A clock receiving this message takes note of the local time when this message is received.
The master may subsequently send a multicast Follow_Up with accurate timestamp. Not all masters 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. Masters with this limitation use the Follow_Up message to convey . Masters 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 master, 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 master. The clocks initiate an exchange with their master designed to measure the transit time . The exchange begins with a clock sending a Delay_Req message at time to the master. The master receives and timestamps the Delay_Req at time and responds with a Delay_Resp message. The master includes the timestamp in the Delay_Resp message.
Through these exchanges a clock learns , , and .
If is the transit time for the Sync message, and is the constant offset between master and follower clocks, then
Combining the above two equations, we find that
The clock now knows the offset during this transaction and can correct itself by this amount to bring it into agreement with their master.
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 master to a follower is equal to the transit time of a message going from the follower to the master. Finally, it is assumed that both the master 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.
IEEE 1588-2008 standard lists the following set of features that implementations may choose to support:
Grand Master Cluster
IEEE 1588-2019 adds additional optional and backward-compatible features:
Modular transparent clocks
Special PTP ports to interface with transports with built-in time distribution
The International IEEE Symposium on Precision Clock Synchronization for Measurement, Control and Communication (ISPCS) is an IEEE organized annual event that includes a plugtest and a conference program with paper and poster presentations, tutorials and discussions covering several aspects of PTP
^In some PTP applications it is permissible to send all PTP messages to 01-1B-19-00-00-00
^Peer delay messages are intended to propagate to the immediately connected neighbor. The multicast addresses for these messages are designed to be link-local in scope and are not passed through a router. IEEE 1588-2008 also recommends setting time to live to 1 (IPv4) or hop limit to 0 (IPv6) as further insurance that the messages will not be routed.
^Peer delay messaging is not present in IEEE 1588-2002
^IEEE 1588-2002 defines a domain as any interconnected set of clocks (regardless of whether they synchronized to one another) and uses subdomain to refer to what is known as a domain in IEEE 1588-2008.
^IEEE 1588-2008 uses 220.127.116.11 as the address for all multicast messages.