37-566: The Precision Time Protocol ( PTP ) is a protocol for clock synchronization throughout a computer network with relatively high precision and therefore potentially high accuracy. In a local area network (LAN), accuracy can be sub-microsecond – making it suitable for measurement and control systems. PTP is used to synchronize financial transactions , mobile phone tower transmissions, sub-sea acoustic arrays , and networks that require precise timing but lack access to satellite navigation signals. The first version of PTP, IEEE 1588-2002 ,
74-403: 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 T 1 ′ {\displaystyle T_{1}'} when this message is received. The leader may subsequently send a multicast Follow_Up with accurate T 1 {\displaystyle T_{1}} timestamp. Not all leaders have
111-462: 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. Via BMCA, PTP selects
148-469: A clock sending a Delay_Req message at time T 2 {\displaystyle T_{2}} to the leader. The leader receives and timestamps the Delay_Req at time T 2 ′ {\displaystyle T_{2}'} and responds with a Delay_Resp message. The leader includes the timestamp T 2 ′ {\displaystyle T_{2}'} in
185-547: A distributed selection of the best clock to act as leader based on the following clock properties: IEEE 1588-2008 uses a hierarchical selection algorithm based on the following properties, in the indicated order: 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
222-520: 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: IEEE 1588-2019 adds additional optional and backward-compatible features: Protocol (computing) Too Many Requests If you report this error to the Wikimedia System Administrators, please include
259-429: A network address to help with routing, a code to identify the type of data in the packet and error-checking information. All this additional information, plus the original service data unit from the higher layer, constitutes the protocol data unit at this layer. The SDU and metadata added by the lower layer can be larger than the maximum size of that layer's PDU (known as the maximum transmission unit ; MTU). When this
296-421: 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. In IEEE 1588-2008, encapsulation
333-401: A service data unit to that layer. The addition of addressing and control information (encapsulation) to an SDU to form a PDU and the passing of that PDU to the next lower layer as an SDU repeats until the lowest layer is reached and the data passes over some medium as a physical signal. The above process can be likened to the mail system in which a letter (SDU) is placed in an envelope on which
370-495: 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. Let the variable t {\displaystyle t} represent physical time. For a given follower device, the offset o ( t ) {\displaystyle o(t)} at time t {\displaystyle t} is defined by: where s ( t ) {\displaystyle s(t)} represents
407-442: 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 leader , a.k.a. master, and a destination is called a follower , a.k.a. slave. 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
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#1732783124023444-582: Is also defined for DeviceNet , ControlNet and PROFINET . 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. The best master clock algorithm (BMCA) performs
481-494: 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
518-409: Is an adaptation of PTP, called gPTP, for use with Audio Video Bridging (AVB) and Time-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 and GPS . IEEE 1588 is designed for local systems requiring accuracies beyond those attainable using NTP. It
555-499: Is called a cell . A media access control protocol data unit ( MAC PDU or MPDU ) is a message that is exchanged between media access control (MAC) entities in a communication system based on the layered OSI model. In systems where the MPDU may be larger than the MAC service data unit (MSDU), the MPDU may include multiple MSDUs as a result of packet aggregation . In systems where
592-407: 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
629-422: Is labeled with the region to which all the bags are to be sent, making the crate a PDU. When the crate reaches the destination matching its label, it is opened and the bags (SDUs) removed only to become PDUs when someone reads the code of the destination post office. The letters themselves are SDUs when the bags are opened but become PDUs when the address is read for final delivery. When the addressee finally opens
666-461: 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 consisting of one or more network segments and one or more clocks. An ordinary clock
703-425: 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
740-475: Is the case, the PDU must be split into multiple payloads of a size suitable for transmission or processing by the lower layer; a process known as IP fragmentation . The significance of this is that the PDU is the structured information that is passed to a matching protocol layer further along on the data's journey that allows the layer to deliver its intended function or service. The matching layer, or "peer", decodes
777-533: Is the transit time for the Sync message, and o ~ {\displaystyle {\tilde {o}}} is the constant offset between leader and follower clocks, then Combining the above two equations, we find that The clock now knows the offset o ~ {\displaystyle {\tilde {o}}} during this transaction and can correct itself by this amount to bring it into agreement with their leader. One assumption
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#1732783124023814-407: Is written an address (addressing and control information) making it a PDU. The sending post office might look only at the postcode and place the letter in a mailbag so that the address on the envelope can no longer be seen, making it now an SDU. The mailbag is labeled with the destination postcode and so becomes a PDU until it is combined with other bags in a crate when it is now an SDU, and the crate
851-452: The Delay_Resp message. Through these exchanges a clock learns T 1 {\displaystyle T_{1}} , T 1 ′ {\displaystyle T_{1}'} , T 2 {\displaystyle T_{2}} and T 2 ′ {\displaystyle T_{2}'} . If d {\displaystyle d}
888-562: The Internet protocol suite , at the Internet layer , the PDU is called a packet , irrespective of its payload type. In the context of packet switching data networks, a protocol data unit (PDU) is best understood in relation to a service data unit (SDU). The features or services of the network are implemented in distinct layers . The physical layer sends ones and zeros across a wire or fiber. The data link layer then organizes these ones and zeros into chunks of data and gets them safely to
925-461: 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 d {\displaystyle d} . The exchange begins with
962-448: The SDU, but the lower layer at the interface does not; moreover, the lower layer treats the SDU as the payload , undertaking to get it to the same interface at the destination. In order to do this, the protocol (lower) layer will add to the SDU certain data it needs to perform its function; which is called encapsulation . For example, it might add a port number to identify the application,
999-519: 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 T 1 {\displaystyle T_{1}} . Leaders with PTP capabilities built into their network hardware are able to present an accurate timestamp in
1036-626: 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. 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
1073-702: 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
1110-451: The data to extract the original service data unit, decide if it is error-free and where to send it next, etc. Unless we have already arrived at the lowest (physical) layer, the PDU is passed to the peer using services of the next lower layer in the protocol "stack". When the PDU passes over the interface from the layer that constructed it to the layer that merely delivers it (and therefore does not understand its internal structure), it becomes
1147-439: The details below. Request from 172.68.168.236 via cp1112 cp1112, Varnish XID 949068348 Upstream caches: cp1112 int Error: 429, Too Many Requests at Thu, 28 Nov 2024 08:38:44 GMT Protocol data units In telecommunications , a protocol data unit ( PDU ) is a single unit of information transmitted among peer entities of a computer network . It is composed of protocol-specific control information and user data . In
Precision Time Protocol - Misplaced Pages Continue
1184-643: The envelope, the top-level SDU, the letter itself, emerges. Protocol data units of the OSI model are: Given a context pertaining to a specific OSI layer, PDU is sometimes used as a synonym for its representation at that layer. Protocol data units for the Internet protocol suite are: On TCP/IP over Ethernet, the data on the physical layer is carried in Ethernet frames . The data link layer PDU in Asynchronous Transfer Mode (ATM) networks
1221-523: The layered architectures of communication protocol stacks, each layer implements protocols tailored to the specific type or mode of data exchange. For example, the Transmission Control Protocol (TCP) implements a connection-oriented transfer mode, and the PDU of this protocol is called a segment , while the User Datagram Protocol (UDP) uses datagrams as protocol data units for connectionless communication . A layer lower in
1258-497: 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
1295-400: The right place on the wire. The network layer transmits the organized data over multiple connected networks, and the transport layer delivers the data to the right software application at the destination. Between the layers (and between the application and the top-most layer), the layers pass service data units (SDUs) across interfaces. The higher layer understands the structure of the data in
1332-588: The time measured by the follower clock at physical time t {\displaystyle t} , and m ( t ) {\displaystyle m(t)} represents the time measured by the leader clock at physical time t {\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 T 1 {\displaystyle T_{1}} with
1369-485: 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. IEEE 802.1AS
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