Meanings of common terms used in IEEE 1588

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1. Boundary clock: A boundary clock is a clock with more than a single PTP port, with each PTP port providing access to a separate PTP communication path. Boundary clocks are used to eliminate fluctuations produced by routers and similar network elements.
2. Clock:   A device providing a measurement of the passage of time since a defined epoch. There are two types of clocks in 1588: boundary clocks and ordinary clocks.
3. Clock timestamp point: 1588 requires the generation of a timestamp on transmission or receipt of all 1588 Sync and Delay_Req messages. The point in the outbound and inbound protocol stacks where this timestamp is generated is called the clock timestamp point.
4. Direct communication: The communication of PTP information between two PTP clocks with no intervening boundary clock is termed a direct communication.
5. External synchronization: It is often desirable to synchronize a single clock to an external source of time, for example to a GPS system to establish a UTC time base. This synchronization is accomplished by means other than those specified by 1588 and is referred to as external synchronization
6. Epoch:   The reference time defining the origin of a time scale is termed the epoch.
7. Grandmaster clock:  Within a collection of 1588 clocks one clock, the grandmaster clock, will serve as the primary source of time to which all others are ultimately synchronized.
8. Master clock: A system of 1588 clocks may be segmented into regions separated by boundary clocks. Within each region there will be a single clock, the master clock, serving as the primary source of time. These master clocks will in turn synchronize to other master clocks and ultimately to the grandmaster clock.
9. Message timestamp point: 1588 Sync and Delay_Req messages contain a distinguished feature, the message timestamp point, serving as a reference point in these messages. When the message timestamp point passes the clock timestamp point, a timestamp is generated that is used by 1588 to compute the necessary corrections to the local clock.
10. Ordinary clock: An ordinary clock is a 1588 clock with a single PTP port.
11. Preferred master clock set: 1588 allows the definition a set of clocks that will be favored over those not so designated in the selection of the grandmaster clock.
12. PTP:   PTP is an acronym for Precision Time Protocol, the name used in the standard for the protocol.
13. PTP domain: A PTP domain is a collection of one or more PTP subdomains. A subdomain is a logical grouping of 1588 clocks that synchronize to each other using the PTP protocol, but that are not necessarily synchronized to PTP clocks in another PTP subdomain. Subdomains provide a way of implementing disjoint sets of clocks, sharing a common network, but maintaining independent synchronization within each set.
14. PTP message: There are five designated messages types defined by 1588: Sync, Delay_Req, Follow-up, Delay_Resp, and Management.
15. Multicast communication: 1588 requires that PTP messages be communicated via a multicast. In this style of communication any node may post a message and all nodes in the same segment of a subdomain will receive this message. Boundary clocks define the segments within a subdomain.
16. PTP port: A PTP port is the logical access point for 1588 communications to the clock containing the port.
17. Synchronized clocks: Two clocks are synchronized to a specified uncertainty if they have the same epoch and measurements of any time interval by both clocks differ by no more than the specified uncertainty. The timestamps generated by two synchronized clocks for the same event will differ by no more than the specified uncertainty.