Preventing the Collision of Requests From Slave Clocks in the Precision Time Protocol (PTP)

Abstract

Abstract-The Precision Time Protocol (PTP) distributes a time reference across a network: it specifically addresses demanding environments, where it can reach sub microsecond precision using appropriate technologies. Its scalability is primarily limited by message packet delay variations induced by packet collisions.While it is possible to avoid collisions with non-PTP packets using appropriate traffic management technologies, collision between PTP packets is an open problem in large systems with critical clock precision requirements.We propose a coordination algorithm that avoids the occurrence of such collisions. It assumes the availability of a multicast facility from the timing reference source, the master clock, to all the slaves: this is not a restrictive hypotesis since PTP itself takes advantage of this kind of connectivity, and it is also compatible with typical wireless environments.The algorithm operates without introducing additional traffic, it ensures an upper bound to the time between two successive synchronizations of any given slave, it does not alter the structure of the standard PTP messages, it envisions a dynamic number of slaves, it tolerates the replacement of the master with a hot spare in case of failure, and does not rely on specialized hardware.The algorithm has a footprint that does not insist on activities that are already time sensitive, and its operation is mostly concentrated on the master.The algorithm inherits the security and fault tolerance limits of PTP: in particular this refers to malicious nodes, and to broken devices that may jam the network.

A Survey of Clock Synchronization Over Packet-Switched Networks

“…Adopting this approach is cost-effective, since only a few configurations are required at each node with QoS support. However, it is worth noting that if multiple PTP messages access the same router at the same time, there will still be non negligible queuing delays, as noted in [90]. To overcome this issue, a token-based mechanism was proposed, whereby a given slave node can transmit PTP messages only when it holds a token [90].…”

“…However, it is worth noting that if multiple PTP messages access the same router at the same time, there will still be non negligible queuing delays, as noted in [90]. To overcome this issue, a token-based mechanism was proposed, whereby a given slave node can transmit PTP messages only when it holds a token [90]. The proposed token-based was shown to be operational in a PTP system without introducing significant additional traffic.…”

Enhancing Accuracy and Robustness of Frequency Transfer Using Synchronous Ethernet and Multiple Network Paths

“…In order to improve accuracy further, especially in high-end industrial and scientific applications, PTP performances have been enhanced by means of various additional features. These include clock state estimators [11], [12], optimal servo-clocks [13], coordination algorithms at the protocol level [14], techniques to estimate and to compensate for the effect of frequency offsets and drifts in multi-hop networks [15], [16], and frequency transfer and control strategies in order to achieve both clock synchronization and syntonization [17]. Two clocks are said to be syntonized if the duration of the second is the same on both, which means that the time, as measured by each, advances at the same rate.…”