PTP (Precision Time Protocol)

networkingprogramming

Precision Time Protocol, IEEE 1588: time synchronisation to sub-microsecond accuracy, achieved by putting timestamps in hardware rather than software.

NTP is accurate to a few milliseconds over a normal network, which is ample for logs and certificates and useless for a power substation, a broadcast facility, a mobile base station or a trading floor. PTP closes the gap by attacking the largest source of error, which is not the network but the operating system: a timestamp taken in software has already been delayed by scheduling, interrupts and buffering by an unknown and variable amount. PTP timestamps in the network interface hardware, as close to the wire as the design allows, and switches along the path participate rather than just forwarding - a transparent clock measures how long a packet spent inside it and writes that into a correction field, while a boundary clock terminates the timing and regenerates it. That participation is the point and the deployment difficulty: PTP accuracy is a property of the whole path, so a single switch that neither corrects nor regenerates will quietly cap what the whole domain can achieve. Industry profiles constrain the many options for particular uses, including telecom profiles for mobile networks and financial ones written against regulatory timestamping requirements. The general lesson transfers beyond timing: when the error you care about is introduced by the layers between your measurement and the wire, the fix is to move the measurement, not to average harder.

Also known as: precision time protocol, ieee 1588, 1588v2

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