Digital Signatures for PTP Using Transparent Clocks

Abstract

Smart grids use synchronous real-time measurements from phasor measurement units (PMU) across portions of a grid to provide grid-wide integrity. Achieving synchronicity requires either accurate GPS clocks at each PMU or a high-resolution clock synchronization protocol, such as the Precision Time Protocol (PTP), specified in IEEE 1588 with the power profile in IEEE C37.238-2011. PTP does not natively include measures to provide authenticity or integrity for timestamps transmitted across an Ethernet network, though there has been recent work in providing end-to-end integrity of transmitted timestamps. However, PTP for use in the smart grid requires a version of the protocol in which network switches update the trusted timestamp in flight, meaning that an end-to-end approach is no longer sufficient. We propose two methods to provide for the integrity of the transmitted and updated timestamps as well as to ensure the authority of all network devices altering the time. In the first, we amend the PTP standard to include signatures as part of the time packet itself at the cost of increased jitter in the system. In the second, we transmit these signatures over a wireless network, reducing congestion on the original network. We test both methods on a simulated PTP switch intended for experimentation only and demonstrate that the use of a second network dedicated to verification-related information is better for current networks, as including signatures in the original packet causes more jitter than is acceptable for synchronizing PMUs in particular.