Clock Synchronization Over Communication Paths With Queue-Induced Delay Asymmetries

摘要

Timing protocols such as the IEEE 1588 Precision Time Protocol and the Network Time Protocol require an accurate measurement of the communication path delay between the time server (master) and the client (slave) in order to provide a precise timing synchronization. The precise time at the client's site is then estimated using an assumption that forward and backward delays due to physical propagation time through network are equal, or any difference between them is calibrated beforehand. Apart from physical link delays, a timing packet experiences queue-induced delay due to switching/routing devices on the path. This queuing delay is usually different in forward and backward directions, thus introducing the queue-induced asymmetry (QIA), which is a major contributor to the time error between master and slave clocks if physical asymmetries are calibrated. This letter proposes a new technique for QIA compensation that does not require any on-path timing support, thus is easily deployed with current network devices.