  High precision, synchronized clocks are essential to a growing number of
Internet applications. Standard protocols and their associated server
infrastructure have been shown to typically enable client clocks to synchronize
on the order of tens of milliseconds. We address one of the key challenges to
high precision Internet timekeeping - the intrinsic contribution to clock error
of path asymmetry between client and time server, a fundamental barrier to
microsecond level accuracy. We first exploit results of a measurement study to
quantify asymmetry and its effect on timing. We then describe three approaches
to addressing the path asymmetry problem: LBBE, SBBE and K-SBBE, each based on
timestamp exchange with multiple servers, with the goal of tightening bounds on
asymmetry for each client. We explore their capabilities and limitations
through simulation and argument. We show that substantial improvements are
possible, and discuss whether, and how, the goal of microsecond accuracy might
be attained.
