[via Mitch]: Tracking PCs everywhere on the Net (paper):

The technique works by "exploiting small, microscopic deviations in device hardware: clock skews." In practice, Kohno's paper says, his techniques "exploit the fact that most modern TCP stacks implement the TCP timestamps option from RFC 1323 whereby, for performance purposes, each party in a TCP flow includes information about its perception of time in each outgoing packet. A fingerprinter can use the information contained within the TCP headers to estimate a device's clock skew and thereby fingerprint a physical device."

Kohno goes on to say: "Our techniques report consistent measurements when the measurer is thousands of miles, multiple hops, and tens of milliseconds away from the fingerprinted device, and when the fingerprinted device is connected to the Internet from different locations and via different access technologies. Further, one can apply our passive and semi-passive techniques when the fingerprinted device is behind a NAT or firewall."

And the paper stresses that "For all our methods, we stress that the fingerprinter does not require any modification to or cooperation from the fingerprintee." Kohno and his team tested their techniques on many operating systems, including Windows XP and 2000, Mac OS X Panther, Red Hat and Debian Linux, FreeBSD, OpenBSD and even Windows for Pocket PCs 2002.

"In all cases," the paper says, "we found that we could use at least one of our techniques to estimate clock skews on the machines and that we required only a small amount of data, although the exact data requirements depended on the operating system in question."