] 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."

Your TCP packets all contain a cookie.

Tracking PCs anywhere on the Net | CNET News.com