This document describes an experimental TCP mechanism called TCP Fast Open (TFO). TFO allows data to be carried in the SYN and SYN-ACK packets and consumed by the receiving end during the initial connection handshake@ and saves up to one full round-trip time (RTT) compared to the standard TCP@ which requires a three-way handshake (3WHS) to complete before data can be exchanged. However@ TFO deviates from the standard TCP semantics@ since the data in the SYN could be replayed to an application in some rare circumstances. Applications should not use TFO unless they can tolerate this issue@ as detailed in the Applicability section.