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.