TCP Fast Open

This RFC 7413 was published in 2014.


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.

RFC 7413 introduction

TCP Fast Open (TFO) is an experimental update to TCP that enables data to be exchanged safely during TCP's connection handshake.
This document describes a design that enables applications to save a round trip while avoiding severe security ramifications.
At the core of TFO is a security cookie used by the server side to authenticate a client initiating a TFO connection.
This document covers the details of exchanging data during TCP's initial handshake, the protocol for TFO cookies, potential new security vulnerabilities and their mitigation, and the new socket API.

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