What is TCP Fast Open and How To Enable it in Microsoft Edge

TCP Fast Open (TFO) is a new feature Microsoft Edge adopted that speeds up the web browser’s page loading time, according to Google Research, by 10% in average and in some cases up to 40%.

So what is TCP Fast Open anyway?

Defined by RFC 7413, TCP Fast Open is an extension to speed up the opening of successive TCP connections between two endpoints. It works by using an encrypted TCP cookie stored on the client side to set upon the initial connection with the server.

Why it’s faster?

Standard TCP requires the client and server to establish a three-way handshake (3WHS) before data can be delivered to the server’s listening application. With TFO, it sends the initial SYN packet along with the TFO cookie data to authenticate itself. If successful, the server may start sending data to the client before the final three-way handshake, skipping a full round-trip delay and lowering the latency in the start of the data transmission.


The theory sounds great, but the adoption hasn’t been there yet. It’s only available in Linux 3.7+ on the server end. And on the client side, only Chrome/Chromium on Linux, ChromeOS, and Android 5.0, and Microsoft Edge on Windows support it. Microsoft Edge is the only browser that supports TCP Fast Open on Windows.

How to enable it on Microsoft Edge?

Microsoft Edge supports TCP Fast Open since Windows 10 Build 14352. However, it’s not enabled by default. To enable the feature and speed up the page load in Edge,

  1. Type about:flags in the address bar and hit Enter.
  2. Scroll down to the bottom of the page where you find the Networking section.
  3. Check the option “Enable TCP Fast Open“, and restart Edge browser.

Developer settings ‎- Microsoft Edge - 2016-07-23 17_22_12

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