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Initcwnd Checker tool

Quickly measure the initial congestion window (initcwnd) of a CDN (or your web server).

The Initcwnd Checker tool is offline. It worked fine for http but everybody is on https nowadays and the tool does not work with https. Follow us on Twitter to hear about the launch of new tools (coming soon!)

Initcwnd Checker enables you to quickly and easily find out how many packets a server sends in the first roundtrip after the first GET request on a fresh connection. Simply enter the full URL (http only, no https!) of a >70 KB file.


The test runs on a EC2 instance in Virginia, US. The receive window is fixed at 65335. The only request header sent is the Host header, which means that if you put a URL for a JS or CSS file, the compressed version is not explicitly requested and the server most likely returns the uncompressed version. Also: redirects are not followed. Packet and byte count is for the payload only. A separate test is run to determine Total bytes received.

Related blog posts:

CDN Tools