Skip to content

Tools » Initcwnd Checker tool

What is the initial congestion window of a CDN (or your web server) ?

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.

Bookmarklet

Notes

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.
Written in Go, publicly available on Github (pull requests are welcome!).

Related blog posts: