Napkin Problem 15: Increase Performance by Fitting In the Initial TCP Slow Start Window

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2021-07-29 21:30:06

Did you know that if your site’s under ~12kb the first page will load significantly faster? Servers only send a few packets (typically 10) in the initial round-trip while TCP is warming up (referred to as TCP slow start). After sending the first set of packets, it needs to wait for the client to acknowledge it received all those packets.

Quick illustration of transferring ~15kb with an initial TCP slow start window (also referred to as initial congestion window or initcwnd) of 10 versus 30:

The larger the initial window, the more we can transfer in the first roundtrip, the faster your site is on the initial page load. For a large roundtrip time (e.g. across an ocean), this will start to matter a lot. Here is the approximate size of the initial window for a number of common hosting providers:

To generate this, I wrote a script that you can use sirupsen/initcwnd to analyze your own site. Based on the report, you can attempt to tune your page size, or tune your server’s initial slow start window size (initcwnd) (see bottom of article). Read on, and we’ll go into far more detail!