Saturday, August 14, 2021

Help me investigate slow download speed from my server

I have slow speed issues with my server that is running on Ubuntu. It has 250Mbits link but when downloading from it my speed is capped around 5Mb/s. I have connection of 1Gb/s.

I'm pretty sure that the issue is related to the tcp window size. Window scaling is enabled on both and tcpdump shows that the scaling is 10 for the server and 9 and 8 for the clients. But the window size value is low. I see 30 or 32 for the depending on which client is connecting and a value around 300 for the clients. . So the calculated window size is low.

Sometimes I don't this issue and in this case the calculated window size is around 30000000

I don't have issues when uploading to the server.

How is this total window size is determined?



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