Comment 0 for bug 1796895

Hardware:

HP DL380 Gen 7 server with Gigabit interface.
Ubuntu release: 18.04.1

Since upgrading from 4.15.0-33 to 4.15.0-36, I have noticed a dramatic slowdown in TCP over IPv6.

Set up: two servers, Server A in US on Ubuntu 18.04.1, Server B in Europe.

Test: Sending 400MB over TCP from Server B to Server A.

With 4.15.0-33 on Server A, the transfer is completed in 24 seconds, with average speed of 139 mbps.

With 4.15.0-36 on Server A, the same amount of data is transferred in 369 seconds, average speed less than 9 mbps.

Looking at the result from tcpdump, I see that the there is a big difference in the TCP Window size.

With 3.15-0.33 the TCP window size is around 30000. But with 3.15-0.36, the window size is only 734.