[strongSwan] one direction slow over strongswan

[Jin Mao]

I used Ubuntu 22 and Strongswan 5.9.5 to setup a WAN connection over a high
latency (240ms) line like this:

A == VPN1 == VPN2 == VPN3 == B

A and B are servers in two servers networks
VPN1 and VPN2 are two Strongswan 5.9.5 on Ubuntu 22, managed by me.
VPN3 is a VPN device not managed by me.

VPN2 has a total network bandwidth of 20Mb/s.

The test is to copy files between these servers using rsync.
A, pulling files from B, can only reach 150Kb/s
A. pushing files to B, can reach 20Mb/s
VPN1, pulling files from B, can only reach 150Kb/s
VPN2, pushing files to B, can reach 20Mb/s
VPN2, pulling files from B, can reach 20Mb/s
VPN2, pushing files to B, can reach 20Mb/s

iperf3 client on A and iperf3 server on B can reach 20Mb/s.

Multiple rsync on A, pulling files from B, can push the network bandwidth
usage higher. I tested 4 concurrent rsync processes and reached 10Mb/s.

During all of these tests, CPU load and memory usage of VPN2 is minimal. No
other workloads.

Does anyone have a clue?

Thank you.

JM.
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