Lots of re-transmit only on 1 direction

[U6xmLH76iA]

I have a R86s with 10G SFP+.

I found something weird that I can get 9.9Gbps from a client to the R86s with 0 (or nearly 0) retransmit running iperf. However, when I ran iperf in reverse mode, I got tons of retransmit.

I tried running both fibre and copper, and I got the same result and the number of retransmit was very similar. Nearly exact same result, so I am pretty sure it is not optics/sfp module/cable issues.

I tried disabling packet filtering on OPNSense and it was the same.

I even started to think maybe something is wrong on my client side, maybe I misconfigured something on my linux box.

Much appreciated if anyone has any idea why it might be happening.

[U6xmLH76iA]

I found that the issue is more on the Linux side, just disabling the checksum with ethtool greatly reduce the retransmit, and speed was much better. Still not quite 0 retransmit, but getting better speed already.