WireGuard unstable speed

Hello.

I’m using WireGuard(UDP) to connect my three VPS together, one of which is DigitalOcean’s droplet. And I noticed that when I’m using iPerf3 to test DigitalOcean’s inbound bandwidth, the speed dropped to nearly zero after 1-2 seconds. Here is the output of iPerf3 (on my another VPS): (10.200.200.2 is the IP of DigitalOcean’s droplet connected with WireGuard)

$ iperf3 -c 10.200.200.2
Connecting to host 10.200.200.2, port 5201
[  4] local 10.200.200.1 port 43684 connected to 10.200.200.2 port 5201
[ ID] Interval           Transfer     Bandwidth       Retr  Cwnd
[  4]   0.00-1.00   sec  33.6 MBytes   281 Mbits/sec    4   2.48 MBytes       
[  4]   1.00-2.00   sec  0.00 Bytes  0.00 bits/sec   27   2.45 MBytes       
[  4]   2.00-3.00   sec  0.00 Bytes  0.00 bits/sec   27   2.41 MBytes       
[  4]   3.00-4.00   sec  4.92 MBytes  41.3 Mbits/sec   24   2.99 MBytes       
[  4]   4.00-5.00   sec  0.00 Bytes  0.00 bits/sec   27   2.95 MBytes       
[  4]   5.00-6.00   sec  0.00 Bytes  0.00 bits/sec   27   2.92 MBytes       
[  4]   6.00-7.00   sec  0.00 Bytes  0.00 bits/sec   27   2.88 MBytes       
[  4]   7.00-8.00   sec  0.00 Bytes  0.00 bits/sec   27   2.85 MBytes       
[  4]   8.00-9.00   sec  0.00 Bytes  0.00 bits/sec   27   2.81 MBytes       
[  4]   9.00-10.00  sec  0.00 Bytes  0.00 bits/sec   27    652 KBytes       
- - - - - - - - - - - - - - - - - - - - - - - - -
[ ID] Interval           Transfer     Bandwidth       Retr
[  4]   0.00-10.00  sec  38.5 MBytes  32.3 Mbits/sec  244             sender
[  4]   0.00-10.00  sec  35.4 MBytes  29.7 Mbits/sec                  receiver

iperf Done.

But there’s no problem to send data:

$ iperf3 -c 10.200.200.2 -R
Connecting to host 10.200.200.2, port 5201
Reverse mode, remote host 10.200.200.2 is sending
[  4] local 10.200.200.1 port 43688 connected to 10.200.200.2 port 5201
[ ID] Interval           Transfer     Bandwidth
[  4]   0.00-1.00   sec  15.3 MBytes   129 Mbits/sec                  
[  4]   1.00-2.00   sec  16.3 MBytes   137 Mbits/sec                  
[  4]   2.00-3.00   sec  15.9 MBytes   133 Mbits/sec                  
[  4]   3.00-4.00   sec  25.1 MBytes   211 Mbits/sec                  
[  4]   4.00-5.00   sec  33.3 MBytes   280 Mbits/sec                  
[  4]   5.00-6.01   sec  30.7 MBytes   256 Mbits/sec                  
[  4]   6.01-7.00   sec  31.0 MBytes   262 Mbits/sec                  
[  4]   7.00-8.00   sec  23.0 MBytes   193 Mbits/sec                  
[  4]   8.00-9.00   sec  33.6 MBytes   281 Mbits/sec                  
[  4]   9.00-10.00  sec  34.3 MBytes   288 Mbits/sec                  
- - - - - - - - - - - - - - - - - - - - - - - - -
[ ID] Interval           Transfer     Bandwidth       Retr
[  4]   0.00-10.00  sec   262 MBytes   220 Mbits/sec   21             sender
[  4]   0.00-10.00  sec   260 MBytes   218 Mbits/sec                  receiver

iperf Done.

Using my 3rd VPS, I can still reproduce the problem, so basically I think this is a problem of DigitalOcean’s droplet.

I guess that maybe some of your DDoS protection system is blocking high-bandwidth UDP, but I’m not sure. Can you help me tracing the problem down?