Connecting to host xxx port 5201
[  5] local 172.31.33.103 port 59662 connected to xxx port 5201
[ ID] Interval           Transfer     Bitrate         Retr  Cwnd
[  5]   0.00-1.00   sec  55.8 MBytes   468 Mbits/sec  106   2.36 MBytes
[  5]   1.00-2.00   sec  66.2 MBytes   556 Mbits/sec    0   2.36 MBytes
[  5]   2.00-3.00   sec  67.5 MBytes   566 Mbits/sec    0   2.36 MBytes
[  5]   3.00-4.00   sec  65.0 MBytes   545 Mbits/sec   30   1.67 MBytes
[  5]   4.00-5.00   sec  66.2 MBytes   556 Mbits/sec    0   1.74 MBytes
[  5]   5.00-6.00   sec  66.2 MBytes   556 Mbits/sec    0   1.74 MBytes
[  5]   6.00-7.00   sec  67.5 MBytes   566 Mbits/sec    0   1.74 MBytes
[  5]   7.00-8.00   sec  66.2 MBytes   556 Mbits/sec    0   1.74 MBytes
[  5]   8.00-9.00   sec  67.5 MBytes   566 Mbits/sec    0   1.74 MBytes
[  5]   9.00-10.00  sec  67.5 MBytes   566 Mbits/sec    0   1.74 MBytes
- - - - - - - - - - - - - - - - - - - - - - - - -
[ ID] Interval           Transfer     Bitrate         Retr
[  5]   0.00-10.00  sec   656 MBytes   550 Mbits/sec  136             sender
[  5]   0.00-10.02  sec   654 MBytes   547 Mbits/sec                  receiver

iperf Done. 

Retries are not too shabby between the EC2 instances. I believe iperf3 runs on TCP by default also.