iperf测试带宽的问题 -- 上下行带宽差距大

今天测试机房的两台服务器之间的网络带宽，发现了一个奇怪的现象：

同时测试上下行带宽：

[root@ptl3m log]# iperf -c 172.28.2.21 -t 60 -i 10  -d -L 8083
------------------------------------------------------------
Server listening on TCP port 8083
TCP window size: 4.00 KByte (default)
------------------------------------------------------------
------------------------------------------------------------
Client connecting to 172.28.2.21, TCP port 5001
TCP window size:  400 KByte (default)
------------------------------------------------------------
[  3] local 172.28.2.23 port 46174 connected with 172.28.2.21 port 5001
[  5] local 172.28.2.23 port 8083 connected with 172.28.2.21 port 39939
[ ID] Interval       Transfer     Bandwidth
[  3]  0.0-10.0 sec  4.37 GBytes  3.75 Gbits/sec
[  5]  0.0-10.0 sec   552 MBytes   463 Mbits/sec
[  3] 10.0-20.0 sec  4.84 GBytes  4.16 Gbits/sec
[  5] 10.0-20.0 sec   221 MBytes   185 Mbits/sec
[  3] 20.0-30.0 sec  4.87 GBytes  4.19 Gbits/sec
[  5] 20.0-30.0 sec   303 MBytes   254 Mbits/sec
[  3] 30.0-40.0 sec  4.01 GBytes  3.45 Gbits/sec
[  5] 30.0-40.0 sec  1.04 GBytes   891 Mbits/sec
[  3] 40.0-50.0 sec  4.81 GBytes  4.14 Gbits/sec
[  5] 40.0-50.0 sec   223 MBytes   187 Mbits/sec
[  3] 50.0-60.0 sec  4.75 GBytes  4.08 Gbits/sec
[  3]  0.0-60.0 sec  27.7 GBytes  3.96 Gbits/sec
[  5] 50.0-60.0 sec   347 MBytes   291 Mbits/sec
[  5]  0.0-60.0 sec  2.65 GBytes   379 Mbits/sec

从上面可以看到这种测试方法上下行带宽差距较大。但是采用下面的命令测试，带宽差距就没有那么大：

[root@ptl3m log]# iperf -c 172.28.2.21 -t 60 -i 10  -r -L 8083
------------------------------------------------------------
Server listening on TCP port 8083
TCP window size: 4.00 KByte (default)
------------------------------------------------------------
------------------------------------------------------------
Client connecting to 172.28.2.21, TCP port 5001
TCP window size:  400 KByte (default)
------------------------------------------------------------
[  3] local 172.28.2.23 port 46130 connected with 172.28.2.21 port 5001
[ ID] Interval       Transfer     Bandwidth
[  3]  0.0-10.0 sec  4.67 GBytes  4.01 Gbits/sec
[  3] 10.0-20.0 sec  4.17 GBytes  3.58 Gbits/sec
[  3] 20.0-30.0 sec  4.70 GBytes  4.04 Gbits/sec
[  3] 30.0-40.0 sec  4.92 GBytes  4.23 Gbits/sec
[  3] 40.0-50.0 sec  4.86 GBytes  4.18 Gbits/sec
[  3] 50.0-60.0 sec  4.28 GBytes  3.68 Gbits/sec
[  3]  0.0-60.0 sec  27.6 GBytes  3.95 Gbits/sec
[  5] local 172.28.2.23 port 8083 connected with 172.28.2.21 port 38653
[  5]  0.0-10.0 sec  3.91 GBytes  3.36 Gbits/sec
[  5] 10.0-20.0 sec  4.02 GBytes  3.45 Gbits/sec
[  5] 20.0-30.0 sec  3.92 GBytes  3.37 Gbits/sec
[  5] 30.0-40.0 sec  4.02 GBytes  3.45 Gbits/sec
[  5] 40.0-50.0 sec  4.06 GBytes  3.49 Gbits/sec
[  5]  0.0-60.0 sec  24.1 GBytes  3.44 Gbits/sec

这种-d 和 -r参数对结果产生了影响，还是网卡的全双工有问题，还是其他配置有问题呢。请大佬指教！谢谢