Hi All,
I was troubleshooting a throughput issue recently on an international
300Mbps link which had a RTT of ~160ms.
I have copied the output below from testing a single TCP session between two
linux servers at either endpoint.
I have the output from two tests from the perspective of SERVER_A (acting
first in server mode and then in client mode). The remote server had similar
output and same window sizes i.e. when in server mode it defaulted to 85.3
Kbytes and when in client mode defaulted to 25.3 Kbytes.
****************************************************
SERVER MODE (receiving connection from remote server)
****************************************************
[root@SERVER_A ~]# iperf -s -i 1
------------------------------------------------------------
Server listening on TCP port 5001
TCP window size: 85.3 KByte (default)
------------------------------------------------------------
[ 4] local 192.168.1.1 port 5001 connected with 192.168.2.1 port 46326
[ ID] Interval Transfer Bandwidth
[ 4] 0.0- 1.0 sec 491 KBytes 4.02 Mbits/sec
[ 4] 1.0- 2.0 sec 3.79 MBytes 31.8 Mbits/sec
[ 4] 2.0- 3.0 sec 9.13 MBytes 76.6 Mbits/sec
[ 4] 3.0- 4.0 sec 9.01 MBytes 75.6 Mbits/sec
[ 4] 4.0- 5.0 sec 9.01 MBytes 75.6 Mbits/sec
[ 4] 5.0- 6.0 sec 10.1 MBytes 84.6 Mbits/sec
[ 4] 6.0- 7.0 sec 9.00 MBytes 75.5 Mbits/sec
[ 4] 7.0- 8.0 sec 9.13 MBytes 76.6 Mbits/sec
[ 4] 8.0- 9.0 sec 9.01 MBytes 75.6 Mbits/sec
[ 4] 9.0-10.0 sec 9.02 MBytes 75.7 Mbits/sec
[ 4] 0.0-10.2 sec 79.8 MBytes 65.6 Mbits/sec
****************************************************
CLIENT MODE (sending data to remote server)
****************************************************
[root@SERVER_A~]iperf -c 192.168.2.1
------------------------------------------------------------
Client connecting to 192.168.2.1, TCP port 5001
TCP window size: 23.2 KByte (default)
------------------------------------------------------------
[ 3] local 192.168.1.1 port 56806 connected with 192.168.2.1 port 5001
[ ID] Interval Transfer Bandwidth
[ 3] 0.0-10.1 sec 116 MBytes 96.7 Mbits/sec
What I did not understand is how we were getting in excess of 65 Mbps using
a single TCP session and default window size of 25.3 Kbytes.
>From my calculations
1000ms/160ms = 6.25 window periods
6.25 x window size (25.3Kbytes) = 158.125 Kbytes
multiplied by 8 to convert into Mbps is 1.265 Mbps
Any ideas on how to explain the logic behind these results given the testing
setup described above?
Cheers
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