Using iPerf to Troubleshoot Speed/Throughput Issues

December 29, 2011

Two of the most common network characteristics we look at when investigating network-related concerns in the NOC are speed and throughput. You may have experienced the following scenario yourself: You just provisioned a new bad-boy server with a gigabit connection in a data center on the opposite side of the globe. You begin to upload your data and to your shock, you see "Time Remaining: 10 Hours." "What's wrong with the network?" you wonder. The traceroute and MTR look fine, but where's the performance and bandwidth I'm paying for?

This issue is all too common and it has nothing to do with the network, but in fact, the culprits are none other than TCP and the laws of physics.

In data transmission, TCP sends a certain amount of data then pauses. To ensure proper delivery of data, it doesn't send more until it receives an acknowledgement from the remote host that all data was received. This is called the "TCP Window." Data travels at the speed of light, and typically, most hosts are fairly close together. This "windowing" happens so fast we don't even notice it. But as the distance between two hosts increases, the speed of light remains constant. Thus, the further away the two hosts, the longer it takes for the sender to receive the acknowledgement from the remote host, reducing overall throughput. This effect is called "Bandwidth Delay Product," or BDP.

We can overcome BDP to some degree by sending more data at a time. We do this by adjusting the "TCP Window" – telling TCP to send more data per flow than the default parameters. Each OS is different and the default values will vary, but most all operating systems allow tweaking of the TCP stack and/or using parallel data streams. So what is iPerf and how does it fit into all of this?

What is iPerf?

iPerf is simple, open-source, command-line, network diagnostic tool that can run on Linux, BSD, or Windows platforms which you install on two endpoints. One side runs in a 'server' mode listening for requests; the other end runs 'client' mode that sends data. When activated, it tries to send as much data down your pipe as it can, spitting out transfer statistics as it does. What's so cool about iPerf is you can test in real time any number of TCP window settings, even using parallel streams. There's even a Java based GUI you can install that runs on top of it called, JPerf (JPerf is beyond the scope of this article, but I recommend looking into it). What's even cooler is that because iPerf resides in memory, there are no files to clean up.

How do I use iPerf?

iPerf can be quickly downloaded from SourceForge to be installed. It uses port 5001 by default, and the bandwidth it displays is from the client to the server. Each test runs for 10 seconds by default, but virtually every setting is adjustable. Once installed, simply bring up the command line on both of the hosts and run these commands.

On the server side:
iperf -s

On the client side:
iperf -c [server_ip]

The output on the client side will look like this:

#iperf -c 10.10.10.5
------------------------------------------------------------
Client connecting to 10.10.10.5, TCP port 5001
TCP window size: 16.0 KByte (default)
------------------------------------------------------------
[  3] local 0.0.0.0 port 46956 connected with 168.192.1.10 port 5001
[ ID] Interval       Transfer     Bandwidth
[  3]  0.0- 10.0 sec  10.0 MBytes  1.00 Mbits/sec

There are a lot of things we can do to make this output better with more meaningful data. For example, let's say we want the test to run for 20 seconds instead of 10 (-t 20), and we want to display transfer data every 2 seconds instead of the default of 10 (-i 2), and we want to test on port 8000 instead of 5001 (-p 8000). For the purposes of this exercise, let's use those customization as our baseline. This is what the command string would look like on both ends:

Client Side:

#iperf -c 10.10.10.5 -p 8000 -t 20 -i 2
------------------------------------------------------------
Client connecting to 10.10.10.5, TCP port 8000
TCP window size: 16.0 KByte (default)
------------------------------------------------------------
[  3] local 10.10.10.10 port 46956 connected with 10.10.10.5 port 8000
[ ID] Interval       Transfer     Bandwidth
[  3]  0.0- 2.0 sec  6.00 MBytes  25.2 Mbits/sec
[  3]  2.0- 4.0 sec  7.12 MBytes  29.9 Mbits/sec
[  3]  4.0- 6.0 sec  7.00 MBytes  29.4 Mbits/sec
[  3]  6.0- 8.0 sec  7.12 MBytes  29.9 Mbits/sec
[  3]  8.0-10.0 sec  7.25 MBytes  30.4 Mbits/sec
[  3] 10.0-12.0 sec  7.00 MBytes  29.4 Mbits/sec
[  3] 12.0-14.0 sec  7.12 MBytes  29.9 Mbits/sec
[  3] 14.0-16.0 sec  7.25 MBytes  30.4 Mbits/sec
[  3] 16.0-18.0 sec  6.88 MBytes  28.8 Mbits/sec
[  3] 18.0-20.0 sec  7.25 MBytes  30.4 Mbits/sec
[  3]  0.0-20.0 sec  70.1 MBytes  29.4 Mbits/sec

Server Side:

#iperf -s -p 8000 -i 2
------------------------------------------------------------
Server listening on TCP port 8000
TCP window size: 8.00 KByte (default)
------------------------------------------------------------
[852] local 10.10.10.5 port 8000 connected with 10.10.10.10 port 58316
[ ID] Interval Transfer Bandwidth
[  4]  0.0- 2.0 sec  6.05 MBytes  25.4 Mbits/sec
[  4]  2.0- 4.0 sec  7.19 MBytes  30.1 Mbits/sec
[  4]  4.0- 6.0 sec  6.94 MBytes  29.1 Mbits/sec
[  4]  6.0- 8.0 sec  7.19 MBytes  30.2 Mbits/sec
[  4]  8.0-10.0 sec  7.19 MBytes  30.1 Mbits/sec
[  4] 10.0-12.0 sec  6.95 MBytes  29.1 Mbits/sec
[  4] 12.0-14.0 sec  7.19 MBytes  30.2 Mbits/sec
[  4] 14.0-16.0 sec  7.19 MBytes  30.2 Mbits/sec
[  4] 16.0-18.0 sec  6.95 MBytes  29.1 Mbits/sec
[  4] 18.0-20.0 sec  7.19 MBytes  30.1 Mbits/sec
[  4]  0.0-20.0 sec  70.1 MBytes  29.4 Mbits/sec

There are many, many other parameters you can set that are beyond the scope of this article, but for our purposes, the main use is to prove out our bandwidth. This is where we'll use the TCP window options and parallel streams. To set a new TCP window you use the -w switch and you can set the parallel streams by using -P.

Increased TCP window commands:

Server side:
#iperf -s -w 1024k -i 2

Client side:
#iperf -i 2 -t 20 -c 10.10.10.5 -w 1024k

And here are the iperf results from two Softlayer file servers – one in Washington, D.C., acting as Client, the other in Seattle acting as Server:

Client Side:

# iperf -i 2 -t 20 -c 10.10.10.5 -p 8000 -w 1024k
------------------------------------------------------------
Client connecting to 10.10.10.5, TCP port 8000
TCP window size: 1.00 MByte (WARNING: requested 1.00 MByte)
------------------------------------------------------------
[  3] local 10.10.10.10 port 53903 connected with 10.10.10.5 port 5001
[ ID] Interval       Transfer     Bandwidth
[  3]  0.0- 2.0 sec  25.9 MBytes   109 Mbits/sec
[  3]  2.0- 4.0 sec  28.5 MBytes   120 Mbits/sec
[  3]  4.0- 6.0 sec  28.4 MBytes   119 Mbits/sec
[  3]  6.0- 8.0 sec  28.9 MBytes   121 Mbits/sec
[  3]  8.0-10.0 sec  28.0 MBytes   117 Mbits/sec
[  3] 10.0-12.0 sec  29.0 MBytes   122 Mbits/sec
[  3] 12.0-14.0 sec  28.0 MBytes   117 Mbits/sec
[  3] 14.0-16.0 sec  29.0 MBytes   122 Mbits/sec
[  3] 16.0-18.0 sec  27.9 MBytes   117 Mbits/sec
[  3] 18.0-20.0 sec  29.0 MBytes   122 Mbits/sec
[  3]  0.0-20.0 sec   283 MBytes   118 Mbits/sec

Server Side:

#iperf -s -w 1024k -i 2 -p 8000
------------------------------------------------------------
Server listening on TCP port 8000
TCP window size: 1.00 MByte
------------------------------------------------------------
[  4] local 10.10.10.5 port 8000 connected with 10.10.10.10 port 53903
[ ID] Interval       Transfer     Bandwidth
[  4]  0.0- 2.0 sec  25.9 MBytes   109 Mbits/sec
[  4]  2.0- 4.0 sec  28.6 MBytes   120 Mbits/sec
[  4]  4.0- 6.0 sec  28.3 MBytes   119 Mbits/sec
[  4]  6.0- 8.0 sec  28.9 MBytes   121 Mbits/sec
[  4]  8.0-10.0 sec  28.0 MBytes   117 Mbits/sec
[  4] 10.0-12.0 sec  29.0 MBytes   121 Mbits/sec
[  4] 12.0-14.0 sec  28.0 MBytes   117 Mbits/sec
[  4] 14.0-16.0 sec  29.0 MBytes   122 Mbits/sec
[  4] 16.0-18.0 sec  28.0 MBytes   117 Mbits/sec
[  4] 18.0-20.0 sec  29.0 MBytes   121 Mbits/sec
[  4]  0.0-20.0 sec   283 MBytes   118 Mbits/sec

We can see here, that by increasing the TCP window from the default value to 1MB (1024k) we achieved around a 400% increase in throughput over our baseline. Unfortunately, this is the limit of this OS in terms of Window size. So what more can we do? Parallel streams! With multiple simultaneous streams we can fill the pipe close to its maximum usable amount.

Parallel Stream Command:
#iperf -i 2 -t 20 -c -p 8000 10.10.10.5 -w 1024k -P 7

Client Side:

#iperf -i 2 -t 20 -c -p 10.10.10.5 -w 1024k -P 7
------------------------------------------------------------
Client connecting to 10.10.10.5, TCP port 8000
TCP window size: 1.00 MByte (WARNING: requested 1.00 MByte)
------------------------------------------------------------
 [ ID] Interval       Transfer     Bandwidth
[  9]  0.0- 2.0 sec  24.9 MBytes   104 Mbits/sec
[  4]  0.0- 2.0 sec  24.9 MBytes   104 Mbits/sec
[  7]  0.0- 2.0 sec  25.6 MBytes   107 Mbits/sec
[  8]  0.0- 2.0 sec  24.9 MBytes   104 Mbits/sec
[  5]  0.0- 2.0 sec  25.8 MBytes   108 Mbits/sec
[  3]  0.0- 2.0 sec  25.9 MBytes   109 Mbits/sec
[  6]  0.0- 2.0 sec  25.9 MBytes   109 Mbits/sec
[SUM]  0.0- 2.0 sec   178 MBytes   746 Mbits/sec
 
(output omitted for brevity on server & client)
 
[  7] 18.0-20.0 sec  28.2 MBytes   118 Mbits/sec
[  8] 18.0-20.0 sec  28.8 MBytes   121 Mbits/sec
[  5] 18.0-20.0 sec  28.0 MBytes   117 Mbits/sec
[  4] 18.0-20.0 sec  28.0 MBytes   117 Mbits/sec
[  3] 18.0-20.0 sec  28.9 MBytes   121 Mbits/sec
[  9] 18.0-20.0 sec  28.8 MBytes   121 Mbits/sec
[  6] 18.0-20.0 sec  28.9 MBytes   121 Mbits/sec
[SUM] 18.0-20.0 sec   200 MBytes   837 Mbits/sec
[SUM]  0.0-20.0 sec  1.93 GBytes   826 Mbits/sec 

Server Side:

#iperf -s -w 1024k -i 2 -p 8000
------------------------------------------------------------
Server listening on TCP port 8000
TCP window size: 1.00 MByte
------------------------------------------------------------
[  4] local 10.10.10.10 port 8000 connected with 10.10.10.5 port 53903
[ ID] Interval       Transfer     Bandwidth
[  5]  0.0- 2.0 sec  25.7 MBytes   108 Mbits/sec
[  8]  0.0- 2.0 sec  24.9 MBytes   104 Mbits/sec
[  4]  0.0- 2.0 sec  24.9 MBytes   104 Mbits/sec
[  9]  0.0- 2.0 sec  24.9 MBytes   104 Mbits/sec
[ 10]  0.0- 2.0 sec  25.9 MBytes   108 Mbits/sec
[  7]  0.0- 2.0 sec  25.9 MBytes   109 Mbits/sec
[  6]  0.0- 2.0 sec  25.9 MBytes   109 Mbits/sec
[SUM]  0.0- 2.0 sec   178 MBytes   747 Mbits/sec
 
[  4] 18.0-20.0 sec  28.8 MBytes   121 Mbits/sec
[  5] 18.0-20.0 sec  28.3 MBytes   119 Mbits/sec
[  7] 18.0-20.0 sec  28.8 MBytes   121 Mbits/sec
[ 10] 18.0-20.0 sec  28.1 MBytes   118 Mbits/sec
[  9] 18.0-20.0 sec  28.0 MBytes   118 Mbits/sec
[  8] 18.0-20.0 sec  28.8 MBytes   121 Mbits/sec
[  6] 18.0-20.0 sec  29.0 MBytes   121 Mbits/sec
[SUM] 18.0-20.0 sec   200 MBytes   838 Mbits/sec
[SUM]  0.0-20.1 sec  1.93 GBytes   825 Mbits/sec

As you can see from the tests above, we were able to increase throughput from 29Mb/s with a single stream and the default TCP Window to 824Mb/s using a higher window and parallel streams. On a Gigabit link, this about the maximum throughput one could hope to achieve before saturating the link and causing packet loss. The bottom line is, I was able to prove out the network and verify bandwidth capacity was not an issue. From that conclusion, I could focus on tweaking TCP to get the most out of my network.

I'd like to point out that we will never get 100% out of any link. Typically, 90% utilization is about the real world maximum anyone will achieve. If you get any more, you'll begin to saturate the link and incur packet loss. I should also point out that Softlayer doesn't directly support iPerf, so it's up to you install and play around with. It's such a versatile and easy to use little piece of software that it's become invaluable to me, and I think it will become invaluable to you as well!

-Andrew