Arnes Web100 based Network Diagnostic Tool (NDT)

Located at Arnes PoP in Ljubljana, Slovenia
1 Gbps GigabitEthernet IPv4 and IPv6 network connection to ARNES backbone


This java applet was developed to test the reliability and operational status of your desktop computer and network connection. It does this by sending data between your computer and this remote NDT server. These tests will determine:
It can also identify 2 serious error conditions:

A test takes about 25 seconds. Click on "start" to begin.


Report Problems         Use "ctrl-C" to copy data onto the clipboard and then paste it into the email message.

The NDT performs 2 TCP throughput tests between your desktop computer and this NDT server. First, data is streamed for 10 seconds from your desktop to the server and then a second 10 second test is performed in the opposite direction. A Web100 modified linux kernel captures detailed statistics on these TCP data flows. This data is then analyzed to determine why the connection achieved the throughput results it reported.

Understanding the test results

End-to-End performance depends upon a number factors. One of the biggest factors is setting the tunable network parametes to the proper value. There are several web sites that provide detailed tuning instructions for various Operating Systems. Two of the best are:

The throughput an application achieves is dependant on the amount of buffer space available in the sending and receiving hosts, packet loss due to errors or congestion, packet size (usually limited to 1500 bytes by Ethernet), and the round trip time between the 2 hosts. If this test reports that the sender or receiver buffer size is the limiting factor, increase the buffer size by changing the default buffer size on your computer. Win 95/98/NT users can easily set and change their default buffer size by using the TCPtune application developed by the NLANR MOAT group. Windows users may find that they perfer the DrTCP tool from the dslreports.com web site.

If a large number of retransmissions occur, check the duplex and speed setting on your host and the network switch it attaches to (duplex mismatches are a serious problem due to broken autonegoation protocols). Extremely long round trip times (over 1 sec) ususally indicate that a network router or switch is congested leading to long queuing times. Contact your local network administrator for help in solving this problem

The Bandwidth * Delay product is reported at the bottom of the "more details" page. Throughput limits for the NDT server's transmit buffer, your clients receive buffer, and the network infrastructure. You may use these numbers as a guide to determining what your client's receive buffer is currently set to. Divide the buffer size by the reported round trip time (RTT) to calculate the throughput value.

For more info on TCP tuning, visit cable/dsl tuning or dslreports.com tweaks or UNIX and Windows TCP/IP tuning tips.

You can find the Web100 variables descriptions gathered on the following web page.


REN accessable NDT servers:

A list of servers known to the Researh and Education Network (REN) community is located on the Internet2 e2epi web page. Note: that not all of these REN servers will be publicly accesible.

Other publicly accessible NDT servers:



Other bandwidth testing sites:

You can see the hops (routers) that your packets pass through from your machine to a target Internet site with the traceroute command (for Windows, use tracert in DOS/command prompt window). The route can actually vary from packet to packet, test to test, and the reverse route (return path) may not be the same. There are several traceroute servers around the world that can show you the route back to your browser. Tom Dunigan at ORNL also has traceroute servers at ORNL and at UT.

The NDT server window size for this Java tester is 64KB. Max window used to be 64KB, but newer OS's now support window scaling, so you may be able to request more than 64 KB. (This NDT server has a window of 16 MBytes, and the network interface is 1 Gbs.)

The NDT software (source and compiled programs) is available via the Internet2 End2End Performance Initiative web server at e2epi.internet2.edu/ndt/download.html. You can also learn more about the NDT system by subscribing to the NDT user discussion or NDT announcement lists hosted by Internet2.

This java applet was originally developed by ORNL and has been extensively modified at ANL.
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Comments to Rich Carlson email:RCarlson@internet2.edu
Created: April 4, 2002 by Rich Carlson
Last Updated: June 8, 2005 by Rich Carlson
Last Updated for Arnes: August 26, 2009 by Arnes NOC