SWITCH NDT server at CERN

Note: This server is configured to handle multiple clients in parallel. So the throughput you get may be limited because of competition from other users. This should only be relevant if you have a very fast Internet connection.

Located at SWITCH's PoP at CERN, Meyrin, Geneva, Switzerland
1 Gbps Ethernet network connection with large-frame support

This java applet was developed to test the reliablity 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:
  • The slowest link in the end-to-end path (Dial-up modem to 10 Gbps Ethernet/OC-192)
  • The Ethernet duplex setting (full or half);
  • If congestion is limiting end-to-end throughput.

It can also identify 2 serious error conditions:

  • Duplex Mismatch
  • Excessive packet loss due to faulty cables.

More information about NDT is available in our eduPERT knowledge base.

There are some well-known problems, please read before submitting a report:

  • Java applet does not load or control buttons do not appear/don't work.

    It appears that the applet does not work with the Microsoft Java Virtual Machine. Sun's JRE 5 or above is recommended.

  • ** Starting test 1 of 1 **
    Server process not running: start web100srv process on remote server
    (ndt.switch.ch:3001)
        
    This error usually means that your local firewall is blocking access to this server. Please make sure that connections to TCP ports 3001, 3002, 3003 of ndt.switch.ch are permitted before submitting a problem report.
  • ** Starting test 1 of 1 **
    unknown server
        
    You may run into this problem if your computer connects to the Internet through a Web proxy but is otherwise restricted to your intranet. For this service to work, your computer needs to be able to look up the address of ndt.switch.ch in the global DNS and connect to it on TCP ports 3001, 3002 and 3003 (see above).

A test takes about 20 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

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. 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 dslreports.com tweaks or UNIX and Windows TCP/IP tuning tips.


Other publicly accessable 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 server at ORNL.

The NDT server window size for this Java tester is 4MB. The maximum window size used to be 64KB, but newer OS's now support window scaling, so you may be able to request more than 64 KB.

The NDT software (source and compiled programs) is available via the Internet2 Performance web site at www.internet2.edu/performance/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.
Complete University of Chicago Copyright Notice


Comments to Rich Carlson email:RACarlson@anl.gov
Created: April 4, 2002 by Rich Carlson
Last Updated: February 4, 2008 by Alexander Gall