Web100 based Network Diagnostic Tool (NDT)

• 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.

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

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.

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 as described at ESnet's Tuning Guide

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.

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

REN accessable NDT servers:

A list of NDT servers known to the Researh and Education Network (REN) community is can be found in the . Note: that not all of these REN servers will be publicly accesible.

Other bandwidth testing sites:

• microsoft's bandwidth tester
• Ookla Speedtest
• upload/download tester
• bt2kfast
• www.dslreports.com speed test browser/throughput tester
• UT web100 bandwidth tester, experimental (not always available)

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.

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.

The NDT software (source and compiled programs) is available via the Internet2 web site at http://software.internet2.edu/ndt/. 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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