UCLA Web100 based Network Diagnostic Tool (NDT)

Located at the University of California, Los Angeles (Los Angeles, CA, USA); 1000 Mbps (Gigabit Ethernet) network connection

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The NDT performs two 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.

Systems behind firewalls must permit TCP ports 3001, 3002, and 3003.

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 two 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 one 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.

Publicly accessible NDT servers in California:

Other publicly accessible NDT servers:



This java applet was originally developed by ORNL and has been extensively modified at ANL.
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