The ICSI Netalyzr
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Network MTU

About this test: Networks cannot transfer arbitrarily big messages. Their paths are limited by their Maximum Transmission Unit (MTU), the largest message the network can transport in one piece. For example, our entrance to the network has an MTU of 1500 bytes, which is the standard size for Ethernet networks. Other devices on the Internet may restrict this size to a smaller amount.

This test attempts to discover the network's MTU for the path between your system and our server. For the path between your system and our server we cannot conduct a complete probe. Instead, we send a single, fragmented message from the applet, to which our server replies with the length of the first fragment. This is often, but not always, the network's MTU.

The path between our server and your network is probed in more detail. In this test, large IP datagrams are sent with the "Don't Fragment" bit set. If the packet would pass through a network with a smaller MTU, the network is supposed to reply back to our server that this is a problem, and our server forwards that on to the client. This allows us to search for the largest permissible packet size. This also detects cases where the fragmenting network does not send the proper notification back to our server.

What if this test reports a problem: If there is a problem, there is unfortunately little that can be done since MTU problems can in principle be located anywhere between your computer and our servers. If you experience spurious connectivity problems for long amounts of time, you can attempt to reduce the MTU on your computer's network interface. If that does not help, please contact your ISP.

For additional information on the results, please consult the Netalyzr help site over at the New Scientist.