WAND Network Research Group

This website enables you to conduct two different Path MTU Discovery (PMTUD) tests, which can be useful when a website appears to be unresponsive but you know other people can use it. Because the tests are typically used when there is a problem, it can take a couple of minutes before the test completes. Therefore, this website is used to launch a test, the results of which will be emailed when they are available. To use the tests, you must first register an email address. The tests are run from a machine in New Zealand.

1. Web server PTB filtering

The first test is used to determine if a webserver will reduce its packet size when asked to by an ICMP Packet Too Big (PTB) message. This test is important because if there is a tunnel in the path back to a client that reduces the maximum transmission unit (MTU) of the path, and the server filters PTB messages, then the server will probably not reduce its packet size so that the client can receive the data. You can read about the algorithm used in the test and some recent results in large-scale testing in this paper. The algorithm was devised by Medina, Allman, and Floyd in 2004.

2. Traceroute with PMTUD

The second test is used to determine the Path MTU towards a particular IP address, to identify where in an IP path tunnels begin, and if any routers do not send PTB messages when they should. Usually, you'd enter your own IPv4/IPv6 address to test to see if there is a tunnel topologically close to you that could explain poor connectivity. The IP address you enter must be responsive to our probe packets so that we can infer whether all packets are being lost, or just large ones. Please ensure you configure your operating system and firewalls to permit UDP probes to port 33435, and to send ICMP destination unreachable messages. You can read more about the algorithm used in the test and some results from testing with jumbo-packet capable hosts in this paper.

Source code

The tests are implemented in scamper, which is freely available.