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.