The MTU Setting Problem - Why Is It So Obscure?

The Maximum Transmission Unit, aka MTU, is a critical setting on your computer.

The MTU setting controls the size of the messages
that it passes to other computers. When you surf the Internet, your computer is in communication with hundreds of other computers, each with their own possibility to contribute to packet loss. Packet loss is caused partly because of an optimistic MTU setting on your computer, and partly because of constantly varying paths between the computers, resulting from packet switching.

The symptoms of an MTU problem - packet loss and inability to connect to the server of your choice - are so transient that you may never be aware of the problem, until 3 conditions are met, simultaneously.
  1. Your computer has an optimistic MTU setting.
  2. You attempt to access a server that is reached (even temporarily) through a network that fragments the packets from your computer, partly because of your optimistic MTU setting.
  3. A distant server - or a network between your computer and the distant server - has a problem with fragmented packets.

Because of packet switching, the path between your computer and the server of your interest can change in milliseconds. With each different path comes a greater or lesser chance of conditions 2 or 3 becoming an active issue. If condition 1 is an active issue, and both conditions 2 and 3 are met, you'll see the symptoms of the MTU problem. If condition 1 is an active issue, and both conditions 2 and 3 are not met, you'll not see symptoms of the problem.

Each different packet from your computer, possibly taking a different route to the server in question, may or may not pass through a network that contributes to the problem, and thus be susceptible to packet loss. If not susceptible, the packet won't be dropped, and the symptom will seem to simply go away.

Any individual server may be more or less susceptible to contributing to conditions 2 and 3.
  1. The server may have an individual problem with fragmented packets.
  2. The server may be connected to a network that has a problem with fragmented packets.
  3. If the server in question has an individual problem with fragmented packets, and you access this server more frequently, you may observe the symptoms more frequently.
  4. If the server in question is simply connected to a network with a problem with fragmented packets, the symptoms may be seen less often.
  5. The likeliness of packet switching causing an alternate path, and bypassing the network in question, will reduce the chances of you seeing the symptoms. This likeliness will vary, depending upon logical and physical distance between the target server, and a network that has a problem with fragmented packets.
    • If the target server itself has the problem, or is directly connected (and not multi homed) to the network with the problem, the symptoms will be seen consistently.
    • A network that is close to the server in question, and having a problem with fragmented packets, will be less likely to consistently cause the symptom.
    • A network that is not very close to the server in question, and having a problem with fragmented packets, will be still less likely to cause the symptom consistently.

All of the above issues should explain, in part, why the MTU problem is so transient, and so seemingly random. Even with identical computers, with identical configurations, in the same location, individual surfing habits (targeting different servers) may make any single computer more, or less, susceptible to exhibiting the symptom of packet loss caused by an optimistic MTU setting.

If your computer needs an MTU setting adjustment, you can only check your MTU setting when the symptoms are observed, and targeting the server which contributes to the symptoms.

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