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MTU debugging
Authored by: acdha on Oct 18, '01 03:47:05AM
The MTU is the size (in bytes) of the maximum packet which can be sent over a link. (the network stack is smart enough to divide data into multiple packets) Because of there is protocol overhead with each packet, you want this size to be as large as the underlying link can handle so that as much of the link's capacity is spent on data as possible - in the degenerate case, you could have something 50% of the data transmitted being packet headers. With ethernet, that size is most commonly 1500 bytes.

If your ADSL connection is using the never-to-be-sufficiently-cursed PPPoE, your traffic is encapsulated over a normal Ethernet link and the protocol overhead adds a few bytes per packet (1492 = 1500 - PPPoE packet header?). This sort of error is extremely annoying as it only happens when you try to send a full packet. Things like ping will work because ping's packets are only 64 bytes by default. Things like telnet/ssh might work if you don't send too much data at time (I've had users report that telnet works until you do something which displays a lot of data quickly with certain misconfigured routers). When this happens, the connection will eventually time out.

You can test the MTU of your connection using ping with the -s option. ping -s 1000 server will send 1008 data bytes to the remote host. If you suspect your MTU needs to be lowered, you can start with a small size and increase it until packets stop making it through and setting your MTU to the last size.

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MTU debugging
Authored by: gb on Oct 18, '01 07:47:53AM

This debugging technique worked very well for me. Even moving the mtu up to 1020, ping started to get flakey. It looks like my ADSL provider needs to lift their game.

Thanks, Gavin.



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