[Help-gnunet] UDP and MTU < 1472

[David Kuehling]

Hi,

I just noticed that when configuring the MTU for UDP to 1462, (almost?)
every incoming packet will be invalid.  Accoding to gnuetd's log:

Nov 26 15:10:56 WARNING: Packet received from nnn.nnn.nnn.nnn:2086 (UDP) 
failed format check.

(such errors being output at a rate of > 20/minute)

All other transports apart from UDP where disabled for testing.  

Looking at the sources (transports/udp.c), it looks like "recvfrom" will
truncat incoming packets down to the locally configured MTU, which might
be the error's cause.  So far I didn't find any kind of negotiation that
would make a host reduce its MTU to match the receiver's MTU.  But where
would then be the point of letting users configure a smaller MTU?

My PPPoE link has an MTU of 1492 instead of the ethernet-MTU 1500, which
is why I thought that reducing gnunetd's MTU would improve performance.
I now changed gnunetd's MTU back to the default 1472 and haven't seen
any errors in the logs since then...

regards,

David
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