Revisiting the weird MTU issue

Roman Mamedov rm at
Wed Apr 10 17:34:31 CEST 2019


I use WireGuard over IPv6 on a PPPoE connection. The Internet interface MTU is
1492. By my calculations MTU 1412 on the WG interface should fit.

However, the following occurs on various MTU combinations between the Remote
(a server in a DC with full 1500 wire MTU) and Local WG interface MTUs:

Fails or not, is whether a within-WG Remote->Local TCP connection (iperf3)
works fine or hangs up after transferring a few initial bits of data.

Remote  Local  Result
  1420   1420  Fails (as expected)

  1420   1412  Fails (weird)

  1412   1412  Works (fair enough)

  1420   1408  Works (super weird!!!)

Now I hope I described the situation clearer than the last time posting about
this, so maybe someone has an idea what could be the culprit?

So far this doesn't cause too much issue, as I'm using on designated p2p links
for when one of the peers is on this PPPoE, I just use 1412 on both sides. But
still, surely the above shouldn't happen?

With respect,

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