Revisiting the weird MTU issue

[Derrick Lyndon Pallas]

MTU discovery isn't supported widely in my experience. My rule has been to set the interface to 1280, which is the minimum allowed.

~Derrick • iPhone

> On Apr 10, 2019, at 8:34 AM, Roman Mamedov <rm at romanrm.net> wrote:
> 
> Hello,
> 
> 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,
> Roman
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