Revisiting the weird MTU issue

Revisiting the weird MTU issue

[Roman Mamedov]

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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Re: 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@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
> _______________________________________________
> WireGuard mailing list
> WireGuard@lists.zx2c4.com
> https://lists.zx2c4.com/mailman/listinfo/wireguard

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