Am 24.06.22 um 12:35 schrieb Gert Doering:
Hi,
On Tue, Jun 21, 2022 at 06:16:48PM +0200, Arne Schwabe wrote:
This changes the default MTU of the tun-mtu to 1420 to avoid MTU related
issues that are even more prominent when DCO server or clients are involved.
I'm not convinced that this change "by default" is a desirable change.
Being able to do that (pushable MTU) is good, but changing the default is
fairly fundamental, and it will break use cases that rely on 1500 inner
MTU (and outside fragmentation).
Originally, it was said that "with DCO, outside fragmentation is not
possible", but if I remember right, this turned out to be incorrect,
and implementations handle outside fragmentation just fine. So there
seems to be no hard pressing need to change the default (I am willing
to be corrected if I misremember something).
Not saying that outside fragmentation is *desirable*, so for TCP,
"mssfix 1492 mtu" will (already) ensure we have no fragmentation
issues - whatever tun-mtu we have.
I understand that this is quite fundamental change but I think
alternative of always using a wrong MTU and hoping that mssfix and
outside fragmentation are going to save the day what we currently do, is
lying to ourselves.
I still think this is a reasonable change. Yes, it might break in some
very obscure setups but for those setups, people can still set the MTU
back to 1500. Tap still uses the 1500 default anyway.
Arne
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