[Bug 173444] [socket] [patch] IPV6_USE_MIN_MTU and TCP is broken

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https://bugs.freebsd.org/bugzilla/show_bug.cgi?id=173444

--- Comment #12 from Andrey V. Elsukov <ae at FreeBSD.org> ---
(In reply to marka from comment #10)
> (In reply to Andrey V. Elsukov from comment #8)
> So what!  Most DNS/TCP response is a few of packets.  What does it
> matter if it is the 3 or 4 packets.

Zone transfers need a lot of such few packets.

> What matters is avoiding PMTUD as it is NOT reliable.  Setting the
> IPv6 packet size to 1280 avoids triggering PMTUD issues.  Limiting
> the packet size avoids timeout and retransmissions due to PTB not
> been generated due to rate limiting or being lost due to stupid
> load balancers and firewalls that drop ICMP.
> 
> Go put your validating resolvers behind a IPv6 in IPv4 link then
> come back and say this is not needed.

When I build the network in the DC, I know better what MTU can be used in my
network. And forcing 1280 bytes size for the network, where 9k is the default
MTU is at least strange in the 2017.

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