[Bug 239749] Issues path MTU discovery on IPv6 and SSH.

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https://bugs.freebsd.org/bugzilla/show_bug.cgi?id=239749

            Bug ID: 239749
           Summary: Issues path MTU discovery on IPv6 and SSH.
           Product: Base System
           Version: CURRENT
          Hardware: Any
                OS: Any
            Status: New
          Severity: Affects Only Me
          Priority: ---
         Component: kern
          Assignee: bugs at FreeBSD.org
          Reporter: zeising at FreeBSD.org

There seem to be an issue with path MTU discovery, and SSH (and possibly
others).

I have a FreeBSD router, running 12.0-RELEASE-p9.  The router terminates an
IPv6 tunnel, using gif (protocol 41).  The MTU on the tunnel is 1480, because
of the encapsulating IPv4 header.
In my internal network, the MTU is the standard 1500.

>From my work station, when I try to SSH over IPv6 to, for example
freefall.freebsd.org, the connection fails.  I can see in package dumps that
the router responds with an ICMP packet too big.  I can understand that this is
the case for the first attempt since the local link has an MTU of 1500 but the
remote link has an MTU of 1480, but ssh does multiple attempts with the same
packet size, and gets ICMP ptb in return.  Eventually, the connection is reset.

I expected that something, either in our TCP stack, IP stack or in SSH, would
adjust to the lower MTU, and resend a shorter packet (possibly splitting it in
multiple TCP packets).

After the first SSH connection attempt has failed, if I try again, the
connection succeeds, so something adjusts so that later connections are not
using the too big MTU.

I have a packet capture of the traffic between my work station, router and
freefall.freebsd.org, that hopefully can shed a little bit of light on what's
going on.  It's available on request.

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