| The result was that my little NetBSD/i386-1.4V running the
| GIF-over-cable-modem tunnel was then forced to fragment my packets.  I
| knew this would happen so I set "net.inet.tcp.mssdflt = 1200" in an
| effort to convince my system so send properly sized packets..

As the manpage says:

		     Do
                     not change this value unless you really know what you are
                     doing.

The advertised MSS is the greater of the max interface mtu less 40 bytes,
or the mssdflt. So setting it is useless if you wish to lower the
advertised MSS (as you do).

--jhawk