In message <20021009062722.GA20007@rek.tjls.com>Thor Lancelot Simon writes
>On Tue, Oct 08, 2002 at 03:02:53PM -0700, Tad Hunt wrote:

>An MTU of less than 576 violates the IPv4 standard, and will cause
>all kinds of obscure (and some not-so-obscure!) problems, even in
>the presence of Path MTU discovery.

Not exactly. One can use IP over links with MTU below 576, provided
one guarantees to reassemble fragmented IP datagrams of up to 576
bytes.  Tiny MTUS, fragmentation, and reassembly, was fairly common in
the days of SLIP/CSLIP over 9600 baud modems. (it helped alleviate
non-intractive traffic from causing head-of-line blocking of
interactive Telnel traffic).

Of course, "Fragmentation COnsidered harmful", usw.

>Since you're going to need to split the packets smaller than the
>tun interface MTU anyway (fragment and reassemble in your "middle
>layer" above LAPD, I suggest), it would be strongly advisable to
>use an MTU of 1500 bytes, which will avoid some _other_ problems.

Much better idea.