ipsec gre mtu

Hi, I am using fedora core 5, with gre over ipsec with ospf. I have
noticed that there are alot of questions regarding packet fragmentation
within the gre tunnel. While taking snapshots of my network during file
transfers, I have noticed the following.

Out side interfaces mtu 1500, but the frame size does not exceed 1510?
Max packet size is 1496

Inside interface Gre0 has a mtu of 1476
Its max frame size is 1440
and its data 1424

I have not seen any PMTU being sent. How does the stack know the proper
size packets to send? I was my thoughts while introducing ipsec-esp
that this overhead would not be accounted for by the mtu size of the
gre interface and introduce fragmentation?

any suggestions would be great.

jason