MTU problems with GRE tunnels (fwd)

Dennis Ferguson

I can tell you for sure that the Cisco routers do send the packets (GRE
or IP protocol 4) with a length which includes the IP header, just like
the RFC. If you look I think you'll find that it is your kernel which is
subtracting out the IP header length before it hands the packet to you on
the raw socket.

BSD Unix converts the length to host order and subtracts the IP header
Linux leaves the length in net order (don't know about subtracting).
Windows leaves the length in net order and does not subtract the IP
   header length.