Dave & Dennis,
thanks a lot for your valuable insights. And of course,
apologies for making the cisco guys look like they didn't read
the RFC. Thank God this was written as a question with a smiley...
# 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.