if_gre, ip_gre and the like (pseudo-interfaces)

[Jerry Toung]

Hello hackers,
for another purpose, I am trying to understand the relationship between 
gre_input and the gre pseudo-device.
My understanding is that gre_input is called by IP everytime it receives a 
packet with a gre headers which after some processing performs 
IF_ENQUEUE(ifq,m) to put it back on the IPqueue so that higher protocol such 
TCP can do their thing. :-)
 I don't see how /dev/gre is ever used on receiving or sending, through  
if_clone_attach??. Somebody educate me on my missing link.
 Thank you all,
 Jerry.