IPIP (protocol 4) requires that the contents of the packet form a
valid IP header which satisfies certain properties wrt. the packet
header. Routers may look at the packet contents. See RFC 2003. You
should use another protocol number.
> doing such things, I suppose I ought to add hooks for encryption,
I have already done this for encryption, see
<URL:http://www.inka.de/~bigred/devel/cipe.html>. Adding hooks for
compression was a thing I thought about for future development. :-)
(Btw., I'm using UDP instead of an own IP protocol number, for various
reasons.)
olaf