I'm working on an IP driver, mounted in io-net with the full TCP/IP stack, under QNX 6.3.2.  If I need to update a piece
 of a packet on its way through my driver, how should I recalculate the IP checksum?

I've read up on RFC 1071, dealing with calculating IP checksums, but if the stack already provides a way to update it 
for me, then I would rather use that then write one of my own.  Since updating the TTL is something that should be done 
on every hop, I was surprised when I couldn't find anything on it when searching the QNX/Foundry27 sites.  Perhaps this 
is typically only done by the stack and not by the drivers... but is there a way for a driver to do it?