linux:networking:ip-tutorial

http://www.kernelnewbies.org/documents/

http://www.kernelnewbies.org/documents/kdoc/kernel-locking.pdf
for kernel locking

source code on web with cross reference
http://lxr.linux.no/source/net/ipv4/ip_forward.c

tutorial on stack
http://www-scf.usc.edu/~rishisin/ngroup/23