LINKS:

[+] Tcpdump and Libpcap official site.

[+] A list of tools based on libpcap.

[+] The journey of a packet through the Linux network stack.

[+] Paper about the BPF filter written by the original authors of libpcap.

[+] A tutorial on libpcap filter expressions.

[+] TCPDump Cheat Sheet, by Tim Keary