8.8 References and Further Study
The man page installed with tcpdump includes information on features of the program not covered here. In particular, tcpdump understands a number of additional protocols, and the filter syntax is capable of more advanced expressions for specifying which packets should be captured.
RFC 791 describes the Internet Protocol (IP), including details of the header format. This explains the significance of the first 20 bytes viewed with the -x option. Further, the UDP protocol is described in RFC 768 and TCP, in RFC 793. ICMP is described in RFC 791. The books Internetworking with TCP/IP (Prentice Hall, 2000) by Douglas Comer and TCP/IP Illustrated (Addison-Wesley, 1994) by W. Richard Stevens both have descriptions and diagrams of all of these protocols, and both are easier reads than the RFCs.
There is a useful page of links to information about other packet analyzers at http://www.tcpdump.org/, under the Related Projects section. This includes pointers to programs such as Ethereal, TCPslice, and Snort.