Appendix A. IPv4, IPv6, ICMPv4, and ICMPv6
Introduction
This appendix is an overview of IPv4, IPv6, ICMPv4, and ICMPv6. This material provides additional background that may be helpful in understanding the discussion of TCP and UDP in Chapter 2. Some features of IP and ICMP are also used in some of the later chapters: IP options (Chapter 27), along with the ping and traceroute programs (Chapter 28), for example.
IPv4 Header
The IP layer provides a connectionless best-effort datagram delivery service (RFC 791 [Postel 1981a]). IP makes its best effort to deliver an IP datagram to the specified destination, but there is no guarantee that the datagram will arrive, will arrive in order relative to other packets, or will arrive only once. Any desired reliability, ...
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