IPv6 Packet Format and Related Data Structures

IPv6 was designed to support a 128-bit address length, four times larger than the 32-bit IPv4 address length [ RFC4291 ] [ RFC8200 ] . This change also meant that many protocols and network functions, such as routing protocols, had to undergo a number of changes to accommodate the 128-bit IPv6 address length. For example, routing protocols need to understand IP addresses and include them in routing updates and other messages that they exchange. This chapter describes the IPv6 packet, the IPv6 header, the fields in the IPv4 header, a comparison between the fields in the IPv6 header and those in the IPv4 header, the IPv6 extension headers, the IPv6 Maximum Transmission Unit, and the IPv6 pseudo-header used for upper-layer checksum computation. A good understanding of the IPv6 packet format and addressing architecture (discussed in the next chapter) helps to provide a proper context for the IPv6 functions discussed in this book. We discuss IPv4 in greater detail in Appendix B .