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The TCP/IP Guide  9  TCP/IP Lower-Layer (Interface, Internet and Transport) Protocols (OSI Layers 2, 3 and 4)       9  TCP/IP Internet Layer (OSI Network Layer) Protocols            9  Internet Protocol (IP/IPv4, IPng/IPv6) and IP-Related Protocols (IP NAT, IPSec, Mobile IP)                 9  Internet Protocol Version 6 (IPv6) / IP Next Generation (IPng)                      9  IPv6 Datagram Encapsulation and Formatting

IPv6 Datagram Main Header Format 1 2 3 4 5 6 IPv6 Datagram Options

After the mandatory “main” header in an IPv6 datagram, one or more extension headers may appear before the encapsulated payload. These headers were created in an attempt to provide both flexibility and efficiency in the creation of IPv6 datagrams. All fields that are needed only for special purposes are put into extension headers and placed in the datagram when needed. This allows the size of the main datagram header to be made small and streamlined, containing only those fields that really must be present all the time.

There is often confusion regarding the role of extension headers, especially compared to datagram options. The IPv4 datagram had only one header, but it included a provision for options, and IPv6 also has options, so why bother with extension headers? Good question.

It would have been possible to do everything using options. However, it was deemed a better design to employ extension headers for certain sets of information that are needed for common functions such as fragmenting. Options are indeed still supported in IPv6; they are used to provide even more flexibility by providing variable-length fields that can be used for any purpose. They are themselves defined using extension headers as we will see below (and in their own topic, which follows.)

When extension headers are included in an IPv6 datagram, they appear one after the other following the main header. Each extension header type has its own internal structure of fields.

IPv6 Datagram Main Header Format 1 2 3 4 5 6 IPv6 Datagram Options