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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 Extension Headers 1 2 IPv6 Datagram Size, Maximum Transmission Unit (MTU), Fragmentation and Reassembly

IPv6 Option Format

Each of these header types has a one-byte Next Header field, and a one-byte Header Extension Length field that indicates the header’s overall length. The rest of the header has one or more option fields. Figure 109 illustrates the overall format of these two headers. The format of each option is similar to that of IPv4 options, and is shown in Table 73.

Table 73: IPv6 Option Format

Subfield Name	Size (bytes)	Description
Option Type	1
Opt Data Len	1	Option Data Length: Specifies the length of the Option Data subfield below. Note that this is a change in semantics from IPv4, where the length field indicated the size of the entire option; in IPv6 the length of the Option Type and Option Data Length fields are not included.
Option Data	Variable	Option Data: The data to be sent as part of the option, which is specific to the option type. Also sometimes referred to as the Option Value.

Since each option has a subfield for type, length and value (data), they are sometimes said to be TLV-encoded. If there are multiple options, they are placed one after each other in the header. At the end of all the options in a Hop-By-Hop Options or Destination Options extension header, padding may be placed to ensure that the header is a multiple of 8 bytes in length.

IPv6 Datagram Extension Headers 1 2 IPv6 Datagram Size, Maximum Transmission Unit (MTU), Fragmentation and Reassembly