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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

In IPv4, all “extra” information required for various purposes is placed into the datagram in the form of options that appear in the IPv4 header. In IPv6, the new concept of extension headers is introduced; these headers take the place of many of the predefined IPv4 options. However, the concept of options is still maintained in IPv6, for a slightly different purpose. Options allow the IPv6 datagram to be supplemented with arbitrary sets of information that aren't defined in the regular extension headers. They provide maximum flexibility, allowing the basic IPv6 protocol to be extended in ways the designers never anticipated, with the goal of reducing the chances of the protocol becoming obsolete.

I said that IPv6 options supplement extension headers; in fact, they are actually implemented as extension headers. There are two different ones used to encode options. These two headers only differ in terms of how the options they contain are to be processed by devices; otherwise, they are formatted the same and used in the same way.

The two extension header types are:

• Destination Options: Contains options that are intended only for the ultimate destination of the datagram (and perhaps a set of routers specified in a Routing header, if present).
  
  
• Hop-By-Hop Options: Contains options that carry information for every device (router) between the source and destination

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