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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)

IPv6 Datagram Options 1 2 3 4 IPv6 Datagram Delivery and Routing

IPv6 Fragmentation Process

The actual mechanics of fragmentation in IPv6 are similar to those in IPv4, with the added complication that extension headers must be handled carefully. For purposes of fragmentation, IPv6 datagrams are broken into two pieces:

• Unfragmentable Part: This includes the main header of the original datagram, as well as any extension headers that need to be present in each fragment. This means the main header, and any of the following headers If present: Hop-By-Hop Options, Destination Options (for those options to be processed by devices along a route) and Routing.
  
  
• Fragmentable Part: This includes the data portion of the datagram, along with the other extension headers if present—Authentication Header, Encapsulating Security Payload and/or Destination Options (for options to be processed only by the final destination).

The Unfragmentable Part must be present in each fragment, while the fragmentable part is split up amongst the fragments. So to fragment a datagram, a device creates a set of fragment datagrams, each of which contains the following, in order:

1. Unfragmentable Part: The full Unfragmentable Part of the original datagram, with its Payload Length changed to the length of the fragment datagram.
   
   
2. Fragment Header: A Fragment header with the Fragment Offset, Identification and M flags set in the same way they are used in IPv4.
   
   
3. Fragment: A fragment of the Fragmentable Part of the original datagram. Note that each fragment must have a length that is a multiple of 8 bytes, because the value in the Fragment Offset field is specified in multiples of 8 bytes.

IPv6 Datagram Options 1 2 3 4 IPv6 Datagram Delivery and Routing