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IPv6 Datagram Size, Maximum Transmission Unit (MTU), Fragmentation and Reassembly
(Page 3 of 4)
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 presentAuthentication
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
- Unfragmentable Part: The full
Unfragmentable Part of the original datagram, with its Payload
Length changed to the length of the fragment datagram.
- Fragment Header: A Fragment
header with the Fragment Offset, Identification and M
flags set in the same way they are used in IPv4.
- 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.
Key Concept: Fragmentation is done in IPv6 in a manner similar to that of IPv4, except that extension headers must be handled specially. Certain extension headers are considered unfragmentable and appear in each fragment; others are fragmented along with the data.
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Version 3.0 - Version Date: September 20, 2005
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