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IPv6 Datagram Size, Maximum Transmission Unit (MTU), Fragmentation and Reassembly
(Page 1 of 4)
The job of the Internet Protocol
is to convey messages across an internet of connected networks. When
datagrams are sent between hosts on distant networks they are carried
along their journey by routers, one hop at a time, over many physical
network links. On each step of this journey the datagram is encoded
in a data link layer frame for transmission.
Overview of IPv6 Datagram Sizing and Fragmentation
In order for a datagram to be successfully
carried along a route, its size must be small enough to fit within the
lower-layer frame at each step on the way. The term maximum transmission
unit (MTU) describes the size limit for any given physical network.
If a datagram is too large for the MTU of a network, it must be broken
into pieces, a process called fragmentation, and then the pieces
reassembled at the destination device. This has been a requirement
since IPv4, and I
explain the concepts and issues related to datagram size, MTUs, fragmentation
and reassembly in detail in a section devoted to these matters in IPv4.
All of these issues apply to sending
datagrams in IPv6 as much as they did in IPv4. However, as in other
areas of the protocol, some important details of how fragmentation and
reassembly are done have changed. These changes were made to improve
the efficiency of the routing process, and also to reflect the realities
of current networking technologies: most can handle average IP datagrams
without needing fragmentation.
The most important differences between
IPv4 and IPv6 with respect to datagram size, MTU, fragmentation and
- Increased Default MTU: In IPv4, the minimum
MTU that routers and physical links were required to handle was 576
bytes. In IPv6, all links must handle a datagram size of at least 1280
bytes. This more-than-doubling in size improves efficiency by increasing
the ratio of maximum payload to header length, and reduces the frequency
with which fragmentation is required.
- Elimination of En Route Fragmentation:
In IPv4, datagrams may be fragmented by either the source device, or
by routers during delivery. In IPv6, only the source node can fragment;
routers do not. The source must therefore fragment to the size of the
smallest MTU on the route before transmission. This has both advantages
and disadvantages, as we will see. Reassembly is of course still done
only by the destination, as in IPv4.
- MTU Size Error Feedback: Since routers
cannot fragment datagrams, they must drop them if they are forced to
try to send a too-large datagram over a physical link. A feedback process
has been defined using ICMPv6 that lets routers tell source devices
that they are using datagrams that are too large for the route.
- Path MTU Discovery: Since source devices
must decide on the correct size of fragments, it is helpful if they
have a mechanism for determining what this should be. This capability
is provided through a special technique called Path MTU Discovery,
which was originally defined for IPv4 but has been refined for IPv6.
- Movement of Fragmentation Header Fields:
To reflect the decreased importance of fragmentation in IPv4, the permanent
fields related to the process that were in the IPv4 header have been
farmed out to a Fragment extension header, included only when
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Version 3.0 - Version Date: September 20, 2005
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