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IP Datagram Size, Maximum Transmission Unit (MTU), Fragmentation and Reassembly
main responsibility is to deliver data between internetworked devices.
As we saw in the
preceding section, this requires that
data received from higher layers be encapsulated into IP datagrams for
transmission. These datagrams are then passed down to the data link
layer where they are sent over physical network links.
In order for this to work properly,
each datagram must be small enough to fit within the frame format of
the underlying technology. If the message is bigger than the maximum
frame size of the underlying network, it may be necessary to break up
an IP message into several datagrams, a process called fragmentation.
The datagrams are then sent individually and reassembled into
the original message.
The Internet Protocol is designed
to manage datagram size, and to allow fragmentation and reassembly in
a seamless manner. In this section I explore issues related to managing
the size of IP datagrams. I start with an overview of datagram size
issues and the important concept of a network's maximum transmission
unit (MTU), discussing why fragmentation is necessary. I then describe
the process by which IP messages to be transmitted are fragmented by
the source device and possibly routers along the path to the destination,
and then outline how they are reassembled by the recipient.
Background Information: Explaining fragmentation and reassembly requires some understanding of the basic format of IP datagrams and some of the fields they contain. If you haven't yet read the topic describing IP datagram general format you may wish to review it before proceeding here.
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The TCP/IP Guide (http://www.TCPIPGuide.com)
Version 3.0 - Version Date: September 20, 2005
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