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IP Datagram Size, the Maximum Transmission Unit (MTU), and Fragmentation Overview
(Page 2 of 4)
Maximum Transmission Unit (MTU) and Datagram Fragmentation
The IP implementation of all devices
on an IP internet needs to be aware of the capacity of the technology
used by that implementation for its immediate data link layer connection
to other devices. This limit is called the maximum transmission unit
(MTU) of the network. This term is also sometimes seen as the maximum
transfer unit.
If an IP layer receives a message
to be sent across the internetwork, it looks at the size of the message
and then computes how large the IP datagram would be after the addition
of the 20 or more bytes needed for the IP header. If the total length
is greater than the MTU of the underlying network, the IP layer will
fragment the message into multiple IP fragments. So, if a host is connected
using an Ethernet LAN to its local network, it may use an MTU of 1,500
for IP datagrams, and will fragment anything larger. Figure 88
shows an example of differing MTUs and fragmentation.
 Key Concept: The size of the largest IP datagram that can be transmitted over a physical network is called that network’s maximum transmission unit (MTU). If a datagram is passed from a network with a high MTU to one with a low MTU, it must be fragmented to fit the network with the smaller MTU.
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Since some physical networks on the
path between devices may have a smaller MTU than others, it may be necessary
to fragment more than once. For example, suppose the source device wants
to send an IP message 12,000 bytes long. Its local connection has an
MTU of 3,300 bytes. It will have to divide this message into four fragments
for transmission: three that are about 3,300 bytes long and a fourth
remnant about 2,100 bytes long. (I'm oversimplifying by ignoring the
extra headers required; the
next topic includes the full details of the fragmentation process.)
Figure 88: IP Maximum Transmission Unit (MTU) and Fragmentation In this simple example, Device A is sending to Device B over a small internetwork consisting of one router and two physical links. The link from A to the router has an MTU of 3,300 bytes, but from the router to B it is only 1,300 bytes. Thus, any IP datagrams over 1,300 bytes will need to be fragmented.
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