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MTU

The MTU is the maximum size of a single data unit (e.g., a frame) of digital communications. MTU sizes are
inherent properties of physical network interfaces, normally measured in bytes. The MTU for Ethernet, for
instance, is 1500 bytes. Some types of networks (like Token Ring) have larger MTUs, and some types have
smaller MTUs, but the values are fixed for each physical technology.
Higher-level network protocols like TCP/IP can be configured with a maximum packet size, a parameter
independent of the physical layer MTU over which TCP/IP runs. Unfortunately, many network devices use the
terms interchangeably. On both home broadband routers and Xbox Live enabled game consoles, for example,
the parameter called MTU is in fact the maximum TCP packet size and not the physical MTU.

In Microsoft Windows, the maximum packet size for protocols like TCP can be set in the Registry. If this value
is set too low, streams of network traffic will be broken up into a relatively large number of small packets that
adversely affects performance. Xbox Live, for example, requires the value of MTU (packet size) by at least
1365 bytes. If the maximum TCP packet size is set too high, it will exceed the network’s physical MTU and
also degrade performance by requiring that each packet be subdivided into smaller ones (a process known as
fragmentation). Microsoft Windows computers default to a maximum packet size of 1500 bytes for broadband
connections and 576 bytes for dialup connections.
Performance problems may also occur if the TCP “MTU” setting on the home broadband router differs from
the setting on individual devices connected to it.