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What is MTU?

Posted On 2008-01-31 by FortyPoundHead
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Tags: Networking Tutorial 
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The MTU is a limit, expressed in bytes, on the size of data sent over a network. It is the maximum size of a single unit (e.g., an Ethernet frame) of digital communications.

MTU sizes are properties of physical network interfaces. 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. Higher-level network protocols like TCP/IP can be configured with a maximum packet size, but this setting is distinct from the physical MTU. The MTU of a network has important implications on the performance of computer communications as explained below.

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. If the maximum TCP packet size is set too high, it will exceed the network''s MTU and also degrade performance by requiring that each packet be subdivided into at least two smaller ones (a process known as fragmentation).

Setting an optimal TCP packet size on home computers can be tricky. For communications within the home LAN, a setting of 1500 bytes to match the Ethernet MTU is optimal. Windows 95 uses a default of 1500 bytes for this reason. However, for communications over a dial-up connection to the Internet, the recommended MTU size is only 576 bytes. Windows 98 uses 576 bytes as its default for this reason. But high-speed connections (including DSL or cable service and home LANs) usually perform better at the higher value. Tuning the maximum packet size of TCP/IP also depends in important ways on two other related parameters named MSS and RWIN.


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