What is MTU

The MTU, also known as Maximum Transmission Unit, 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.
Ethernet Frame Size

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. However, for communications over a dial-up connection to the Internet, the recommended MTU size is only 576 bytes. This size is also sometimes recommended for VoIP implementations because there is less likelyhood of a large packet causing interuption of the voice traffic.

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.

If you are using your broadband connection for VPN access you may also be better off with a lower MTU setting.

See the article MTU issues when using PPPoE for more information.

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