Maximum Transmission Unit “MTU”

What is MTU?

IT is the maximum transmission unit which 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.

How can MTU affects the performance?

A larger MTU brings greater efficiency because each packet carries more user data while protocol overheads, such as headers or underlying per-packet delays, remain fixed; the resulting higher efficiency means a slight improvement in bulk protocol throughput. A larger MTU also means processing of fewer packets for the same amount of data. In some systems, per-packet-processing can be a critical performance limitation.

Large packets can occupy a slow link for some time, causing greater delays to following packets and increasing lag and minimum latency. For example, a 1500-byte packet, the largest allowed by Ethernet at the network layer (and hence over most of the Internet), ties up a 14.4k modem for about one second.

Large packets are also problematic in the presence of communications errors. Corruption of a single bit in a packet requires that the entire packet be retransmitted. At a given bit error rate larger packets are more likely to be corrupted. Retransmissions of larger packets take longer.

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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Osama Aboelfath is co-founder at Recursive-lookup. Osama is a network engineer and developer with over 10 years of production network engineering, deployment & operation.

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