When you send a file or message via Transmission Control Protocol (TCP), it is divided into packets that are reassembled after reaching the intended destination. The maximum transmission unit (MTU) is the maximum size of a single data unit that can be transmitted over a digital communications network. Higher-level network protocols, like TCP/IP, can be configured with a maximum packet size, which is a parameter that’s independent of the physical layer MTU over which TCP/IP runs. While it’s possible to set the maximum TCP packet size to almost any value, it should never exceed the network’s MTU.

MTU Size Pros and Cons

The MTU size is a property of a physical network interface and is usually measured in bytes. The MTU for Ethernet, for example, is 1500 bytes. Some types of networks, such as token rings, have larger MTUs. Some networks have smaller MTUs, but the value is fixed for each physical technology. A larger MTU means that more data fits into fewer packets, which generally allows for faster and more efficient transmission. However, if a communications error occurs, the packet takes longer to re-transmit. Since larger packets are more prone to corruption and delays, a smaller MTU can improve network latency.

Maximum TCP Packet Size Pros and Cons

In Microsoft Windows, the maximum packet size for protocols such as TCP can be set in the Windows Registry. If this value is set too low, streams of network traffic are broken up into a relatively large number of small packets, which adversely affects performance. Being on the Xbox network, for example, requires the value of the packet size to be at least 1365 bytes. If the maximum TCP packet size is set too high, it exceeds the network’s physical MTU and degrades performance by requiring that each packet be subdivided into smaller ones. This process is called fragmentation. Microsoft Windows computers default to a maximum TCP packet size of 1500 bytes for broadband connections and 576 bytes for dial-up connections to avoid exceeding the MTU.

Ethernet’s MTU of 1500 bytes limits the size of packets that traverse it. Sending a packet that is larger than the maximum transmission window for Ethernet is called jabbering. If unaddressed, jabbering can disrupt a network. Usually, jabber is detected by repeater hubs or network switches. The simplest way to prevent jabber is to set the maximum size of a TCP packet to no more than 1500 bytes. In theory, the maximum size limit of a TCP packet is 64K (65,525 bytes), which is much larger than you’ll ever use. Nonetheless, performance problems may also arise if the TCP maximum transmission settings on your home broadband router differ from the settings on individual devices connected to it.