Network MTU vs. Maximum TCP

Maximum transmission unit (MTU) and maximum TCP packet size are computer networking terms that often get confused. Learn about the differences between network MTU vs. maximum TCP packet size and how these are related.

Network MTU Size

• Limited by network hardware.

• Cannot be adjusted without hardware changes.

• Measured in bytes.

Maximum TCP Packet Size

• Can be set to any value.

• Should never be higher than the MTU.

• Measured in bytes.

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.

Some network devices incorrectly use these terms interchangeably. For example, on some home broadband routers, the parameter called MTU is actually the maximum TCP packet size.

MTU Size Pros and Cons

Advantages

• A larger MTU allows for faster data transmission.

• A smaller MTU results in decreased network latency.

Disadvantages

• A larger MTU can increase network latency.

• Increasing MTU may require expensive hardware upgrades.

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

Advantages

• Can be adjusted through the operating system.

• A lower max TCP packet size can improve network latency.

Disadvantages

• Setting it higher than the MTU can cause jabbering.

• A lower max TCP packet size results in slower transmission.

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.

MTU and Max TCP Related Problems

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.