What Is Maximum Transmission Unit?

Estimated reading time: 3 minutes

The maximum transmission unit (MTU) defines the largest packet size a device can send across a network without fragmentation. It’s typically configured at the network interface or router level.

Each network packet is a structured bundle of binary data consisting of:

• Protocol headers: Information added by network layers, such as:
  • IP header: IPv4 or IPv6
  • Transport layer header: Transmission Control Protocol (TCP) or User Datagram Protocol (UDP)
  • Data link layer header: Ethernet frame header
• Payload: The actual application data

The maximum packet size allowed varies by network type and protocol. Larger MTUs reduce the number of packets needed, while smaller MTUs increase it. Packet size directly affects the quality of Internet of Things (IoT) connectivity and the overall user experience. Packets that are too small reduce efficiency since more packets are required to transmit the same amount of data.  

Why Does MTU Matter? 

IoT devices frequently operate over constrained networks such as LPWA and Wi-Fi. Narrowband IoT (NB-IoT) MTU can be as low as 512 bytes. This affects the transmission of firmware updates and sensor data. Correctly configuring MTU is vital for optimal network performance.

Optimizing MTU helps ensure smooth communication and reduces unnecessary delays. This is crucial for IoT deployments and real-time applications for which bandwidth and latency matter.

A properly configured MTU:

• Reduces fragmentation
• Minimizes retransmissions
• Improves throughput

If a network packet exceeds the maximum transmission unit of the network path, several outcomes are possible. The behavior depends on the IP version.

IPv4:

The IP layer can fragment packets into smaller pieces to fit the MTU. However, fragmentation adds overhead and increases delays. Fragmentation also increases the chance of packet loss. If one fragment is lost, the entire message may need to be retransmitted, degrading reliability.

IPv6:

Routers do not fragment packets. Oversized packets are dropped. The source device must use path MTU discovery to send appropriately sized packets. A misconfigured MTU can lead to connectivity failures rather than fragmentation.

Impact on Use Cases

MTU has practical implications across industries, shaping how IoT systems perform under varying network conditions. Misconfigured MTU settings can disrupt real-world applications, like:

• Smart agriculture: Sensors in remote fields often rely on cellular networks with varying MTU sizes. A misconfigured MTU may cause data from soil sensors to fragment, delaying the transmission of critical irrigation alerts.
• Connected vehicles: Automotive telematics systems send frequent status updates. An MTU mismatch between the vehicle’s onboard unit and the cloud server can cause packet loss. This can affect real-time navigation and safety alerts.
• Industrial IoT: Factories that use Ethernet-based networks for machine monitoring require consistent MTU settings. An incorrect MTU can slow down predictive maintenance alerts, risking costly downtime.

How to Measure MTU

MTU is measured in bytes. To calculate the MTU, network administrators often use the ping command with fragmentation turned off. This process involves sending packets of varying sizes to a destination and verifying they pass through without fragmentation. The largest successful size becomes the MTU. You can determine the MTU for your network by adjusting the packet size until you find the largest one that does not fragment.

Seamless IoT Connectivity with Telit Cinterion

Don’t let misconfigured MTU settings compromise your IoT performance. Select LPWA modules and connectivity solutions from a trusted IoT leader to ensure your devices communicate seamlessly.

Partner with Telit Cinterion for expert guidance on MTU configuration and network optimization. Our modules and connectivity services handle diverse network conditions. We ensure your devices stay online and function reliably in any environment.