MTU

Maximum Transmission Unit (MTU) and Path MTU Discovery (PMTUD)

Overview

Maximum Transmission Unit (MTU) is the largest size of a packet that can be transmitted over a network interface without fragmentation. Properly configuring MTU is essential for optimizing network performance, reducing latency, and avoiding excessive fragmentation.

Path MTU Discovery (PMTUD) is a mechanism that enables a sender to determine the optimal MTU size across an entire network path. By avoiding packet fragmentation, PMTUD helps improve efficiency and reduces overhead on network devices.

Understanding MTU

MTU is typically measured in bytes and varies across different network types:

Ethernet: 1500 bytes (default)
Jumbo Frames (Ethernet): Up to 9000 bytes
PPPoE: 1492 bytes
VPN Tunnels (IPsec, GRE): Typically lower due to encapsulation overhead

An incorrectly set MTU can lead to fragmentation or packet loss, causing performance issues.

How Path MTU Discovery (PMTUD) Works

PMTUD is used to determine the maximum MTU along the network path by leveraging the **Don't Fragment (DF) bit** in the IP header: 1. The sender transmits packets with the DF bit set. 2. If a router along the path has a lower MTU, it drops the packet and sends back an ICMP "Fragmentation Needed" message. 3. The sender lowers the packet size and retransmits until a successful MTU size is found.

This method is commonly used in both IPv4 and IPv6, though **IPv6 strictly prohibits fragmentation by routers**, making PMTUD even more critical.

Enabling and Testing PMTUD

PMTUD is generally enabled by default in modern operating systems. However, firewall rules that block ICMP messages can disrupt PMTUD, leading to connectivity issues.

To test PMTUD manually:

On Linux/macOS:

 ```sh
 ping -M do -s <size> <destination>
 ```
 Example:
 ```sh
 ping -M do -s 1472 8.8.8.8
 ```
 (1472 bytes + 28-byte ICMP header = 1500 MTU)

On Windows:

 ```powershell
 ping -f -l <size> <destination>
 ```

Adjust the packet size until you find the largest value that does not result in fragmentation.

Best Practices for MTU and PMTUD

1. **Do not block ICMP Type 3 Code 4 (Fragmentation Needed)** messages in firewalls. 2. **Use Jumbo Frames wisely** – ensure all devices on the network path support them. 3. **Set MTU appropriately for VPNs and tunnels** to avoid excessive fragmentation. 4. **Use MSS Clamping** to control TCP Maximum Segment Size (MSS) when dealing with smaller MTUs. 5. **Test connectivity** with tools like `tracepath` or `ping` to confirm MTU behavior.

Conclusion

Properly managing MTU and enabling PMTUD ensures efficient network performance and prevents issues caused by fragmentation. By understanding how PMTUD operates and following best practices, network engineers can optimize connectivity across complex network infrastructures.