What are the best practices for MTU discovery and negotiation on a LAN?

1 Ping and traceroute

Ping and traceroute are two common tools that can help you determine the MTU of a network path. Ping sends a packet with a specified size and waits for a reply, while traceroute sends a series of packets with increasing TTL values and records the hops along the way. By using the -s option in ping and the -F option in traceroute, you can specify the size of the packet and the do-not-fragment flag, which tells the routers not to fragment the packet. If the packet is larger than the MTU of any hop, you will receive an error message indicating that the packet was too big or that the fragmentation was needed. By gradually reducing the packet size until you receive a successful reply, you can find the MTU of the network path.

2 Path MTU discovery

Path MTU discovery (PMTUD) is a protocol that allows a sender to discover the MTU of a network path by using the ICMP protocol. The sender starts with the MTU of its own interface and sends packets with the do-not-fragment flag. If a router along the way cannot forward the packet without fragmentation, it will send back an ICMP message with the type "destination unreachable" and the code "fragmentation needed and DF set". The message will also contain the MTU of the next-hop network. The sender will then reduce its packet size to match the reported MTU and try again. This process will continue until the sender reaches the destination or the minimum MTU of 68 bytes.

3 Best practices

To ensure successful MTU discovery and negotiation on a LAN, it is important to use ping and traceroute to test the MTU of different network segments and devices. Additionally, PMTUD should be used to adjust the MTU of packets according to the network path. To prevent black holes, configure routers and firewalls to allow ICMP messages related to PMTUD. Furthermore, avoid using arbitrary or non-standard MTU values on your LAN and monitor your network performance for any issues related to MTU and fragmentation. Tools such as wireshark, tcpdump, or iperf can be used to capture and analyze your network traffic.

4 Here’s what else to consider

This is a space to share examples, stories, or insights that don’t fit into any of the previous sections. What else would you like to add?