Path MTU discovery in practice

What this post added

This post details a specific networking challenge encountered with Path MTU discovery, particularly affecting users with IPv6 tunnels. It explains the underlying mechanisms of MTU, fragmentation, and ICMP messages, and how Cloudflare's internal ECMP routing inadvertently broke Path MTU discovery by misrouting ICMP messages. The post outlines temporary fixes (reducing MTU for IPv6, enabling RFC4821 for IPv4) and introduces their open-sourced Path MTU Daemon (pmtud) as a comprehensive solution to ensure correct ICMP routing within their datacenters.