Path MTU discovery in practice

Last week, a few users experienced issues accessing Cloudflare services due to a broken "path MTU discovery" on the company's servers. This issue affected those using IP tunnels, primarily for IPv6 over IPv4. The problem was caused by a networking change that impacted path MTU discovery. To fix this, Cloudflare implemented a comprehensive solution called Path MTU Daemon (PMTUD), which is now open-source and available on GitHub. Additionally, the company enabled RFC4821 Path MTU Discovery for IPv4 to help with true ICMP black holes.