Fixing an old hack - why we are bumping the IPv6 MTU

In 2015, Cloudflare deployed ECMP routing within their datacenters to distribute traffic across multiple servers. However, they encountered a problem with Path MTU discovery where ICMP packets destined for their Anycast IPs were being dropped. To solve this issue, they created pmtud software and reduced the IPv6 MTU to 1280 bytes. Recently, they increased the IPv6 MTU to 1400 bytes to support UDP traffic in Cloudflare Spectrum while minimizing the probability of encountering Path MTU issues. They continue monitoring ICMP PTB packets and encourage users with misconfigured MSS or weird MTUs to report any issues.