Fixing an old hack – why we are bumping the IPv6 MTU
I recently spent some time with IPv4 MTU. I don&#x27;t know how much it applies to IPv6, but my TLDR is MSS (+ size of minimum header) is usually a good indicator of acheivable MTU, but some small fraction of people are misconfigured. A lot of that is syns indicating 1500, but actual MTU is 1492 (pppoe is a scourge). But there&#x27;s some other issues too, it&#x27;s hard to track down. Some client oses are capable of probing for mtu black holes, but Android doesn&#x27;t do it (thanks Google), even though the kernel has it available, and there&#x27;s no way an app could turn it on (thanks Google). If you want to avoid this problem, you can send back MTU - X, depending on how conservative you want to be. 8, 20 and 50 were good values of X, but you&#x27;re adding some overhead too.