DNS/FCP/UDPpayloadについて、ここに記述してください。

https://stackoverflow.com/questions/1098897/what-is-the-largest-safe-udp-packet-size-on-the-internet

The maximum safe UDP payload is 508 bytes. 
This is a packet size of 576, minus the maximum 60-byte IP header and the 8-byte UDP header.
Any UDP payload this size or smaller is guaranteed to be deliverable over IP (though not guaranteed to be delivered). 
Anything larger is allowed to be outright dropped by any router for any reason.
Except on an IPv6-only route, where the maximum payload is 1,212 bytes.
As others have mentioned, additional protocol headers could be added in some circumstances. 
A more conservative value of around 300-400 bytes may be preferred instead.