edns-buffer-size

[T.Suzuki]

unbound.conf
# EDNS reassembly buffer to advertise to UDP peers (the actual buffer
# is set with msg-buffer-size). 1480 can solve fragmentation (timeouts)
edns-buffer-size:

Why does this comment recommend
 1480 = 1500 - 20 ? (UDP datagram size over IPv4)

Why is this value not to become
 1472 = 1500 - 20 - 8 ? (UDP payload size over IPv4)

Or, Is this a value of another meaning?

(I verified that the edns-buffer-size is used for udp payload size of EDNS0.)

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T.Suzuki