Internet Minimum MTU: 576 Bytes

文章出處：http://blog.chinaunix.net/space.php?uid=10392839&do=blog&id=1988876

Each router must be able to fragment as needed to handle IP datagrams upto the size of the largest MTU used by networks to which they attach. Routersare also
required, as a minimum, to handle an MTU of at least 576 bytes. Thisvalue is specified in RFC 791, and was chosen to allow a “reasonable sized”data block of at least 512 bytes, plus room for the standard IP header andoptions. Since it is the minimum size specified
in the IP standard, 576 byteshas become a common default MTU value used for IP datagrams. Even if a host isconnected over a local network with an MTU larger than 576, it may choose touse an MTU value of 576 anyway, to ensure that no further fragmentation will
berequired by intermediate routers.