The IPv4 specification requires that IPv4 devices must be able to reassemble a datagram with a length of a minimum of 576 bytes.
Looking back into the creation of the internet...
RFC 791 in defining the IP protocol states:
Every internet destination must be able to receive a datagram of 576octets either in one piece or in fragments to be reassembled.
And by RFC879 it has become:
HOSTS MUST NOT SEND DATAGRAMS LARGER THAN 576 OCTETS UNLESS THEYHAVE SPECIFIC KNOWLEDGE THAT THE DESTINATION HOST IS PREPARED TOACCEPT LARGER DATAGRAMS.
This is a long established rule.
At what point prior to the internet being created did this become an established rule ? Was this 576 byte limit set to meet the capabilities of an already existing system ? for example hardware such as a computer or a modem, or a transport layer protocol such as a dial-up protocol ?
On further research Xerox Network Systems also specifies a value of 576 bytes. However as a maximum packet length. So while unusual that these numbers are identical I would rule out the thought that the intention was to tunnel XNS over IP as there would be no capacity to wrap the XNS packet.
