However, such datagram fragments must not combine to a size larger than allowed by the destination host. There are some conditions where source host fragmentation would be necessary. If the host receives ICMP Echo messages with data it is required to send an ICMP Echo-Reply message with the same data. Gateway Fragmentation Gateways must be prepared to do fragmentation. It is not an optional feature for a gateway.
Gateways have no information about the size of datagrams destination hosts are prepared to accept.
It would be inappropriate for gateways to attempt to keep such information. Gateways must be prepared to accept the largest datagrams that are allowed on each of the directly attached networks, even if it is larger than 576 octets. Gateways must be prepared to fragment datagrams to fit into the packets of the next network, even if it smaller than 576 octets. If a source host thought to take advantage of the local network's ability to carry larger datagrams but doesn't have the slightest idea if the destination host can accept larger than default datagrams and expects the gateway to fragment the datagram into default size RFC 879 November 1983 TCP Maximum Segment Size fragments, then the source host is misguided. If indeed, the destination host can't accept larger than default datagrams, it probably can't reassemble them either.