The IPv4 RFC doesn't have anything precluding DF on a fragment. Since each fragment is a datagram in its own right, adding a DF just means that it can't be frag

The IPv4 RFC doesn't have anything precluding DF on a fragment. Since each fragment is a datagram in its own right, adding a DF just means that it can't be fragmented even further. The example fragmentation algorithm only considers DF on a per-datagram basis, regardless of whether or not it is a fragment of some original datagram. (It does take care to forward the original offsets and MF flag correctly, to account for the datagram being a fragment.)