It is actually more complicated than that. It proposes the use of one of the reserved IP flag bits to indicated whether the packet includes "IPv4+" addresses an

It is actually more complicated than that. It proposes the use of one of the reserved IP flag bits to indicated whether the packet includes "IPv4+" addresses and then re-use of the already used DF and MF flag bits to indicated whether the source or destination addresses are "IPv4+" Since these flags are already used for packet fragmentation IPv4+ packets could not be fragmented so it also requires implementing a path mtu discovery protocol for udp since current path mtu discovery using ICMP is broken.