Limitations in IPv4 Packet Re-assembly

[patacongo]

IPv4 packet re-assembly is implemented in net/devif/ipv4_input.c. The current implementation, however, has too many limitations to be useful:

1. IPv4 reassembly is untested.
2. Currently can only work with Ethernet due to the use of Ethernet packet size in the definition of IPv4_REASS_BUFSIZE.
3. Since there is only a single reassembly buffer, IPv4 reassembly cannot be used in a context where there may be multiple, concurrent re-assembly packets.
4. The IPv4 header is variable-length depending up on the options in the header. The logic here assumes that the size of the IPv4 header is a constant (this is a bug and not a limitation).