- ETHER_ADDR_LEN: length of Ethernet address (actually, we already defined
this).
- ETHER_TYPE_LEN: length of the Ethernet header `type' field.
- ETHER_CRC_LEN: length of the Ethernet CRC (explorer got this already, mostly
because I forgot to commit these changes earlier).
- ETHER_HDR_LEN: total length of the Ethernet header
- ETHER_MAX_LEN: maximum length of an Ethernet frame, including header and CRC
- ETHER_MIN_LEN: minimum length of an Ethernet frame, including header and CRC
Define ETHERMTU and ETHERMIN (payload sizes) in terms of the above constants.