Tasks not implemented

Link-layer: Packet I/O On Ethernet

When joining a multicast group, checks about whether the address is a valid multicast address are not performed.
- We only join IPv4 multicast groups (for RIP), no need for the complication.
Ethernet II checksum not implemented.
- The sent frames may have a random CRC field of length 4.
Ethernet minimum-length padding not implemented.
- The sent frames may be too short for Ethernet.
MTU-related checks not performed.
- Reason: pcap_sendpacket will fail if a constructed frame was too large for the device. Besides, as we use getifaddrs for getting physical address, it's not elegant to again use ioctl for getting the MTU of the interface.

Network-layer: IP Protocol

ICMP messages not implemented.
- May be implemented in the future, not what we focus on now ("may" in RFC791).
IP fragmenting/reassembly not implemented.
- Per instruction as not necessary.
- As a result, the Identification field is always left blank.
- All packets sent have DF set.
IP options not supported and will be ignored on receiving.
- Per instruction as not necessary.
Routing of multicast packets are not implemented.
- The rules for routing IPv4 multicast are complicated and routing them rarely works.
Not all RIPv2 options supported:
- Next Hop and Route Tag not supported.
- Authentication not supported.
- RIPv1 compatibility not supported.

Transport-layer: UDP Protocol

UDP checksum not implemented.
- Took too long but still can't get it right... Optional per RFC768, so set as all zero when sending and not verified when receiving.