TODO

1. TCP implementation
 not implemented:
  timer(retransmission/...)
  packet retransmission
  tcp option handling
---
 implemented:
  tcp three-way handshake connection
  tcp connection terminal
  tcp data receiving and sending
  tcp TIME-WAIT timer

2. Application support
 network debug tools: traceroute, arping, ss, route, arp, ip [...]

3.
 In TOP1, l2 cannot receive its reply packet sometimes!
 (Should we use a single thread to receive packet and
  another thread to parse it?)

5. Add bridge programming, then we will not use brctl and can delete doc/brctl.sh

6. test udp port allocation alogrithm

7. multicast/broadcast ip address handling
 ip option handling

8. icmp error report to l4(TCP/UDP/RawIP)

9. add cache layer for internal structure

10. remove timer thread. use poll timeout to implement it

11. support select, epoll, kqueue ...
    or use a third-party event library