SYN flood

[Tong YU]

Could some body explain what do the following log messages mean ?
Why there's SYN flood and cookies from various IPs  ?
Thanks.

Tong
------------------------------------------------------------------------------
Sep 25 03:04:45 ns kernel: Warning: possible SYN flood from 203.184.204.170 on 2
06.55.157.112:1288.  Sending cookies.
Nov  7 12:07:16 ns kernel: Warning: possible SYN flood from 209.141.239.71 on 20
6.55.157.103:111.  Sending cookies.
Nov 11 19:55:19 ns kernel: Warning: possible SYN flood from 208.176.108.8 on 206
.55.157.101:111.  Sending cookies.
Nov 12 00:54:35 ns kernel: Warning: possible SYN flood from 209.53.2.63 on 206.5
5.157.103:111.  Sending cookies.
Nov 12 07:38:03 ns kernel: Warning: possible SYN flood from 63.65.248.126 on 206
.55.157.2:53.  Sending cookies.