Q4 TCP Flags: Sってなんぞや? →TCPFlags There are six TCP flags. They are: URG - urgent pointer is valid ACK - acknowledge data received (set in all packets but first) PSH - push data to app RST - reset connection (hard close) SYN - connection request (& returned by server w/ ack to accept) FIN - end connection (normal close) The flag as noted in the log file is the first letter of the flag abbreviation. Therefore, "AP" means ACK plus PSH, "S" means SYN.