TCP SYN Flood attack uses the three-way handshake mechanism.

1. An attacker at system A sends a SYN packet to victim at system B.

2. System B sends a SYN/ACK packet to victim A.

3. As a normal three-way handshake mechanism system A should send an ACK packet to system B, however, system A does not send an ACK packet to system B. In this case client B is waiting for an ACK packet from client A.

This status of client B is called _________________