 When a client sends back a TCP ACK packet to the server in response to the server's SYN+ACK packet, the client must (according to the TCP spec) use "n+1" in the packet's "Acknowledgement number", where "n" is the initial sequence number sent by the server.The server then subtracts 1 from the acknowledgement number to reveal the SYN cookie sent to the client.The server then performs the following operations.