When connection is established,
a client sends a synchronous (syn = j) to the server, and enters the SYN_SENT state; Synchronize Squence Numbers synchronization SEQ ID NO
2. The server receives syn packets, must confirm the customer SYN (ack = j + 1) , while themselves sends a SYN packet (syn = k), i.e., SYN + ACK packet, then the server enters a state SYN_RECV;
Reproduced in: https: //my.oschina.net/yourpapa/blog/621595