Some views on TCP/IP protocol (2): TCP protocol

The above describes the overall architecture of the TCP protocol, today introduces the TCP protocol's famous three-time handshake and four-time handshake.

The server and client must establish a connection before communicating, which is done through three handshakes, thus ensuring the reliability of the communication.

The steps for the three-time handshake are as follows:

Suppose A is the client, B is the server, and a actively communicates with B, the steps are as follows:

1.A send SYN message and Mark Seq=x,ack=0

2.B after receiving the SYN message, if it can communicate, send a confirmation message, and Mark Seq=y,ack=x+1

3.A after receiving confirmation message, send confirmation message, Mark Seq=x+1,ack=y+1

This establishes the connection, where the presence of the state includes

Endpoint	State	Describe
A, b	CLOSED	Connection closed, no in-progress communication
B	LISTEN	Listening status
B	SYN RCVD	B received the connection request but not confirmed
A	SYN SENT	A Send a connection request and wait for confirmation
A, b	Established	Both establish a connection

The four-time handshake close link is as follows:

The specific steps are as follows:

1.A send fin message, Mark Seq=m

2.B after receiving fin message, send confirmation message, Mark Seq=m+1

Fin messages are also sent after 3.B, labeled Seq=n

4.A after receiving fin message, send confirmation message, Mark Seq=n+1

At this point the two sides are disconnected, and the states involved are

Endpoint	State	Describe
A	FIN WAIT 1	Send a close request, wait for confirmation
A	FIN WAIT 2	Receipt of B's confirmation request, waiting for the fin message of B
A, b	CLOSING	Both sides try to close simultaneously, waiting for the other party to confirm
B	CLOSE WAIT	Received a close request, has been confirmed
A	Time waited	Complete two-way shutdown, waiting for all packets to die
B	Last ACK	Wait for the last confirmation

The reason for the 4 handshake is that TCP communication is full-duplex and requires both confirmation.

Some views on TCP/IP protocol (2): TCP protocol