Six pictures show you why TCP shakes three times?

Three handshakes

picture

picture

Two handshakes （ situation 1）

picture

Two handshakes （ situation 2）

picture

OK, Let's answer this question seriously , We need to find out the problem , First of all, understand TCP How to ensure reliable transmission .

picture

PS：TCP Agreement , The end that initiates the request is called 『 client 』, The end of the passive connection is called 『 Server side 』. Whether it's the client or the server ,TCP Data can be sent and received after the connection is established .

At first , The server And client are both CLOSED state . Before communication begins , Both sides have to create their own transmission control blocks （TCB）. The server is created TCB After entering LISTEN state , At this time, you are ready to receive the connection request from the client .

The first handshake

The client sends the connection request message segment to the server . In the header of the segment SYN=1,ACK=0,seq=x. After the request is sent , The client enters SYN-SENT state .

• PS1：SYN=1,ACK=0 Indicates that the segment is a connection request message .
• PS2：x For this time TCP The initial sequence number of the byte stream of communication .TCP Regulations ：SYN=1 The message segment of cannot have data part , But to consume a sequence number .

The second handshake

After the server receives the connection request message segment , If you agree to connect , A response will be sent ：SYN=1,ACK=1,seq=y,ack=x+1. After the response is sent, it will enter SYN-RCVD state .

• PS1：SYN=1,ACK=1 Indicates that the message segment is the reply message of connection agreement .
• PS2：seq=y When the server is the sender , Send the initial sequence number of the byte stream .
• PS3：ack=x+1 Indicates that the server wants to send the next datagram sequence number from x+1 Start byte .

The third handshake

When the client receives the reply of the connection consent , It also sends an acknowledgement message segment to the server , Express ： The connection consent response from the server has been successfully received . The header of the message segment is ：ACK=1,seq=x+1,ack=y+1. After the client sends this message segment, it will enter ESTABLISHED state , After receiving this response, the server will enter ESTABLISHED state , At this point, the connection is established ！

Why does it take three handshakes to establish a connection , Not two handshakes ？

To prevent the invalid connection request message segment from being received by the server , So there's a mistake .

PS： Invalid connection requests ： If the connection request sent by the client to the server is lost , The client will send the connection request again after waiting for a response , here , The last connection request was 『 Invalid 』.

If a connection is established, only two handshakes are needed , The client doesn't change much , You still need to get the response from the server before entering ESTABLISHED state , The server enters after receiving the connection request ESTABLISHED state .

At this point, if the network is congested , The connection request sent by the client fails to reach the server , The client will time out to resend the request , If the server receives and confirms the response correctly , Double convenient start communication , Release the connection when communication is over . here , If the invalid connection request reaches the server , Because there were only two handshakes , When the server receives the request, it will enter ESTABLISHED state , Waiting to send data or actively sending data . But the client has already entered CLOSED state , The server will always wait , This wastes the connection resources on the server side .

TCP Four waves

picture

TCP The release of the connection takes four steps , So called 『 Four waves 』.

We know ,TCP The connection is two-way , So in four waves , The first two waves are used to disconnect in one direction , The last two waves are used to disconnect the connection in the other direction .

First wave

if A Think data transmission is complete , Then it needs to be directed to B Send connection release request . The request has only header , The main parameters carried in the head are ：FIN=1,seq=u. here ,A Will enter the FIN-WAIT-1 state .

• PS1：FIN=1 Indicates that the segment is a connection release request .
• PS2：seq=u,u-1 yes A towards B The sequence number of the last byte sent .

Second wave

B After receiving the connection release request , The corresponding application will be notified , Tell it A towards B The connection in this direction has been released . here B Get into CLOSE-WAIT state , And to A Send a connection release response , Its header contains ：ACK=1,seq=v,ack=u+1.

• PS1：ACK=1： except TCP Outside the connection request segment ,TCP All datagrams in the process of communication ACK All for 1, To answer .
• PS2：seq=v,v-1 yes B towards A The sequence number of the last byte sent .
• PS3：ack=u+1 Wish to receive from u+1 A message segment starting with bytes , And has successfully received the front u Bytes .

A Received the response , Get into FIN-WAIT-2 state , wait for B Send connection release request .

After the second wave ,A To B The connection in the direction has been released ,B No more data will be received ,A No more data will be sent . but B To A The connection of directions still exists ,B You can go on to A send data .

Third wave

When B towards A After sending all the data , towards A Send connection release request , Request header ：FIN=1,ACK=1,seq=w,ack=u+1.B Then enter LAST-ACK state .

Fourth wave

A After receiving the release request , towards B Send a confirmation response , here A Get into TIME-WAIT state . This state will last 2MSL Time , If there is no B If you want to resend the request , To get into CLOSED state , revoke TCB. When B After receiving the confirmation reply , And then I went into CLOSED state , revoke TCB.

Why? A Enter first TIME-WAIT state , wait for 2MSL It's time to enter CLOSED state

In order to ensure B To receive A The confirmation response . if A After sending the confirmation response, directly enter CLOSED state , Then if the reply is lost ,B After the timeout, the connection release request will be sent again , But this time A It's closed , There will be no response , therefore B Never shut down properly .