TCP triple handshake

Preface

TCP When the protocol passed down data , client （Client） And the service side （Server） Will establish a connection , Then segment the files to be transferred , And provide reliable transmission and flow control ！ After the data transfer is complete , The current session should also be disconnected , Avoid waste of resources . all TCP The third handshake is the process of establishing a connection , And four waves are the process of disconnecting ！

TCP Basic knowledge

Transmission control protocol （ English ：Transmission Control Protocol, abbreviation ：TCP） It's a connection oriented 、 reliable 、 Transport layer communication protocol based on byte stream

TCP Header format

• Serial number ： The random number generated by the computer when establishing the connection is used as the initial value , adopt SYN The packet is sent to the receiving host , Every time it's sent , The size of the number of bytes is accumulated once . It is used to solve the network packet disorder problem
• Confirm answer signal ： Next time [ expect ] The serial number of the data received , After receiving the acknowledgement, the sender can consider that the data before the serial number has been received normally . To solve the problem of no packet loss
• Control bits

ACK： This bit is 1 when , Confirm that the reply field becomes valid ,TCP Except when the connection is initially established SYN This bit outside the package must be set to bit 1

RST： This bit is 1 when , Express TCP When an exception occurs during connection, the connection must be forcibly disconnected

SYN： This bit is 1 when , Indicates that you want to establish a connection , And set the initial value of the serial number in the field of the serial number

FIN： This bit is 1 when , No more data will be sent in the future , Want to disconnect . When communication ends and you want to disconnect , The hosts of both sides can exchange with each other FIN Position as 1 Of TCP paragraph .

Three handshakes

• The first handshake ： When establishing a connection , client A send out SYN package (SEQ_NUMBER=x) To the server B, And enter SYN_SEND state , Waiting for server B confirm .
• The second handshake ： The server B received SYN package , Customer must be confirmed A Of SYN(ACK_NUMBER=x+1), At the same time, I also send a SYN package (SEQ_NUMBER=y), namely SYN+ACK package , At this time, the server B Get into SYN_RECV state .
• The third handshake ： client A Receive server B Of SYN＋ACK package , To the server B Send confirmation package ACK(ACK_NUMBER=y+1), This package has been sent , client A And the server B Get into ESTABLISHED state , Complete three handshakes .
• Complete three handshakes , Client and server begin to transmit data .

Four waves

• First wave ： The client intends to close the connection , A TCP The first one FIN The flag bit is set to 1, Its serial number is the sequence number of the last byte of the previously transmitted data plus 1(seq=x)
• Second wave ： After the client receives the message , Send it to the client ACK Reply message ,ACK=1,ack=x+1, And bring your own serial number seq=y, here , The server enters CLOSE-WAIT（ Turn off waiting ） state .
• After the client receives the confirmation request from the server , here , The client enters FIN-WAIT-2（ Stop waiting 2） state , Wait for the server to send the connection release message （ Before that, you need to accept the last data sent by the server ）.
• Third wave ： After the server sends the last data , Send the connection release message to the client ,FIN=1,ack=x+1, Because it's half closed , The server probably sent some more data , Suppose the serial number at this time is seq=z, here , The server is in LAST-ACK（ Final confirmation ） state , Wait for the client to confirm .
• Fourth wave ： After the client receives the connection release message from the server , Confirmation must be sent ,ACK=1,ack=z+1, And my serial number is seq=h, here , The client enters TIME-WAIT（ Time waits ） state . Note that this time TCP The connection has not been released , After the longest message segment life , When the client cancels the corresponding TCB after , Only enter CLOSED state .
• As long as the server receives the confirmation from the client , Enter immediately CLOSED state . Again , revoke TCB after , It's the end of this TCP Connect . You can see , End of server TCP The connection time is earlier than the client .

Why not use two handshakes to connect ？

answer ： Three handshakes complete two important functions , Both sides should be ready to send data , Also know that both sides know that each other is ready ！ If you change to two handshakes, it will lead to deadlock . Suppose the client sends a connection request to the server , The server received this packet , And sent a confirmation response packet . According to the agreement of two handshakes , The server thinks that the connection has been successfully established , You can start sending data packets . But , When the response signal of the server is lost during transmission , The client does not know whether the server is ready , I don't know what serial number the server will create , The client even doubts whether the server has received its own connection request packet . under these circumstances , The client thinks that the connection has not been established successfully , Any data packets sent by the server will be ignored , Only wait for the connection to confirm the reply group . The server sends out packets after timeout , Send the same packet over and over again . This creates a deadlock .

The second of the four waves 、 What happens when you merge three times ？

answer ： Because when the server receives a request from the client to disconnect , There may still be some data not sent out , Then reply first ACK, Indicates that a disconnect request has been received . Wait until the data is sent FIN, Disconnect the data transfer from the server to the client .

Only when all the messages on the server end are sent , To send FIN message , So we can't send . So it takes four waves .