TCP protocol

1、TCP brief introduction

• TCP(Transmission Control Protocol) Transmission control protocol , seeing the name of a thing one thinks of its function , Is to control the transmission of data .

Source port ：
We can see from the source port that it refers to the port number sent by the sender's computer

Destination port ：
The same goes for the destination port , It refers to the port number of the data received by the receiving computer

Serial number （seq）：
seq Number , We know that a data may consist of multiple packets , The function of serial number is to identify the sequence of each packet , Serial number occupation 32bit, Then the serial number is at most 2 Of 32 Power -1 individual , What if it exceeds this number ？ From 0 Start renumbering .

Confirmation no. （ack）：
ack Number , Be careful ack Number and below ACK The logo is different ,ack=seq+1 It means that the sender and receiver are paired successfully . This confirmation number is an attribute that the data receiver feeds back to the sender , If the confirmation number is N, Then it means that the receiver has received N-1 And previous data

Data migration ：
This data offset is for TCP Message segment , Means from TCP Message segment to TCP Distance between data parts , That is to say TCP The size of the head , Generally speaking, it is 20 Bytes , But we can find that the sixth row has a variable length , So sometimes it's not 20 Bytes .

Retain ：
Reserved means that it is not used now , This length can be used .

URG：
URG=1 Indicates that this message is an emergency message , So this message has the highest priority , It will be put at the beginning of transmission by the sender , It should be used together with the emergency pointer we will talk about next

ACK：
ACK Is a property used to indicate whether a connection is established , When ACK=1 It indicates that the connection has been established , All messages after the connection is established ACK All must be 1, And only when ACK=1 The confirmation number will only be valid

PSH：
This attribute is used when the application process at one end wants to receive a response from the other immediately after typing a command ,TCP Push operation is available

RST：
This field is actually reset, When our TCP When a serious error occurs in the connection , We need to reset the connection , You need to release the connection at this time , Re establish a new connection

SYN：
This attribute is more important , When it's equal to 1 It is used to indicate that this message is a connection request or connection acceptance message , When SYN=1 And ACK=0 when , That means it is a connection request message . When SYN=1 And ACK=1 when , It indicates that this is the connection acceptance message , And when the SYN=1 It is not allowed to carry data , And will consume a serial number .

FIN：
This property is used to release the connection , yes finnish Abbreviation , When the sender will FIN Set up 1 when , It indicates that the data has been completely sent , Request release connection

window ：
Window is an attribute used to control the transmission rate , It indicates the amount of data the receiver allows the other party to send , To solve the problem of inconsistent bandwidth

The checksum ：
It is used to verify the header and data

Pointer to an emergency ：
When URG=1 when , It indicates that this message is an emergency message , The emergency pointer indicates the number of bytes of emergency data in this report , Emergency data processing is completed TCP Will tell the application to resume normal operation , also , The window is 0 You can also send emergency data

2、 Three handshakes and four waves

2.1 Three handshakes

The first handshake ： When establishing a connection , The client sends SYN package （SYN=1,seq=x） To the server , And enter SYN_SEND state , Wait for server to confirm ;
The second handshake ： Server received SYN package , Must confirm the client's SYN(ACK=1,ack=x+1), At the same time, I also send a SYN package （SYN=1,seq=y）, namely SYN+ACK package , At this time, the server enters SYN_RECV state ;
The third handshake ： Client receives server's SYN+ACK package , Send confirmation package to server ACK(ACK=1,ack=y+1,seq=x+1), This package has been sent , Client and server access established state , Complete three handshakes .

 2.2 Four waves

1. First wave ： The client sends one FIN, Used to close client to server data transfer , Client access fin_wait_1 state
2. Second wave ： Server received FIN after , Send a ACK To the client , Confirmation No. is receipt No +1, Server access Close_wait state . here TCP The connection is half closed , That is, the client has no data to send , But if the server sends data , Then the client still needs to accept
3. Third wave ： The server sends a FIN, It is used to close the data transmission from the server to the client , Server access Last_ack state
4. Fourth wave ： Client received FIN after , Client access Time_wait state , And then send a ACK To the server , After the confirmation , Server access Closed state , Finish four waves