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TCP：Transmission Control Protocol
The goal is ：process to process Process to process
Ensure reliability , But not to ensure safety
What is reliability ？
1. TCP I will do my best , As far as possible Send data to the other party ; But there is no guarantee 100% You can send it to the other party .
2. TCP When the data is not sent to the other party , Give the application layer a Error notification .
Application layer sends data , Or send it to the other party , Or you will know that the data is lost .
3. TCP It can guarantee the receiver ( application layer ) Strictly follow the data at the time of sending Sequential reception .
4. TCP Protect data from unintentional damage (UDP This is also done ).
5. TCP Maintain the network quality as much as possible .

TCP The mechanism provided （ Ensure reliability ）

About number

TCP Segment The two roles of

About retransmission

Packet loss occurred , How to retransmit ？

Fast retransmission

When a message segment is lost , The sender will always receive 1001 In this way ACK, It's like reminding the sender " I think
What I want is 1001" equally ;
If the sending host receives the same message three times in a row "1001" Such a response , The corresponding data will be 1001 -
2000 To resend ;
At this time, the receiver receives 1001 after , Return again ACK Namely 7001 了（ because 2001 - 7000） The receiver
In fact, I have received it before , It is placed in the receive buffer of the receiving operating system kernel ;
This mechanism is called " High speed retransmission control "（ Also called " Fast retransmission "）.

Timeout time

TCP Of Header

TCP The buffer

TCP The connection of

TCP Connect (Connection) The concept of , And connection management
( A connected life = Creation at the beginning ＋ For official use ＋ The destruction )

Three handshakes

syn ack Sign a

Three complete handshakes

Wireshark Caught tools , demonstration TCP Three handshakes

4 Bit head length (header length)
Because the first may contain optional content , therefore TCP The length of the header is uncertain , If the header does not contain any optional fields, the length is 20 byte ,4 The maximum value that can be represented by the bit header length field is 1111, Turn into 10 Into the system for 15,15*4= 60, So the maximum length of the header is 60 byte . The head length is also called data offset , Because the header length actually indicates the starting offset of the data area in the message segment .

Three handshake state transitions

Summary of three handshakes

Four wave phase

Sign a ：(FIN) Final/Finish
Active wave side ： The party who actively disconnects
Passive wave side ： Passively disconnected party
Simultaneous closure ： Both sides take the initiative to wave their hands
The active waving party must be the active connecting party ? no !( Those who take the initiative to break up are not necessarily the people who took the initiative to pursue at that time )

Four wave state changes

CLOSE_WAIT state
TIME_WAIT state
Why keep ？
Examples of why to keep
A lot of TIME_WAIT What happened

Summary of four waves

TCP Exception in

Situation 1

Situation two

Situation three

Situation four

OS Is it hardware or software ？
OS It's software .
Software is a program , It's data + The instruction is to run in CPU On command , And the data to be processed by these instructions .
Mechanism to solve dead links

You can view the TCP Connection （ understand ）

flow control

flow control
1. You need to know the receiving ability of the other party , It is better to perceive in real time .
2. Get the receiving ability of the other party -> Send traffic
3. What mechanism is used to control the sending volume