TCP fast retransmission sack mechanism

First, let's look at a netizen question ：
Questions from netizens

Here is an explanation for the blogger about the doubling of overtime retransmission time , Because after timeout retransmission ,RTT Time will double , Previously, it was thought that doubling was only for the current TCP package , I didn't expect that it was all connected TCP Of RTT Time doubling .
Another problem with overtime retransmission is , Find out 2 The number is out of time , that 3 Do you want to retransmit the number , The answer is yes , Need to retransmit , That's why a bag is lost , All subsequent packets are retransmitted , And when retransmitted RTT It's doubled , This leads to very poor efficiency .
And the timeout retransmission will cause the sliding window to change .
Another strange thing is , Why is there no trigger for fast retransmission , See the blog text for details , Fast retransmission means that the receiver will send three packets when it finds a packet loss ack To tell the sender about the packet loss , The package needs to be sent again , This will be shorter than the timeout , If it's sending sack Of ACK package , It will be more efficient . Specifically SACK The following is about .

About SACK Introduce 1

About SACK Introduce 2

TCP Overtime retransmission and fast retransmission

ACK Delay confirmation mechanism ：

One 、ACK Definition
TCP Agreement , After the receiver successfully receives the data , Will reply one ACK Data packets , It means that you have confirmed the receipt of ACK All the data in front of the confirmation number .
ACK The length of the field is 32 position , Can express 0~2^32-1 Between the value of the .

Two 、ACK effect
The sender did not receive the server's ACK After confirming the package , It will resend TCP Data packets . The sender received it ACK, Indicates that the receiver has received the data , Ensure the reliability of the data .

3、 ... and 、ACK Mechanism
After receiving the data , Not immediately to the sender ACK Of . This can be caused by ：
1、 Before the serial number of the received data packet, there are packets to be received . Because when the sender sends data , You don't have to wait until the data was sent last time Ack You can continue sending TCP package , And these TCP The order in which packets arrive is not guaranteed , In this way, the receiver may receive and then send TCP package （ Note that the submission to the application layer is sequential ）.
2、 In order to reduce network traffic ,ACK There is a delay confirmation mechanism .
3、ACK After reaching the maximum value of , And from 0 Start .

Four 、ACK Delay confirmation mechanism
After receiving the data , Not immediately ACK, It's a delay . commonly ACK The delay time is 200ms, But this 200ms It's not the amount of time you need to delay after you receive the data . The system has a fixed timer every 200ms Will check if you need to send ACK package . There are two purposes to do this .
1、 The purpose of this is ACK It can be combined , That is, if you receive two in a row TCP package , It doesn't have to be ACK two , Just reply to the final ACK That's all right. , Can reduce network traffic .
2、 If the receiver has data to send , Then the data will be sent TCP In the packet , close ACK Information . To do so , Can avoid a lot of ACK With a single TCP Packet sending , Reduced network traffic .

Delay confirmation (Delayed Ack)

setsockopt(fd, IPPROTO_TCP, TCP_QUICKACK, (int[]){1}, sizeof(int))
Generally speaking, it is related to nagle Use... In specific situations