About “tcp How protocols ensure reliable transmission ” This problem , The information on the Internet is uneven . Recently, I began to read some textbooks in the library , Read the following bibliography with questions about TCP Chapter of , The summary is recorded .

• University of Potsdam, Germany 《internetworking Technische Grundlagen und Anwendungen》/《 Network technology foundation and Application 》（christoph meinel,harald sack）
• 《 In depth understanding of computer networks 》（ Wang Da ）
• 《 Computer network tutorial top-down method 》（Behrouz A.Forouzan Firouz Mosharraf）
• 《 computer network 》（ Zhang Zengke 、 Yang Xianhui ）
• 《 computer network 》 The first 7 edition （ Xie Xiren ）

Why? TCP We need reliable transmission

• The network layer is unreliable . The task of the network layer is only to select the appropriate inter network routing and switching nodes , Make sure the data is transmitted in time .
• Due to the nature of the transport layer , The upper level needs reliability . Therefore, the main task of transport layer is to make the best use of network resources according to the characteristics of communication subnet , And Reliability and economy The way , Between the conversation layers , Provide to build 、 Function of maintaining and canceling transmission connection , Responsible for reliable transmission of data .
• UDP By sacrificing reliability , In exchange for best effort delivery 、 High communication efficiency 、 Ensure the real-time characteristics of the data . Natural needs TCP Appearance , Come on Use needs complement each other .

So the above 3 spot , The burden of reliable transmission has been placed on TCP above .

 

TCP How to achieve reliable transmission

For the purpose of reliable transmission ,TCP Use 4 Solutions ： Connection oriented transport mechanism 、 Timeout retransmission control 、 Variable sliding window flow control 、  Congestion control .

 

Connection oriented transport mechanism

Used Three handshakes 、 Four waves , This is more common , Everybody is familiar with , There is no redundancy .

The checksum , Confirm response （ACK）, Serial number .

And it doesn't work for repetition ACK To deal with , Take it and discard it , Realize automatic retransmission request .

 

Timeout retransmission control

When the message is sent, the receiver's confirmation is not received within a certain period of time , The sender will retransmit . Every time a packet is sent , Just set up a Timeout retransmission timer , When the other party returns ack, To undo the set timeout timer . To achieve this , There are three points to note ：

• Keep after sending copy , For retransmission , Received the return of the other party ack after , To clear the copy .
• Number .
• The time setting of the timeout retransmission timer takes the delay into account 、 Congestion and other uncertain factors , thus Retransmission time The setting will be longer than the average round-trip time of data packets .

 

Variable slide window flow control

The window is divided into sending window and receiving window . Briefly , The receiver puts the acceptable data size （ Accept window size ）, adopt ACK Feedback to the sender , To set the coordinated sending content size . The details are as follows ：

• When the sending data exceeds the size of the receiver window , Retransmit the excess ; 
• When the sending data is smaller than the size of the receiver window , The sender continues to send the content ;
• Not for every segment ACK Reply to , When possible, one or more segments , Send a ACK;
• When the number of received message segments is missing , The received segment is stored in the cache , Wait for the sender to retransmit the missing segment ; Merge complete , Back again ACK; If the missing segment is not received for a long time , Then the message segments in the cache will be emptied .

 

  Congestion control

There are four kinds of control algorithms ： Slow start 、 Congestion avoidance 、 Fast retransmission and fast recovery .

• Slow start ： Gradually increase the value of congestion window from small to large .

        Initial value of congestion window = 1 The value of the maximum segment to two senders ;

        Congestion window increases each time = min( The number of bytes of the newly received acknowledgement message , The value of the maximum message segment of the sender ).

• Congestion avoidance : Let the congestion window slow down and increase .

      Every time I pass by RTT Just slide the window size and just add 1;

• Fast retransmission ： The data received by the receiver shall be confirmed immediately , A packet returns a confirmation ACK, In order to know the loss of individual messages as soon as possible .

      for example , When sending M3 Packet loss , The receiver only received M2 And M4 Packet time , The receiver returns in succession 3 individual M2 Of ACK To the sender （ The sender is missing M3 Little brother ）;

      The sender received 3 A continuous M2 Of ACK after , immediately Retransmission M3 Data packets .

• Fast recovery ： After encountering network congestion , Reduce congestion window immediately .

      Set up a ssthresh As a starting point for using congestion avoidance algorithms , Through this ssthresh value It is half of the maximum threshold of sliding window .

 

Understand the above four algorithms , It's easy to look at the picture below ：

 

summary ：TCP Because it's in the transport layer , The essential , Solve the unreliable problem of the lower network layer , Provides a reliable connection to the session layer ; And realize the right to UDP The use of complementary .

TCP Reliability is mainly through connection oriented transport mechanism 、 Timeout retransmission control 、 Variable sliding window flow control 、  Congestion control implementation .