Sliding window summary of TCP connections

< One >1, Recently I encountered a problem in my work ,tcp Connection establishment is normal , But sending packets to the receiving end in the sending direction is , The receiver program is in a suspended state , Unable to receive data , After checking, it is found that the receiver has sent... To the sender ZeroWindows Data packets of , That is, the sliding window is 0 The situation of , This situation causes the sender not to send the relevant data .
2, Slide window Introduction
stay tcp There are two cases in the connection. One is the fixed window , The other is sliding window , The following describes the principles of the two windows .
a> Fixed window , Is the size of the buffer agreed by both sides of the connection , Used to receive the required data , But this method is not good , Or waste , Or not enough ,( Insufficient conditions will block the transmission of link data, resulting in inefficiency ) In a word, the data transmission mode is not very good .
b> The sliding window
Sliding window is a kind of flow control technology .
It essentially describes the receiver TCP Datagram buffer size data , According to this data, the sender can calculate how much data it can send , If the sender receives the receiver with a window size of 0 Of TCP The datagram , Then the sender will stop sending data , Wait until the receiver sends the window whose size is not 0 The arrival of the datagram
c> working principle
When sending data for the first time, the window size is determined according to the link bandwidth .

Suppose the window is 3. At this time, after receiving the data, the receiver will confirm the data and tell the sender how much data I hope to receive next time .

In the diagram above ： We see that the receiver sent ACK = 3（ This is the sending sequence to the sender 2 Yes , The next time the receiver expects to receive 3 Sequence signal ）, At this time, the sender will know the first time I sent after receiving this data 3 The other party only received two data , I knew the third data was not received by the other party , The next time you return it, start from 3 Data starts to be sent . At this time, the window size becomes 2.

As shown in the figure below ：

See... Sent by the receiver ACK yes 5 It means that the next data he wants to receive is 5, The sender knows what I just sent 2 Data received by the other party , This is the time to start sending 5 Data .

The above is the introduction of sliding window .
Be careful : Attitude data transmission process , The corresponding data transmission processing should be handled according to the actual situation , Can split packets , Send the actual data required in batch . It can be determined by the length of the transmitted data according to the corresponding structure . It can better avoid sending sticky packets .

< Two >socket Communication connection .
Client connection denied , appear Connection refused. It was found that , On the client side SYN After the newspaper , The server sent... To the client RST,ACK The data packet , Means to reestablish the connection . Common phenomena , The server does not listen to the corresponding port . Because the ports used by the client and server are the same ip Inconsistency leads to . Such problems , This will cause reconnection failure , Connection denied , situation .
resolvent ： Make sure that both engines are used ip The port is consistent with ( The precondition that the port is not occupied ).