Yyds dry goods inventory TCP & UDP

difference

TCP Who initiated the connection in , stay UDP Who sent the message in .

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TCP stay IP Based on the message , Added such as retransmission 、 confirm 、 Orderly transmission 、 Congestion control and other capabilities , Both sides of the communication work in a certain context .

UDP It is an unreliable communication protocol , No retransmission and confirmation , No orderly control , There's no congestion control , Effective transmission and orderly message are not guaranteed .

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TCP The sending and receiving of are always in The same definite context in

UDP Every time you receive or send a Independent context

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Socket （socket）

When communication , The endpoint in the respective communication connection

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​ Definition and advantages ：

TCP（ Stream socket ： Connection oriented “ Data flow ” agreement  ）： Is reliable , Two way connected communication stream

UDP（ Socket datagram ： No connection “ The datagram ” agreement ）： Fast

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The server receives the connection

1、 establish socket

2、bind  Associate socket address and port

3、listen  Convert socket to passive receive （ The socket created by default is Initiatively requesting ）

4、accept   After the connection with the client is established , The operating system kernel notifies the application .

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TCP

Connection process

1、 Server initialization

2、 Client initialization socket, And the server  TCP Three handshakes

3、 Client side and server side Establishing a connection （ Once the connection is established , Data transmission is no longer one-way , It's two-way , This is also TCP One of the salient features of ）

4、 client close , send out FIN Package to the server , After the server receives it Perform passive shutdown , The server side will also execute close function , The entire link is completely shut down

The client initiates the connection process

1、 establish socket

2、connect Server side  TCP Connection required

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TCP Three handshake process

1. The protocol stack of the client sends... To the server SYN package , And tell the server the serial number currently sent j, Client access SYNC_SENT state ;
2. After the server-side protocol stack receives the packet , Communicate with the client ACK The reply , The value of the response is j+1, Said to SYN package j The confirmation of , At the same time, the server also sends a SYN package , Tell the client that my current sending serial number is k, Server access SYNC_RCVD state ;
3. Client protocol stack received ACK after , Make the application from connect Call return , Indicates that the one-way connection from the client to the server is successfully established , The status of the client is ESTABLISHED, At the same time, the client protocol stack will also affect the server SYN Packet response , The response data is k+1;
4. After the response packet arrives at the server , The server-side protocol stack makes accept Blocking call return , At this time, the one-way connection from the server to the client is also established successfully , The server also enters ESTABLISHED state .

Send buffer

TCP After the connection is successfully established , The operating system kernel will create for each connection Send buffer . call write Function time , The actual thing to do is to put the data The send buffer copied from the application to the operating system kernel in , It's not necessarily to write data through sockets .

about send Come on , Returning success only means that the data is written to the send buffer successfully , It does not mean that the opposite end has successfully received .

about read Come on , Need to read data circularly , And you need to think about EOF Other abnormal conditions .

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UDP

apply

For delay 、 Packet loss is not a particularly sensitive scenario

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