[socket] ① overview of socket technology

One 、 summary

When we design software systems , There is often a need for network communication with other software . Now? , Most network communications are based on TCP/IP Agreement to proceed , And at the bottom of the TCP/IP agreement 、 The process is cumbersome ,socket The programming interface provides a simple TCP/IP Mode of operation .

• demand ： Network communication is needed for information exchange between software processes .
• TCP/IP agreement ： It is a data transmission scheme .
• Socket Interface ： It is the abstraction of network communication endpoint , One can shield TCP/IP Cumbersome low-level operation tools .
  Information communication flow of application , As shown in the figure ：
  

Two 、TCP And UDP Workflow 、 difference

Reference resources ：https://blog.csdn.net/weixin_47021806/article/details/124883559

（ One ）TCP/UDP Workflow

1、UDP Communication workflow

Server side	establish socket	Binding address / port	send out / receive data	close
client	establish socket	----	send out / receive data	close

2、TCP Communication workflow

Server side	establish socket	Binding address / port	Convert to passive connection （ monitor ）	Receive client connections	receive / send data	close
client	establish socket	–	–	Connect to server	send out / receive data	close

• socket() Method ： Is used to create a socket , With sockets, you can send and receive data through the network . This is why a program that communicates over a network must first create a socket . Specify the type of service to use when creating a socket , Use TCP Protocol selection streaming service （SOCK_STREAM）.
• bind() Method ： It is used to specify socket usage IP Address and port .IP The address is the address of your host , If the host is not connected to the network , The loopback address can be used when testing the program “127.0.0.1”. The port is a 16 Bit shaping value , commonly 0-1024 Is a well-known port , Such as HTTP The use of 80 Port no. . This kind of port is generally not used by users . secondly ,1024-4096 Reserved port for , Users generally do not use .4096 These are temporary ports , Users can use . stay Linux On ,1024 Port number within , Only root Users can use .
• listen() Method ： It is used to create a listening queue . There are two types of listening queues , One is to store connections that have not completed three handshakes , One is to store the connection that has completed three handshakes .listen() The second parameter is to specify the length of the queue that has completed three handshakes .
• accept()： Method processing is stored in listen Create a connection in the queue that has completed three handshakes . Each connection processed , be accept()： Returns the socket descriptor corresponding to the connection . If the queue is empty , be accept Blocking .
• connect()： Methods are generally executed by client programs , You need to specify the server side of the connection IP Address and port . After the method is executed , There will be three handshakes , Establishing a connection .
• send() Method ： Used to direct to TCP The opposite end of the connection sends data .send() Successful implementation , It can only indicate that the data is successfully written to the sending buffer of the sending end , It does not mean that the data has been sent to the opposite end .send() The return value of is the length of data actually written into the transmit buffer .
• recv() Method ： For reception TCP Data sent by the opposite end of the connection .recv() Read data from the local receive buffer , If there is no data in the receive buffer , be recv() Method will block . The return value is the number of bytes actually read , If
• recv() The return value is 0, It means that the other party has closed TCP Connect .
• close()： Method to close TCP Connect . here , Will wave four times .

3、TCP And UDP Communication differences

（1） Difficult and easy to operate
UDP It's a connectionless protocol , So there's no need to look like TCP equally listen and accept, It's easy to operate
UDP The server ： establish socket、 Binding port 、 Transceiver operation 、 close socket
TCP The server ： establish socket、 Binding port 、 Listening port 、 Receiving connection 、 Transceiver operation 、 close socket
（2） Retransmission mechanism and delay
UDP If the transmitted message is lost on the network , It's lost .UDP The protocol itself has no retransmission mechanism .
and TCP There is a message at the bottom of the protocol to verify whether it arrives , If you lose , The mechanism that the sender will retransmit .
UDP： It can ensure real-time
TCP： Can guarantee integrity
（3） Message order problem
TCP The information data of both sides of the protocol communication is like flowing in the pipeline , There is a clear order .
Sender Application The message sent first is Ken First received by the receiving application . and UDP The protocol sends independent messages one by one , The receiving order of the receiving application is not necessarily the same as the sending order .
（4）TCP Advantages and disadvantages ：
advantage ：
1．TCP Provides for explicitly creating and terminating connections in an approved way .
2．TCP Be sure to be reliable 、 Sequential （ Packets are received in the order they are sent ） And data transmission that won't repeat .
3．TCP Process flow control .
4． Allow data first
5． If the data is not transmitted to , be TCP Socket interface returns an error status condition .
6．TCP Processing large data blocks by keeping them continuous and dividing them into smaller pieces .— No need for programmers to know
shortcoming ： TCP You must create... When transferring data （ And keep ） A connection . This connection adds overhead to the communication process , Let it compare UDP Slow down .
（5）UDP Advantages and disadvantages ：
1．UDP There is no requirement to keep a connection
2．UDP No packet received due to receiver's acceptance （ Or automatic retransmission when the packet does not arrive correctly ） And the cost .
3． Design UDP The purpose of the is for short applications and control messages
4． On the basis of a packet connecting a packet ,UDP Required network bandwidth ratio TDP smaller .

3、 ... and 、C/S The choice of server and client solutions in the architecture

1、 The server needs to be bound IP And port
2、Tcp You need to start the server first