4.13, TCP communication process

1. The difference between TCP and UDP (transport layer protocol)

UDP: User Datagram Protocol, connectionless-oriented, unicast, multicast, broadcast, datagram-oriented, unreliable:
TCPTransmission Control Protocol, connection-oriented, reliable, byte stream-based, only supports unicast transmission

	UDP	TCP
Whether to create a connection	no connection	connection-oriented
Is it reliable	Unreliable	reliable
number of connected objects	One-to-one, one-to-many, many-to-one, many-to-many	Support one-on-one
way of transmission	Datagram Oriented	stream-oriented
head overhead	8 bytes	20 bytes minimum
Applicable scene	Real-time applications (video conferencing, live streaming)	Applications with high reliability (file transfer)

2. TCP communication process

① Server side (the role of passively accepting connections)

1. Create a socket for listening
   • Listening : Listening for connections from clients
   • Socket : This socket is actually a file descriptor
2. Bind this listening file descriptor to the local IP and port (the IP and port are the address information of the server)
   • The client uses this IP and port when connecting to the server.
3. Set up the monitor and fdstart
4. Blocking waiting , when a client initiates a connection, unblocks, accepts the client's connection, and will get a communication with the client套接字（fd）
5. communication
   • Receive data
   • send data
6. Communication ends, disconnect

②Client (initiate connection actively)

1. create a communication套接字（fd）
2. To connect to the server, you need to specify the server to connect to IP 和 端口
3. The connection is successful, and the client can communicate directly with the server
   • Receive data
   • send data
4. Communication ends, disconnect