[Python Network Programming] Comparison of TCP and UDP

TCP transport method

1. TCP transmission characteristics

  • Connection Oriented Transport Service

    • Transmission characteristics: Provides reliable data transmission, reliability refers to no loss, no disorder, no error, no duplication during data transmission.
    • Reliability guarantee mechanism (all are automatically completed by the operating system network service for the application):
      • A data connection needs to be established before communication
      • Acknowledgment response mechanism
      • When the communication ends, the connection should be disconnected normally

  • Three-way handshake (connection establishment)

    • The client sends a message to the server requesting a connection
    • After the server receives the request, the reply message confirms that it can be connected
    • The client receives the reply and sends the final message to establish the connection

  • Four waves (disconnect)
    • The active party sends a message requesting to disconnect
    • After receiving the request, the passive party immediately responds, indicating that it is ready to disconnect
    • The passive party is ready, and sends a message again to indicate that it can be disconnected
    • The active party receives the confirmation and sends the final message to complete the disconnection

2. TCP server

  • create socket
 sockfd=socket.socket(family,type)
    Function: create socket
    Parameters: family network address type AF_INET means ipv4
         type socket type SOCK_STREAM means tcp socket (also called streaming socket)
    Return value: socket object

  • bind address (same as udp socket)

  • set monitor

 sockfd. listen(n)
    Function: Set the socket as a listening socket and determine the size of the listening queue
    Parameters: listener queue size

  • Handle client connection requests
 connfd,addr = sockfd. accept()
    Function: Block waiting to process client requests
    Return value: connfd client connection socket
            addr client address of the connection
  • messaging
 data = connfd. recv(buffersize)
    Function: accept client message
    Parameters: the size of the maximum message received each time
    Return value: received content

    n = connfd. send(data)
    Function: send message
    Parameters: the content to be sent in bytes format
    Return value: the number of bytes sent
  1. close socket (same as udp socket)

3. TCP client

  • Create a TCP socket
  • request connection
 sockfd.connect(server_addr)
    Function: connect to server
    Parameters: tuple server address
  • send and receive messages

Note: To prevent blocking at both ends, recv send should cooperate

  • close socket

4. TCP socket details

  • When one end of the tcp connection exits, if the other end is blocked in recv, recv will immediately return an empty string.

  • If one end of the tcp connection does not exist, if you still try to send data to it through send, a BrokenPipeError will occur

  • A server can connect to multiple clients at the same time, and can also be connected repeatedly

  • tcp sticky packet problem

    • cause

      • In order to solve the problem of speed inconsistency that may occur during data retransmission, the operating system sets a buffer
      • The actual network working process is more complicated, resulting in inconsistent message sending and receiving speeds
      • TCP transmits data in a byte stream, and does not distinguish message boundaries when receiving

    • the impact

      • If the content sent each time has an independent meaning, the receiving end needs to analyze it independently. At this time, the sticky packet will have an impact.

    • Approach

      • Message formatting, such as artificially adding message boundaries, used as a division between messages
      • Control the speed of sending

5. Comparison between TCP and UDP

  • transmission characteristics

    • TCP provides reliable data transmission, but UDP does not guarantee the reliability of transmission
    • TCP transmission data is processed as a byte stream, while UDP is processed as a data packet
    • TCP transmission needs to establish a connection to transmit data, and the efficiency is relatively low. UDP is relatively free, no connection is required, and the efficiency is high.

  • Socket programming difference

    • Different types of sockets created
    • The tcp socket will have sticky packets, and the udp socket will not have sticky packets if there are message boundaries
    • The tcp socket relies on listen accept to establish a connection to send and receive messages, but the udp socket does not need it
    • tcp socket uses send, recv to send and receive messages, udp socket uses sendto, recvfrom

  • scenes to be used

    • TCP is more suitable for scenarios that require high accuracy and transmit large data
      • File transfer: such as downloading movies, visiting web pages, uploading photos
      • mail sending and receiving
      • Point-to-point data transmission: such as point-to-point chat, login request, remote access, sending red envelopes
    • UDP is more suitable for scenarios where reliability requirements are not so high and transmission methods are relatively free
      • Transmission of video stream: such as live broadcast, video chat
      • Broadcasting: such as webcasting, mass messaging
      • Real-time transmission: such as game screen
    • In a large project, both TCP and UDP networks may be involved

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