UDP & TCP Servers: Sockets & Client-Server Interaction

• The following notes cover examples of UDP and TCP server applications, socket programming, and client/server socket interactions.

UDP Server Example

• This is a Python UDP server example
• It imports the socket library
• The server listens on port 12000
• It creates a UDP socket and binds it to the specified port
• The server then enters an infinite loop to receive and process incoming messages
• When a message is received, it's decoded, converted to uppercase, and sent back to the client
• The serverSocket.recvfrom(2048) function reads UDP packets, getting the message and client address
• Use serverSocket.sendto() to send the modified message back to the client

Socket Programming with TCP

• When programming with TCP a client must contact the server
• The server process must be running before a client can connect
• The server must have also created a socket that welcomes client connections
• Clients can contact the server by creating a TCP socket, specifying the IP address and port number of the server process
• When a client creates a socket it establishes a TCP connection to the server
• When a client contacts a server, the server TCP creates a new socket for the server process
• The new socket enables communication with that particular client, allowing the server to handle multiple clients
• Source port numbers are used to distinguish between clients
• From an application viewpoint, TCP provides reliable, in-order byte-stream transfer (a "pipe") between client and server processes

Client/Server Socket Interaction with TCP

• On the server-side, a socket is created with a specified port for incoming requests
• serverSocket = socket() creates this socket
• The server waits for incoming connection requests using connectionSocket = serverSocket.accept()
• A TCP connection setup occurs at this point
• The server reads the request from the connectionSocket, then writes a reply to the same socket
• The connection is closed via the connectionSocket
• On the client-side, a socket is created and connected to the server's host ID and port with clientSocket = socket()
• The client sends a request using clientSocket and reads the reply from clientSocket
• Finally, the client closes the socket (clientSocket)

TCP Client Example

• This is a Python TCP client example
• It imports the socket library
• The server's name is defined as 'servername' and the port as 12000
• The client creates a TCP socket using clientSocket = socket(AF_INET, SOCK_STREAM)
• It connects to the server using clientSocket.connect((serverName, serverPort))
• The client prompts the user to input a lowercase sentence and sends it to the server
• The client then receives a modified sentence from the server and prints it
• The client closes the socket with clientSocket.close()
• A server name and port are not needed to attach to the server process

TCP Server Example

• This is a Python TCP server example
• It imports the socket library
• The server listens on port 12000
• It creates a TCP welcoming socket and binds it to the specified port
• Use serverSocket = socket(AF_INET,SOCK_STREAM)
• serverSocket.bind(('',serverPort))
• The server begins listening for incoming TCP requests via serverSocket.listen(1)
• The server waits on accept() for incoming requests, which returns a new socket for the connection
• The server reads bytes from the socket of the new connection, decodes them, capitalizes the sentence, and sends it back
• The connection to this client is closed, but not the welcoming socket using connectionSocket.close()