Write a simple TCP program for a server that accepts lines of input from a client and prints the lines onto the servers standard output. (You can do this by modifying the TCPServer.py program in the text.) Compile and execute your program. On any other machine that contains a Web browser, set the proxy server in the browser to the host that is running your server program; also configure the port number appropriately. Your browser should now send its GET request messages to your server, and your server should display the messages on its standard output. Use this platform to determine whether your browser generates conditional GET messages for objects that are locally cached.

Now lets take a look at the server program. from socket import * serverPort = 12000 serverSocket = socket(AF_INET,SOCK_STREAM) serverSocket.bind((,serverPort)) serverSocket.listen(1) print The server is ready to receive while 1: connectionSocket, addr = serverSocket.accept() sentence = connectionSocket.recv(1024) capitalizedSentence = sentence.upper() connectionSocket.send(capitalizedSentence) connectionSocket.close()

Lets now take a look at the lines that differ significantly from UDPServer and TCPClient. As with TCPClient, the server creates a TCP socket with: serverSocket=socket(AF_INET,SOCK_STREAM) Similar to UDPServer, we associate the server port number, serverPort, with this socket: serverSocket.bind((,serverPort))

But with TCP, serverSocket will be our welcoming socket. After establishing this welcoming door, we will wait and listen for some client to knock on the door: serverSocket.listen(1)

This line has the server listen for TCP connection requests from the client. The parameter specifies the maximum number of queued connections (at least 1). connectionSocket, addr = serverSocket.accept()

When a client knocks on this door, the program invokes the accept() method for serverSocket, which creates a new socket in the server, called connectionSocket, dedicated to this particular client. The client and server then complete the handshaking, creating a TCP connection between the clients clientSocket and the servers connectionSocket. With the TCP connection established, the client and server can now send bytes to each other over the connection. With TCP, all bytes sent from one side not are not only guaranteed to arrive at the other side but also guaranteed arrive in order. connectionSocket.close()

In this program, after sending the modified sentence to the client, we close the connection socket. But since serverSocket remains open, another client can now knock on the door and send the server a sentence to modify.

This completes our discussion of socket programming in TCP. You are encouraged to run the two programs in two separate hosts, and also to modify them to achieve slightly different goals.