Introduction to Network Programming Asst. Prof. Chaiporn Jaikaeo, Ph.D. Computer Engineering Department Kasetsart University, Bangkok, Thailand

2 Outline End-to-end communication Direct serial communication Internet communication Simple client-server applications

End-to-End Communication Direct communication Communicating devices are directly connected Internet communication Communication is performed over the Internet 3 TX RX GND Internet

4 Client-Server Paradigm Client Initiates communication Issues command Server Awaits and respond to commands Most common model for Internet applications Client Server

Our Simple Protocol Request Issued by Client Response by Server id\r\n Your student ID, followed by \r\n name\r\n Your name, followed by \r\n 5 Server responds with "ERROR" when receiving an unknown command. Client Server command response

Hands-on Activity 1: Serial Comm. Most basic form of device-to-device communication However, most recent computers do not come with a serial port Use USB-Serial dongle instead 6 TX RX GND USB- Serial Dongle

Python's Serial API Opening a serial port Sending data with newline characters Receiving data (blocking call) Wait until new line characters are received and return the whole line Wait until 100 bytes are received 7 from serial import Serial ser = Serial("/dev/ttyUSB0") ser.write('Hello\r\n') ser.readline() ser.read(100)

8 Internet Comm. - App's Viewpoint Two network applications should interact as if they were directly connected Internet A App B App writeread

99 Internet Comm. – Socket API API for developing applications that perform inter- process communication most commonly for communications across a computer network Example functions listen – used by server to wait for contact from client connect – used by client to contact server send – used by either client or server to send data recv – used by either client or server to receive data close – close the connection

10 Services Provided by Socket API Connection-oriented, stream-like service Provides virtual stream-oriented pipe Data transfer is reliable No loss, in-order arrival Both machines use Transmission Control Protocol (TCP) to transfer data Internet A App B App

11 Services Provided by Socket API Connectionless, datagram service User must prepare packet of data before sending Data transfer is NOT reliable Loss possible, out-of-order arrival possible Both machines use User-Datagram Protocol (UDP) to transfer data Internet A App B App

12 Port Addressing IP addresses are used to identify hosts (i.e., machines) on the Internet Port numbers are used to distinguish different processes running on the same host Internet AB Proc1 Proc2 Proc3 Proc4 Proc5 IP Address 1 IP Address 2 Ports

Hands-on Activity 2: Internet Comm. Implement Activity 1 over the Internet using stream-oriented service

14 close() recv() send() connect() send()receive() TCP Socket: Flow in Python Client bind() listen() Server accept()

15 Server: Creating Socket Create a socket object and bind it to port on any available network interface from socket import * listen_sock = socket() listen_sock.bind((' ',12345)) listen_sock.listen(1) # max. of 1 client can wait sock,info = listen_sock.accept() Listen on any interface Port 12345

16 Client: Create Socket On another machine, create a client socket and connect to the server's IP and port from socket import * sock = socket() server = (" ", 12345) sock.connect(server)

17 Finding Out Your IP Address Windows – Run ipconfig from a command prompt MacOS/Linux/Unix – Run ifconfig from a terminal

18 Server: Check Client's IP and Port The method getpeername() tells us about client's IP address and port number The method returns a tuple (ip-addr, port) addr,port = sock.getpeername() print "Client is connected from",addr

19 Transferring Data Use send() and recv() methods to send and receive data, respectively E.g., At the server, enter the command The server will block until it receives data At the client, enter the command sock.send('hello') msg = sock.recv(1024) specify the max number of bytes to be received

20 Closing Connection Server: Client: sock.close() listen_sock.close() sock.close()

21 Assignment: Throw me your name I will be running a server on my machine For each of you Create a socket to connect to my machine Then send a line containing,,, Server will close connection immediately