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Socket programming. What is a socket? goal: learn how to build client/server applications that communicate using sockets socket: door between application.

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Presentation on theme: "Socket programming. What is a socket? goal: learn how to build client/server applications that communicate using sockets socket: door between application."— Presentation transcript:

1 Socket programming

2 What is a socket? goal: learn how to build client/server applications that communicate using sockets socket: door between application process and end-end-transport protocol 2-2 Internet controlled by OS controlled by app developer transport application physical link network process transport application physical link network process socket

3 What is a socket? Socket is an interface between application and network (the lower levels of the protocol stack) The application creates a socket The socket type dictates the style of communication reliable vs. best effort connection-oriented vs. connectionless Once a socket is setup the application can pass data to the socket for network transmission receive data from the socket (transmitted through the network, received from some other host) 3

4 Most popular types of sockets TCP socket Type: SOCK_STREAM reliable delivery in-order guaranteed connection-oriented bidirectional UDP socket Type: SOCK_DGRAM unreliable delivery no order guarantees no notion of connection – app indicates destination for each packet can send or receive

5 Ports 5 Each host machine has an IP address (or more!) Each host has 65,536 ports (2 ? ) As you know some ports are reserved for specific apps 20,21: FTP 23: Telnet 80: HTTP see RFC 1700 (about 2000 ports are reserved) Port 0 Port 1 Port A socket provides an interface to send data to/from the network through a port

6 Addresses, Ports and Sockets Like apartments and mailboxes You are the application Your apartment building address is the address Your mailbox is the port The post-office is the network The socket is the key that gives you access to the right mailbox (one difference: assume outgoing mail is placed by you in your mailbox) Q: How do you choose which port a socket connects to? 6

7 Socket programming 2-7 Application Example: 1. Client reads a line of characters (data) from its keyboard and sends the data to the server. 2. The server receives the data and converts characters to uppercase. 3. The server sends the modified data to the client. 4. The client receives the modified data and displays the line on its screen.

8 Ports 8 Each host machine has an IP address (or more!) Each host has 65,536 ports (2 ? ) As you know some ports are reserved for specific apps 20,21: FTP 23: Telnet 80: HTTP see RFC 1700 (about 2000 ports are reserved) Port 0 Port 1 Port A socket provides an interface to send data to/from the network through a port

9 Addresses, Ports and Sockets Like apartments and mailboxes You are the application Your apartment building address is the address Your mailbox is the port The post-office is the network The socket is the key that gives you access to the right mailbox (one difference: assume outgoing mail is placed by you in your mailbox) Q: How do you choose which port a socket connects to? 9

10 Socket programming with UDP UDP: no connection between client & server no handshaking before sending data sender explicitly attaches IP destination address and port # to each packet rcvr extracts sender IP address and port# from received packet UDP: transmitted data may be lost or received out-of-order Application viewpoint: UDP provides unreliable transfer of groups of bytes (datagrams) between client and server 2- 10

11 Client/server socket interaction: UDP close clientSocket read datagram from clientSocket create socket: clientSocket = socket(AF_INET,SOCK_DGRAM) Create datagram with server IP and port=x; send datagram via clientSocket create socket, port= x: serverSocket = socket(AF_INET,SOCK_DGRAM) read datagram from serverSocket write reply to serverSocket specifying client address, port number Application 2-11 server (running on serverIP ) client

12 2- 12 Example app: UDP client from socket import * serverName = hostname serverPort = clientSocket = socket(socket.AF_INET, socket.SOCK_DGRAM) message = raw_input(Input lowercase sentence:) clientSocket.sendto (message,(serverName, serverPort)) modifiedMessage, serverAddress = clientSocket.recvfrom(2048) print modifiedMessage clientSocket.close() Python UDPClient include Pythons socket library create UDP socket for server get user keyboard input Attach server name, port to message; send into socket print out received string and close socket read reply characters from socket into string

13 2- 13 Example app: UDP server from socket import * serverPort = serverSocket = socket(AF_INET, SOCK_DGRAM) serverSocket.bind(('', serverPort)) print The server is ready to receive while 1: message, clientAddress = serverSocket.recvfrom(2048) modifiedMessage = message.upper() serverSocket.sendto(modifiedMessage, clientAddress) Python UDPServer create UDP socket bind socket to local port number loop forever Read from UDP socket into message, getting clients address (client IP and port) send upper case string back to this client

14 Socket programming with TCP client must contact server server process must first be running server must have created socket (door) that welcomes clients contact client contacts server by: Creating TCP socket, specifying IP address, port number of server process when client creates socket: client TCP establishes connection to server TCP when contacted by client, server TCP creates new socket for server process to communicate with that particular client allows server to talk with multiple clients source port numbers used to distinguish clients TCP provides reliable, in-order byte-stream transfer (pipe) between client and server application viewpoint:

15 Client/server socket interaction: TCP wait for incoming connection request connectionSocket = serverSocket.accept() create socket, port= x, for incoming request: serverSocket = socket() create socket, connect to hostid, port= x clientSocket = socket() server (running on hostid ) client send request using clientSocket read request from connectionSocket write reply to connectionSocket TCP connection setup close connectionSocket read reply from clientSocket close clientSocket

16 2- 16 Example app: TCP client from socket import * serverName = servername serverPort = clientSocket = socket(AF_INET, SOCK_STREAM) clientSocket.connect((serverName,serverPort)) sentence = raw_input(Input lowercase sentence:) clientSocket.send(sentence) modifiedSentence = clientSocket.recv(1024) print From Server:, modifiedSentence clientSocket.close() Python TCPClient create TCP socket for server, remote port No need to attach server name, port

17 2- 17 Example app: TCP server from socket import * serverPort = 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() Python TCPServer create TCP welcoming socket server begins listening for incoming TCP requests loop forever server waits on accept() for incoming requests, new socket created on return read bytes from socket (but not address as in UDP) close connection to this client (but not welcoming socket)

18 Socket Creation in C int s = socket(domain, type, protocol); s : socket descriptor, an integer (like a file-handle) domain : integer, communication domain e.g., PF_INET (IPv4 protocol) – typically used type : communication type SOCK_STREAM : reliable, 2-way, connection-based service SOCK_DGRAM : unreliable, connectionless, other values: need root permission, rarely used, or obsolete protocol : specifies protocol (see file /etc/protocols for a list of options) - usually set to 0 NOTE: socket call does not specify where data will be coming from, nor where it will be going to - it just creates the interface. 18

19 The bind function The bind function associates and (can exclusively) reserves a port for use by the socket int status = bind(sockid, &addrport, size); status : error status, = -1 if bind failed sockid : integer, socket descriptor addrport : struct sockaddr, the (IP) address and port of the machine (address usually set to INADDR_ANY – chooses a local address) size : the size (in bytes) of the addrport structure bind can be skipped for both types of sockets. When and why? 19

20 20 Skipping the bind SOCK_DGRAM : if only sending, no need to bind. The OS finds a port each time the socket sends a pkt if receiving, need to bind SOCK_STREAM : destination determined during conn. setup don t need to know port sending from (during connection setup, receiving end is informed of port)

21 On the connecting end When connecting to another host (i.e., connecting end is the client and the receiving end is the server), the OS automatically assigns a free port for the outgoing connection. During connection setup, receiving end is informed of port) You can however bind to a specific port if need be. 21

22 Connection Setup A connection occurs between two ends Server: waits for an active participant to request connection Client: initiates connection request to passive side Once connection is established, server and client ends are similar both can send & receive data either can terminate the connection 22

23 Server and clients 23 socket() bind() listen() accept() write() read() TCP Server close() socket() TCP Client connect() write() read() close() connection establishment data request data reply end-of-file notification

24 Connection setup steps Server end: step 1: listen (for incoming requests) step 3: accept (a request) step 4: send/recv The accepted connection is on a new socket The old socket continues to listen for other active participants Client end: step 2: request & establish connect ion step 4: send/recv 24 Server l-socka-sock-1a-sock-2 Client1 socket Client2 socket From: Dan Rubensteins slides:

25 Server Socket: listen & accept Called on server side: int status = listen(sock, queuelen); status : 0 if listening, -1 if error sock : integer, socket descriptor queuelen : integer, # of active participants that can wait for a connection listen is non-blocking : returns immediately int s = accept(sock, &addr, &addrlen); s : integer, the new socket (used for data-transfer) sock : integer, the orig. socket (being listened on) addr : struct sockaddr, address of the active participant addrlen : sizeof(addr): value/result parameter must be set appropriately before call adjusted by OS upon return accept is blocking : waits for connection before returning 25

26 connect int status = connect(sock, &addr, addrlen); status : 0 if successful connect, -1 otherwise sock : integer, socket to be used in connection addr : struct sockaddr : address of passive participant addrlen : integer, sizeof(addr) connect is blocking 26

27 Sending / Receiving Data int count = send(sock, &buf, len, flags); count : # bytes transmitted (-1 if error) buf : void*, buffer to be transmitted len : integer, length of buffer (in bytes) to transmit flags : integer, special options, usually just 0 int count = recv(sock, &buf, len, flags); count : # bytes received (-1 if error) buf : void*, stores received bytes len : # bytes received flags : integer, special options, usually just 0 Calls are blocking [returns only after data is sent (to socket buf) / received] 27

28 close When finished using a socket, the socket should be closed: status = close(s); status : 0 if successful, -1 if error s : the file descriptor (socket being closed) Closing a socket closes a connection frees up the port used by the socket 28 From: Dan Rubensteins slides:

29 The struct sockaddr The struct to store the Internet address of a host: struct sockaddr_in { short sin_family; u_short sin_port; struct in_addr sin_addr; char sin_zero[8]; }; sin_family = AF_INET // Specifies the address family sin_port : // Specifies the port #( ) sin_addr : // Specifies the IP address sin_zero : unused // unused! 29 From: Dan Rubensteins slides:

30 SOCKADDR Example struct sockaddr_in server; // Definition memset(&server, 0, sizeof(server)); // Initilize to 0 server.sin_family = AF_INET; // Set address family server.sin_port = htons(MYPORTNUM); // Set port server.sin_addr.s_addr = htonl(INADDR_ANY);// Set address Host Byte-Ordering: the byte ordering used by a host (big- endian or little-endian) Network Byte-Ordering: the byte ordering used by the network – always big-endian Any words sent through the network should be converted to Network Byte-Order prior to transmission (and back to Host Byte- Order once received) 30

31 Network Byte-Ordering u_long htonl(u_long x); u_short htons(u_short x); u_long ntohl(u_long x); u_short ntohs(u_short x); 31 On big-endian machines, these routines do nothing On little-endian machines, they reverse the byte order Big-Endian machine Little-Endian machine htonl ntohl From: Dan Rubensteins slides:

32 TIPS 1 Sometimes, an ungraceful exit from a program (e.g., ctrl-c ) does not properly free up a port Eventually (after a few minutes), the port will be freed You can kill the process, or To reduce the likelihood of this problem, include the following code: In header include: #include void cleanExit(){exit(0);} In socket code: signal(SIGTERM, cleanExit); signal(SIGINT, cleanExit); 32

33 Tips 2 Check Beej's Guide to Network Programming Using Internet Sockets Search the specification for the function you need to use for more info, or check the man pages. 33

34 Tips 3 How to find the IP address of the machine my server program is running on? Use or localhost for accessing a server running on your local machine. For a remote server running linux use the bash shell command: $ /sbin/ifconfig For windows, use ipconfig in cmd 34

35 summary application architectures client-server P2P application service requirements: reliability, bandwidth, delay Internet transport service model connection-oriented, reliable: TCP unreliable, datagrams: UDP our study of network apps now complete! specific protocols: HTTP FTP SMTP, POP, IMAP DNS P2P: BitTorrent, DHT socket programming: TCP, UDP sockets

36 typical request/reply message exchange: client requests info or service server responds with data, status code message formats: headers: fields giving info about data data: info being communicated important themes: control vs. data msgs in-band, out-of-band centralized vs. decentralized stateless vs. stateful reliable vs. unreliable msg transfer complexity at network edge summary most importantly: learned about protocols!


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