Socket programming

1. Socket programming 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 application application socket controlled by app developer process process transport transport controlled by OS network network link Internet link physical physical

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 typedictates 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)

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. Each host machine has an IP address (or more!) Each host has 65,536 ports (2?) As you know some ports are reservedfor specific apps 20,21: FTP 23: Telnet 80: HTTP see RFC 1700 (about 2000 ports are reserved) Ports Port 0 Port 1 Port 65535 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?

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

8. Each host machine has an IP address (or more!) Each host has 65,536 ports (2?) As you know some ports are reservedfor specific apps 20,21: FTP 23: Telnet 80: HTTP see RFC 1700 (about 2000 ports are reserved) Ports Port 0 Port 1 Port 65535 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?

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

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

12. Example app: UDP client Python UDPClient include Python’s socket library from socket import * serverName = ‘hostname’ serverPort = 12000 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() create UDP socket for server get user keyboard input Attach server name, port to message; send into socket read reply characters from socket into string print out received string and close socket

13. Example app: UDP server Python UDPServer from socket import * serverPort = 12000 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) create UDP socket bind socket to local port number 12000 loop forever Read from UDP socket into message, getting client’s address (client IP and port) send upper case string back to this client

14. application viewpoint: Socket programming with TCP client must contact server • server process must first be running • server must have created socket (door) that welcomes client’s 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

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

16. Example app: TCP client Python TCPClient from socket import * serverName = ’servername’ serverPort = 12000 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() create TCP socket for server, remote port 12000 No need to attach server name, port

17. Example app: TCP server Python TCPServer 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() 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.

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?

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.

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

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

24. l-sock a-sock-1 socket socket a-sock-2 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 connection • step 4: send/recv Server Client1 Client2 From: Dan Rubenstein’s slides: http://www.cs.columbia.edu/~danr/courses/6761/Summer03/intro/6761-1b-sockets.ppt

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: structsockaddr, 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

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

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]

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 From: Dan Rubenstein’s slides: http://www.cs.columbia.edu/~danr/courses/6761/Summer03/intro/6761-1b-sockets.ppt

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 #(0-65535) • sin_addr: // Specifies the IP address • sin_zero: unused // unused! From: Dan Rubenstein’s slides: http://www.cs.columbia.edu/~danr/courses/6761/Summer03/intro/6761-1b-sockets.ppt

30. SOCKADDR Example structsockaddr_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)

31. 12 40 119 128 htonl ntohl 128 128 128 119 119 119 40 40 40 12 12 12 Network Byte-Ordering • u_long ntohl(u_long x); • u_short ntohs(u_short x); • u_long htonl(u_long x); • u_short htons(u_short x); • On big-endian machines, these routines do nothing • On little-endian machines, they reverse the byte order Big-Endian machine Little-Endian machine 128.119.40.12 128.119.40.12 From: Dan Rubenstein’s slides: http://www.cs.columbia.edu/~danr/courses/6761/Summer03/intro/6761-1b-sockets.ppt

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 <signal.h> void cleanExit(){exit(0);} • In socket code: signal(SIGTERM, cleanExit); signal(SIGINT, cleanExit);

33. Tips 2 • Check Beej's Guide to Network Programming Using Internet Socketshttp://beej.us/guide/bgnet/output/html/multipage/index.html • Search the specification for the function you need to use for more info, or check the man pages.

34. Tips 3 • How to find the IP address of the machine my server program is running on? • Use 127.0.0.1 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

35. summary • specific protocols: • HTTP • FTP • SMTP, POP, IMAP • DNS • P2P: BitTorrent, DHT • socket programming: TCP, UDP sockets • 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!

36. summary most importantly: learned about protocols! • 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” • 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