Socket Programming. Sungkyunkwan University. Hyunseung Choo Copyright Networking Laboratory

Transcription

1 Socket Programming Sungkyunkwan University Hyunseung Choo Copyright Networking Laboratory

2 Contents Goals Client-Server mechanism Introduction to socket Programming with socket on UNIX Stream socket (TCP) Datagram socket (UDP) Programming with socket on WINDOWS Stream socket Datagram socket Practice examples Networking Laboratory 2/61

3 Goals Understanding Client-Server mechanism Understanding stream socket programming on Linux (Windows) Understanding datagram socket programming on Linux (Windows) Can do some simple client server applications: Echo: checking if a server is available by sending a message to a server and check if the server sends back the same message Mathematic Server: creating a simple mathematic server to solve simple expressions received from clients Chatting service: creating a simple chatting channel for 2 users Networking Laboratory 3/61

4 Client-Server mechanism Let us assume that a person is trying to start two programs on separate networked machines (M1 and M2) and these programs must communicate with each other When the person starts the first program on machine M1, this program sends a message to the program on machine M2 Since the computer works much faster than human beings, the program on M1 waits a few milliseconds for response from M2 and then gives up displaying an error message Networking Laboratory 4/61

5 Client-Server mechanism Meanwhile, our friend reaches M2 and types the second program name to start it on M2 The second program waits for a message from M1 for a few milliseconds and then concludes that the first program is not active yet. It displays an error message and exits. Our friend would keep starting the programs but they will never be able to communicate. Networking Laboratory 5/61

6 Client-Server mechanism The solution is to let one program open up communications and wait indefinitely for a message from the other program The first program to start running is called the server. The server must listen for any message from the client The other program is called the client and the client is supposed to initiate the communication by sending a message to the server Networking Laboratory 6/61

7 Client-Server mechanism Typical network app has two pieces: client and server Client: initiates contact with server ( speaks first ) typically requests service from server, for Web, client is implemented in browser; for , in mail reader Server: provides requested service to client e.g., Web server sends requested Web page, mail server delivers e- mail application transport network data link physical request reply application transport network data link physical Networking Laboratory 7/61

8 Introduction to socket What is a socket? A socket is an interface for an application to connect to a host s network stack (part of the OS). After connecting, an application is able to bidirectionally exchange data with other processes on the same or another host. Networking Laboratory 8/61

9 Introduction to socket The Computer Systems Research Group (CSRG) at the University of California Berkeley gave birth to the Berkeley Socket API (along with its use of the TCP/IP protocol) release in A Socket is comprised of: a 32-bit node address (IP address) a 16-bit port number (like 7, 21, 13242) Example: :1051 The host address is in IPv4 dotted-quad format Networking Laboratory 9/61

10 Two essential types of sockets SOCK_STREAM a.k.a. TCP reliable delivery in-order guaranteed connection-oriented bidirectional App SOCK_DGRAM a.k.a. UDP unreliable delivery no order guarantees no notion of connection app indicates dest. for each packet can send or receive App D socket Dest socket D2 Q: why have type SOCK_DGRAM? D3 Networking Laboratory 10/61

11 TCP vs UDP TCP is used for services with a large data capacity, and a persistent connection UDP is more commonly used for quick lookups, and single use query-reply actions. Some common examples of TCP and UDP with their default ports: Networking Laboratory 11/61

12 Socket programming Client-Server communication Server passively waits for and responds to clients passive socket Client initiates the communication must know the address and the port of the server active socket Networking Laboratory 12/61

13 Socket programming Sockets - Procedures Networking Laboratory 13/61

14 Socket programming (Unix) Client Server communication (Unix) Networking Laboratory 14/61

15 Socket programming (Unix) Socket creation in C: socket() sockid: socket descriptor, an integer (like a file-handle) family: integer, communication domain, e.g., PF_INET, IPv4 protocols, Internet addresses (typically used) PF_UNIX, Local communication, File addresses type: communication type SOCK_STREAM - reliable, 2-way, connection-based service SOCK_DGRAM - unreliable, connectionless, messages of maximum length protocol: specifies protocol IPPROTO_TCP IPPROTO_UDP usually set to 0 (i.e., use default protocol) upon failure returns -1 NOTE: socket call does not specify where data will be coming from, nor where it will be going to it just creates the interface! Networking Laboratory 15/61

16 Socket programming (Unix) Socket close in C: close() Networking Laboratory 16/61

17 Socket programming (Unix) Socket close in C: close() When finished using a socket, the socket should be closed sockid: the file descriptor (socket being closed) status: 0 if successful, -1 if error Closing a socket closes a connection (for stream socket) frees up the port used by the socket Networking Laboratory 17/61

18 Socket programming (Unix) Specifying Addresses Socket API defines a generic data type for addresses: Particular form of the sockaddr used for TCP/IP addresses: Note: sockaddr_in can be casted to a sockaddr Networking Laboratory 18/61

19 Socket programming (Unix) Assign address to socket: bind() Networking Laboratory 19/61

20 Socket programming (Unix) Assign address to socket: bind() associates and reserves a port for use by the socket sockid: integer, socket descriptor addrport: struct sockaddr, the (IP) address and port of the machine for TCP/IP server, internet address is usually set to INADDR_ANY, i.e., chooses any incoming interface size: the size (in bytes) of the addrport structure status: upon failure -1 is returned Networking Laboratory 20/61

21 Socket programming (Unix) bind()- Example with TCP Networking Laboratory 21/61

22 Socket programming (Unix) Skipping the bind() bind can be skipped for both types of sockets Datagram socket: if only sending, no need to bind. The OS finds a port each time the socket sends a packet if receiving, need to bind Stream socket: destination determined during connection setup don t need to know port sending from (during connection setup, receiving end is informed of port) Networking Laboratory 22/61

23 Socket programming (Unix) Assign address to socket: listen() Networking Laboratory 23/61

24 Socket programming (Unix) Assign address to socket: listen() Instructs TCP protocol implementation to listen for connections sockid: integer, socket descriptor queuelimit: integer, # of active participants that can wait for a connection status: 0 if listening, -1 if error listen() is non-blocking: returns immediately The listening socket (sockid) is never used for sending and receiving is used by the server only as a way to get new sockets Networking Laboratory 24/61

25 Socket programming (Unix) Establish Connection: connect() Networking Laboratory 25/61

26 Socket programming (Unix) Establish Connection: connect() The client establishes a connection with the server by calling connect() sockid: integer, socket to be used in connection foreignaddr: struct sockaddr: address of the passive participant addrlen: integer, sizeof(name) status: 0 if successful connect, -1 otherwise connect() is blocking Networking Laboratory 26/61

27 Socket programming (Unix) Incoming Connection: accept() The server gets a socket for an incoming client connection by calling accept() s: integer, the new socket (used for data-transfer) sockid: integer, the orig. socket (being listened on) clientaddr: struct sockaddr, address of the active participant filled in upon return addrlen: sizeof(clientaddr): value/result parameter must be set appropriately before call adjusted upon return accept() is blocking: waits for connection before returning dequeues the next connection on the queue for socket (sockid) Networking Laboratory 27/61

28 Socket programming (Unix) Exchanging data with stream socket Networking Laboratory 28/61

29 Socket programming (Unix) Exchanging data with stream socket msg: const void[], message to be transmitted msglen: integer, length of message (in bytes) to transmit flags: integer, special options, usually just 0 count: # bytes transmitted (-1 if error) recvbuf: void[], stores received bytes buflen: # bytes received flags: integer, special options, usually just 0 count: # bytes received (-1 if error) Calls are blocking returns only after data is sent / received Networking Laboratory 29/61

30 Socket programming (Unix) Exchanging data with datagram socket msg, msglen, flags, count: same with send() foreignaddr: struct sockaddr, address of the destination addrlen: sizeof(foreignaddr) recvbuf, buflen, flags, count: same with recv() clientaddr: struct sockaddr, address of the client addrlen: sizeof(clientaddr) Calls are blocking returns only after data is sent / received Networking Laboratory 30/61

31 Socket programming (Windows) Using WinSock 2 library for socket programming WinSock 2 - to provide a protocol-independent transport interface that is fully capable of supporting emerging networking capabilities including real-time multimedia communications Importing Winsock2 library Networking Laboratory 31/61

32 Socket programming (Windows) Initialize Winsock DLL //Create a WSADATA object called wsadata. WSADATA wsadata; // Initialize the use of the Windows Sockets DLL before making other // Winsock functions calls. // This also makes certain that Winsock is supported on the system. int err; err = WSAStartup(WSVERS, &wsadata); if (err!= 0) { WSACleanup(); // Tell the user that we could not find a usable WinsockDLL printf("wsastartup failed with error: %d\n", err); } Networking Laboratory 32/61

33 Socket programming (Windows) Socket creation in C: socket() Similar to creating a socket on Unix but the function return the SOCKET data type instead of an socket id Close a socket Specifying Addresses Networking Laboratory 33/61

34 Socket programming (Windows) bind() and listen() Networking Laboratory 34/61

35 Socket programming (Windows) connect() and accept() Connect Accept Networking Laboratory 35/61

36 Socket programming (Windows) Exchanging data with stream socket Send data Receive data Networking Laboratory 36/61

37 Socket programming (Windows) Exchanging data with datagram socket Send Receive Networking Laboratory 37/61

38 Practice examples Echo A client communicates with an echo server The server simply echoes whatever it receives back to the client Library for socket programming: Networking Laboratory 38/61

39 Practice examples Echo using stream socket Networking Laboratory 39/61

40 Practice examples Echo using stream socket Networking Laboratory 40/61

41 Practice examples Echo using stream socket Networking Laboratory 41/61

42 Practice examples Echo using stream socket Networking Laboratory 42/61

43 Practice examples Echo using stream socket Networking Laboratory 43/61

44 Practice examples Echo using stream socket Networking Laboratory 44/61

45 Practice examples Echo using stream socket Networking Laboratory 45/61

46 Practice examples Echo using stream socket Networking Laboratory 46/61

47 Practice examples Echo using stream socket Networking Laboratory 47/61

48 Practice examples Echo using stream socket Networking Laboratory 48/61

49 Practice examples Echo using stream socket Networking Laboratory 49/61

50 Practice examples Echo using stream socket Networking Laboratory 50/61

51 Practice examples Echo using stream socket Networking Laboratory 51/61

52 Practice examples Echo using datagram socket Networking Laboratory 52/61

53 Practice examples Echo using datagram socket Networking Laboratory 53/61

54 Practice examples Echo using datagram socket Networking Laboratory 54/61

55 Practice examples Echo using datagram socket Networking Laboratory 55/61

56 Practice examples Echo using datagram socket Networking Laboratory 56/61

57 Practice examples Echo using datagram socket Networking Laboratory 57/61

58 Practice examples Mathematic Server Networking Laboratory 58/61

59 Practice examples Mathematic Server Sample Source Codes: Expression calculation: Input: a string has 3 characters where first and third characters are digits, second character is + or - operator. Networking Laboratory 59/61

60 Practice examples Chatting service Chatting service: creating a simple chatting channel for 2 users using datagram socket Networking Laboratory 60/61

61 THANK YOU Networking Laboratory 61/61