Programming with Sockets

1. Programming with Sockets

2. Client-Server Communication • Sockets • Remote Procedure Calls • Remote Method Invocation (Java)

3. Client Server Computing • Although the Internet provides a basic communication service, the protocol software cannot initiate contact with, or accept contact from, a remote computer. Instead, two application programs must participate in any communication with one application initiates communication and the one accepts it.

4. In network applications, a SERVER application waits passively for contact after informing local protocol software that a specific type of message is expected, while a CLIENT application initiates communication actively by sending a matched type of message.

5. Identifying A Particular Service • Transport protocols assign each service a unique identifier. • Both client and server specify the service identifier; protocol software uses the identifier to direct each incoming request to the correct server. • In TCP/IP, TCP uses 16-bit integer values known as protocol port numbers to identify services.

6. Concurrent Server • Concurrent execution is fundamental to servers because concurrency permits multiple clients to obtain a given service without having to wait for the server to finish previous requests. • In concurrent server designs, the server creates a new thread or process to handle each client. • Transport protocols assign an identifier to each client as well as to each service. • Protocol software on the server’s machine uses the combination of client and server identifiers to choose the correct copy of a concurrent server.

7. The Socket API • The interface between an application program and the communication protocols in an operating system (OS) is known as the Application Program Interface or API. • Sockets provide an implementation of the SAP (Service Access Point) abstraction at the Transport Layer in the TCP/IP protocol suite, which is part of the BSD Unix.

8. A socket library can provide applications with a socket API on an operating system that does not provide native sockets (e.g. Windows 3.1). When an application calls one of the socket procedures, control passes to a library routine that makes one or more calls to the underlying OS to implement the socket function. • A socket may be thought of as a generalization of the BSD Unix file access mechanism (open-read-write-close) that provides an end-point for communication.

9. When an application creates a socket, the application is given a small integer descriptor used to reference the socket. If a system uses the same descriptor space for sockets and other I/O, a single application can be used for network communication as well as for local data transfer. • An application must supply many details for each socket by specifying many parameters and options (e.g. an application must choose a particular protocol, provide address of remote machine, specify whether it is a client or server, etc.)

10. To avoid having a single socket function with separate parameters for each options, designers of the socket API chose to define many functions, each with a few parameters.

11. Functions needed • Specify local and remote communication endpoints • Initiate a connection • Wait for incoming connection • Send and receive data • Terminate a connection gracefully • Error handling

12. Server (connection-oriented protocol) socket() bind() listen() accept() Client blocks until connection from client socket() connection establishment connect() data (request) read() write() process request data (reply) write() read() Socket system calls for connection-oriented protocol

13. Server (connectionless protocol) Socket system calls for connectionless protocol socket() bind() Client recvfrom() socket() blocks until data received from client bind() data (request) sendto() process request sendto() data (reply) revfrom() Not necessary in UDP!!

14. Data communication between two hosts on the Internet require the five components of what is called an association to be initialized: {protocol,local-addr, local-process, foreign-addr, foreign-process} • The different system calls for sockets provides values for one or more of these components.

15. Socket system call • The first system call any process wishing to do network I/O has to call is the socket system call. • int sockfd = socket (int family, int type, int protocol) • Examples of Family include: • AF_UNIX • AF_INET • Examples of Type include • SOCK_STREAM • SOCK_DGRAM • SOCK_RAW

16. The protocol argument is typically zero, but may be specified to request an actual protocol like UDP, TCP, ICMP, etc. • The socket system call just fills in one element of the five-tuple we’ve looked at - the protocol. The remaining are filled in by the other calls as shown in the figure.

17. local_addr, local_process foreign_addr, foreign_process accept() socket() bind() Connection-Oriented Server socket() connect() Connection-oriented Client socket() bind() recvfrom() Connectionless Server socket() bind() sendto() Connectionless Client protocol

18. Specifying an Endpoint Address • Remember that the sockets API is generic • There must be a generic way to specify endpoint addresses • TCP/IP requires an IP address and a port number for each endpoint address. • Other protocol suites(families) may use other schemes. • Generic socket addresses • (The C function that make up the sockets API expect structures of type sockaddr.) : struct sockaddr { unsigned short sa_family; //specifies the address type char sa_data[14]; //specifies the address value };

19. AF_INET--TCP/IP address • For AF_INET we need: • 16 bit port number • 32 bit IP address (IPv4 only) struct sockaddr_in{ short sin_family; unsigned short sin_port; struct in_addr sin_addr; char sin_zero[8]; }; • how these fields to be set and interpreted?

20. Network Byte Order Functions Example: struct sockaddr_in sin; sin.sin_family = AF_INET; sin.sin_port = htons(9999); sin.sin_addr.s_addr = inet_addr; unsigned short htons(unsigned short); unsigned short ntohs(unsigned short); unsigned long htonl(unsigned long); unsigned long ntohl(unsigned long);

21. Bind System Call • The bind system call assigns an address to an unnamed socket. Example: • int bind(int sockfd, struct sockaddr_in *myaddr, int addrlen)

22. What is bind used for ? • Servers (both connection oriented and connectionless) NEED to register their well-known address to be able to accept connection requests. • A client can register a specific address for itself. • A connectionless client NEEDS to assure that it is bound to some unique address, so that the server has a valid return address to send its responses to – However, it does not have to bind to a particular port! WHY?

23. The bindsystem call provides the values for the local_addr and local_process elements in the five_tuple in an association. • An address for the Internet domain sockets is a combination of a hostname and a port number, as shown below: struct sockaddr_in { short sin_family ; /*typically AF_INET*/ u_short sin_port; /* 16 bit port number, network byte ordered */ struct in_addr sin_addr ; /* 32 bit netid/hostid, network byte ordered */ char sin_zero[8]; /* unused*/ }

24. Connect/Listen/Accept System Calls • Connect • A client process connects a socket descriptor after a socket system call to establish a connection with the server. • int connect(int sockfd, struct sockaddr_in *servaddr, int addrlen) • For a connection-oriented client, the connect (along with an accept at the server side) assigns all four addresses and process components of the association.

25. Listen • The listen system call is used by a connection-oriented server to indicate it is willing to receive connections. • int listen(int socket, int qlength) • allows servers to prepare a socket for incoming connections • puts the socket in a passive mode ready to accept connections • informs the OS that the protocol software should enqueue multiple simultaneous requests that arrive at the socket • applies only to sockets that have selected reliable stream delivery service

26. Accept • After the connection-oriented server executes a listen, it waits for connection requests from client(s) in the accept system call, e.g., newsockfd = accept(sockfd, peer, addrlen) • needs to wait for a connection • blocks until a connection request arrives • addrlen is a pointer to an integer; • when a request arrives , the system fills in argument addr with the address of the client that has placed the request and sets addrlen to the length of the address. • system creates a new socket, returns the new socket descriptor

27. accept returns a new socket descriptor, which has all five components of the association specified - three (protocol, local addr, local_process) are inherited from the existing sockfd (which however has its foreign address and process components unspecified, and hence can be re-used to accept another request. This scenario is typical for concurrent servers.

28. Sending and Receiving Data • Here’s how you might read from a socket: • num_read = read(sockfd, buff_ptr, num_bytes) • And here’s how you read from an open file descriptor in Unix: • num_read = read(fildes, buff_ptr, num_bytes) • There are other ways (with different parameters) to send and receive data: read, readv, recv, recvfrom, recvmsg to receive data through a socket; and write, writev, send, sendto, sendmsg to send data through a socket.

29. sendto()--UDP Sockets • int sendto(int socket, char *buffer, int length, int flags, struct sockaddr *destination_address, int address_size); • For example: struct sockaddr_in sin; sin.sin_family = AF_INET; sin.sin_port = htons(12345); sin.sin_addr.s_addr = inet_addr("128.227.22.43"); char *msg = "Hello, World"; sendto(s, msg, strlen(msg)+1, 0, (struct sockaddr *)sin, sizeof(sin));

30. recvfrom()--UDP Sockets • Int recvfrom(int socket, char *buffer, int length, int flags, struct sockaddr *sender_address, int *address_size) • For example: struct sockaddr_in sin; char msg[10000]; int ret; int sin_length; sin_length = sizeof(sin); ret = recvfrom(s, msg, 10000, 0, (struct sockaddr *)sin, &sin_length); printf("%d bytes received from %s (port %d)\n", ret, inet_ntoa(sin.sin_addr), sin.sin_port);

31. send() and recv() -- TCP Sockets • int send(int s, const char *msg, int len, int flags) • connected socket • argument flags controls the transmission. • allows the sender to specify that the message should be sent out-of- band messages correspond to TCP’s urgent data • allows the caller to request that the message be sent without using local routine tables (take control of routine) • int recv(int s, char *buf, int len, int flags) • connected socket • argument flags allow the caller to control the reception • look ahead by extracting a copy of the next incoming message without removing the message from the socket

32. close() and shutdown() • close(int socket) • For UDP sockets, this will release the ownership on the local port that is bound to this socket • For TCP, this will initiate a two-way shutdown between both hosts before giving up port ownership. • shutdown(int socket, int how) • f the how field is 0, this will disallow further reading (recv) from the socket. • If the how field is 1, subsequent writes (send) will be disallowed. The socket will still need to be passed to close.

33. Relationship Between Sockets and File Descriptors • Socket handles are integer values. In UNIX, socket handles can be passed to most of the low-level POSIX I/O functions. • read(s, buffer, buff_length); //s could be a file descriptor too • write(s, buffer, buff_length) ; • Calling read on an open socket is equivalent to recv and recvfrom • if the socket is UDP, then information about the sender of the datagram will not be returned • Similarly the write function call is equivalent to send and sendto • UDP sockets may call connect to use send and write • use the socket library functions instead of the file I/O equivalents.

34. Utility Functions • unsigned int inet_addr(char *str) • str represents an IP address(dotted-quad notation); inet_addr will return it's equivalent 32-bit value in network byte order. • This value can be passed into the sin_addr.s_addr field of a socketaddr_in structure • -1 is returned if the string can not be interpreted • char *inet_ntoa(struct in_addr ip) • Converts the 32-bit value which is assumed to be in network byte order and contained in ip to a string • The pointer returned by inet_ntoa contains this string. However, subsequent calls to inet_ntoa will always return the same pointer, so copying the string to another buffer is recommended before calling again.

35. Utility Functions ( cont’d ) • int gethostname(char *name, int length) • Copies the name (up to length bytes) of the hostname of the local computer into the character array pointed to by name • struct hostent *gethostbyname(char *strHost) • int select (int nfds, fd_set *readfds, fd_set *writefds, fd_set *exceptfds, const struct timeval *timeout)

36. Other Socket API • JAVA • platform independence; • Java API for network programming • java.net Pakage • classes • Socket • ServerSocket • DatagramSocket • DatagramPacket • InetAddress • etc. • compile and run

37. Others • Include files • #include <sys/types.h>; #include <sys/socket.h>; #include <netinet/in.h>; #include <arpa/inet.h>; #include <netdb.h>; #include <unistd.h>; #include <signal.h>; #include <stdio.h>; #include <fcntl.h>; #include <errno.h; #include <sys/time.h>; #include <stdlib.h>; #include <memory.h>; • Compiling and Linking • Under most versions of UNIX (Linux, BSD, SunOS, IRIX) compiling is done as usual: • gcc my_socket_program.c -o my_socket_program • Solaris: • cc my_socket_program.c -o my_socket_program -lsocket -lnsl • Programming tips • always check the return value for each function call • consult the UNIX on-line manual pages ("man") for a complete description

38. Summary • Network Application Programming Interface (API) • TCP/IP basic • UNIX/C Sockets • socket() ; bind() ; connect() ; listen() ; accept() ; sendto() ; recvfrom(); send() ; recv() ; read() ; write(); • some utility functions • Java Socket API