1 Programming with TCP/IP Ram Dantu
2 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.
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. 3
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. 4
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. 5
6 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
Server (connection-oriented protocol) socket() bind() listen() accept() blocks until connection from client read() connection establishment data (request) Socket system calls for connection-oriented protocol Client socket() connect() write() process request write() data (reply) read() 7
Server (connectionless protocol) socket() bind() recvfrom() Socket system calls for connectionless protocol Client socket() blocks until data received from client process request data (request) bind() sendto() sendto() data (reply) revfrom() 8
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 9
The protocol argument is typically zero, but may be specified to request an actual protocol like UDP, TCP, ICMP, etc. 10
11 Connection-Oriented Server Connection-oriented Client Connectionless Server Connectionless Client protocol socket() socket( ) socket( )socket( ) local_addr, local_process bind() bind() bind() foreign_addr, foreign_process accept() connect() recvfrom() sendto()
12 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?
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) 13
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. 14
Listen The listen system call is used by a connectionoriented 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 15
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 16
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. 17
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 18
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. 19
20 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
Summary Network Application Programming Interface (API) TCP/IP basic UNIX/C Sockets socket() ; bind() ; connect() ; listen() ; accept() ; sendto() ; recvfrom(); send() ; recv() ; read() ; write(); 21