Socket Programming in C Slides Adapted on Jörn Altmann‘s Slides

CEN4500C2 Questions that will be Addressed What mechanisms are available for a programmer who writes network applications? How to write a network application that sends packets between hosts (client and server) across an IP network? Answer: socket API Client Server IP Network

CEN4500C3 Socket Programming Table of Contents 1. Network Application Programming Interface: Sockets and Internet Sockets Network Application Programming Interface 2. Network Programming Tips Network Programming Tips 3. Client-Server Architecture Client-Server Architecture 4. Example: Client Programming Example: Client Programming 5. Example: Server Programming Example: Server Programming 6. Network Programmer ’ s Mistakes Network Programmer ’ s Mistakes

CEN4500C4 Layers of the IP Protocol Suite Link Layer Transport Layer Network Layer Application Layer Link Layer Transport Layer Network Layer Application Layer Ethernet e.g. ftp e.g. TCP, UDP e.g. IP

CEN4500C5 Protocol Suite Location Internet Protocol Layer Link Layer Transport Layer (TCP, UDP) Network Layer (IP) Application Layer Network Card & Device Driver (e.g. Ethernet card) Operating System (e.g. Unix) Applications (e.g. browser, game, ftp) Application Programming Interface (API) (e.g. network API) Interface to the Network Card Location

CEN4500C6 Network API Operating system provides Application Programming Interface (API) for network application API is defined by a set of function types, data structures, and constants Desirable characteristics of the network interface Simple to use Flexible  independent from any application  allows program to use all functionality of the network Standardized  allows programmer to learn once, write anywhere Application Programming Interface for networks is called socket

CEN4500C7 Sockets Sockets provide mechanisms to communicate between computers across a network There are different kind of sockets DARPA Internet addresses (Internet Sockets) Unix interprocess communication (Unix Sockets) CCITT X.25 addresses and many others Berkeley sockets is the most popular Internet Socket runs on Linux, FreeBSD, OS X, Windows fed by the popularity of TCP/IP

CEN4500C8 Internet Sockets Support stream and datagram packets (e.g. TCP, UDP, IP) Is Similar to UNIX file I/O API ( provides a file descriptor) Based on C, single thread model does not require multiple threads

CEN4500C9 Types of Internet Sockets Different types of sockets implement different communication types (stream vs. datagram) Type of socket: stream socket connection-oriented two way communication reliable (error free), in order delivery can use the Transmission Control Protocol (TCP) e.g. telnet, ssh, http Type of socket: datagram socket connectionless, does not maintain an open connection, each packet is independent can use the User Datagram Protocol (UDP) e.g. IP telephony Other types exist: similar to the one above

CEN4500C10 Network Programming Tips Byte Ordering Naming Addressing

CEN4500C11 Byte Ordering of Integers Different CPU architectures have different byte ordering D3 high-order bytelow-order byte memory address A memory address A +1 Stored at little-endian computer Stored at big-endian computer low-order bytehigh-order byte F2 Integer representation (2 byte)

CEN4500C12 Message in Memory of of big-endian Computer Message is sent across Network C 4C 6F Byte Ordering Problem Question: What would happen if two computers with different integer byte ordering communicate? Message is: [Hello,1] Message is: [Hello,256] Processing C 4C 6F Message in Memory of little-endian Computer Processing Answer:  Nothing if they do not exchange integers!  But: If they exchange integers, they would get the wrong order of bytes, therefore, the wrong value! Example:

CEN4500C13 Byte Ordering Solution There are two solutions if computers with different byte ordering system want to communicate They must know the kind of architecture of the sending computer (bad solution, it has not been implemented) Introduction of a network byte order. The functions are: uint16_t htons(uint16_t host16bitvalue) uint32_t htonl(uint32_t host32bitvalue) uint16_t ntohs(uint16_t net16bitvalue) uint32_t ntohs(uint32_t net32bitvalue) Note: use for all integers (short and long), which are sent across the network Including port numbers and IP addresses

CEN4500C14 Network Programming Tips Byte Ordering Naming Addressing

CEN4500C15 Naming and Addressing Host name identifies a single host (see Domain Name System slides) variable length string (e.g. is mapped to one or more IP addresses IP Address written as dotted octets (e.g ) 32 bits. Not a number! But often needs to be converted to a 32-bit to use. Port number identifies a process on a host 16 bit number

CEN4500C16 Client-Server Architecture Client requests service from server Server responds with sending service or error message to client ClientServer request response

CEN4500C17 Simple Client-Server Example ClientServer request response socket() connect() send() recv() close() socket() bind() listen() accept() recv() send() recv() close() Connection establishment Data response Data request End-of-file notification

CEN4500C18 Example: Client Programming Create stream socket ( socket() ) Connect to server ( connect() ) While still connected: send message to server (send() ) receive (recv() ) data from server and process it Close TCP connection and Socket (close())

CEN4500C19 socket(): Initializing Socket Getting the file descriptor int chat_sock; if ((chat_sock = socket(AF_INET, SOCK_STREAM, 0)) < 0) { perror("socket"); printf("Failed to create socket\n"); abort (); } 1.parameter specifies protocol/address family 2.parameter specifies the socket type Other possibilities: SOCK_DGRAM 3.parameter specifies the protocol. 0 means protocol is chosen by the OS.

CEN4500C20 IP Address Data Structure struct sockaddr_in { short intsin_family; // Address family unsigned short intsin_port; // Port number struct in_addr sin_addr; // Internet address unsigned char sin_zero[8]; }; struct in_addr { unsigned long s_addr; // 4 bytes }; Padding of sin_zeros : struct sockaddr_in has same size as struct sockaddr

CEN4500C21 connect(): Making TCP Connection to Server struct sockaddr_in sin; struct hostent *host = gethostbyname (argv[1]); unsigned int server_address = *(unsigned long *) host->h_addr_list[0]; unsigned short server_port = atoi (argv[2]); memset (&sin, 0, sizeof (sin)); sin.sin_family = AF_INET; sin.sin_addr.s_addr = server_address; sin.sin_port = htons (server_port); if (connect(chat_sock, (struct sockaddr *) &sin, sizeof (sin)) < 0) { perror("connect"); printf("Cannot connect to server\n"); abort(); }

CEN4500C22 send(): Sending Packets int send_packets(char *buffer, int buffer_len) { sent_bytes = send(chat_sock, buffer, buffer_len, 0); if (send_bytes < 0) { perror (“send"); } return 0; } Needs socket descriptor, Buffer containing the message, and Length of the message Can also use write()

CEN4500C23 Receiving Packets: Separating Data in a Stream Use records (data structures) to partition the data stream B Fixed length record 013 C D 5 A receive buffer slide through

CEN4500C24 Receiving Packets int receive_packets(char *buffer, int buffer_len, int *bytes_read) { int left = buffer_len - *bytes_read; received = recv(chat_sock, buffer + *bytes_read, left, 0); if (received < 0) { perror (“recv"); } if (received <= 0) { return close_connection(); } *bytes_read += received; while (*bytes_read > RECORD_LEN) { process_packet(buffer, RECORD_LEN); *bytes_read -= RECORD_LEN; memmove(buffer, buffer + RECORD_LEN, *bytes_read); } return 0; } Can also use read() buffer *bytes_read buffer_len

CEN4500C25 Server Programming: Simple Create stream socket ( socket() ) Bind port to socket ( bind() ) Listen for new client ( listen() ) While accept user connection and create a new socket (accept() ) data arrives from client (recv() ) data has to be send to client (send() )

CEN4500C26 bind(): Assign IP and Port struct sockaddr_in sin; struct hostent *host = gethostbyname (argv[1]); unsigned int server_address = *(unsigned long *) host->h_addr_list[0]; unsigned short server_port = atoi (argv[2]); memset (&sin, 0, sizeof (sin)); sin.sin_family = AF_INET; sin.sin_addr.s_addr = server_address; sin.sin_port = htons (server_port); if (bind(chat_sock, (struct sockaddr *) &sin, sizeof (sin)) < 0) { perror("bind"); printf("Cannot bind server application to network\n"); abort(); }

CEN4500C27 bind(): bind() tells the OS to assign a local IP address and local port number to the socket. Many applications let the OS choose an IP address. Use wildcard INADDR_ANY as local address in this case. At server, user process must call bind() to assign a port At client, bind() is not required since OS may assign available port and IP address The server will get the port number of the client through the UDP/TCP packet header Note: Each application is represented by a server port number

CEN4500C28 listen(): Wait for Connections int listen(int sockfd, int backlog); Puts socket in a listening state, willing to handle incoming TCP connection request. Backlog: number of TCP connections that can be queued at the socket.

CEN4500C29 Server Example #define MYPORT 3490 // the port users will be connecting to #define BACKLOG 10 // how many pending connections queue will hold int main(void) { int sockfd, new_fd; // listen on sockfd, new connection on new_fd struct sockaddr_in my_addr; // my address information struct sockaddr_in their_addr; // connector's address information int sin_size; if ((sockfd = socket(AF_INET, SOCK_STREAM, 0)) == -1) { perror("socket"); exit(1); } my_addr.sin_family = AF_INET; // host byte order my_addr.sin_port = htons(MYPORT); // short, network byte order my_addr.sin_addr.s_addr = INADDR_ANY; // auto. filled with local IP memset(&(my_addr.sin_zero), '\0', 8); // zero the rest of the struct

CEN4500C30 if (bind(sockfd, (struct sockaddr *)&my_addr, sizeof(struct sockaddr)) == -1) { perror("bind"); exit(1); } if (listen(sockfd, BACKLOG) == -1) { perror("listen"); exit(1); } while(1) { // main accept() loop sin_size = sizeof(struct sockaddr_in); if ((new_fd = accept(sockfd, (struct sockaddr *)&their_addr, &sin_size)) == -1) { perror("accept"); continue; } printf("server: got connection from %s\n", inet_ntoa(their_addr.sin_addr)); if (send(new_fd, "Hello, world!\n", 14, 0) == -1) perror("send"); close(new_fd); } return 0; }

CEN4500C31 Client Example #include #define PORT 3490 // the port client will be connecting to #define MAXDATASIZE 100 // max number of bytes we can get // at once int main(int argc, char *argv[]) { int sockfd, numbytes; char buf[MAXDATASIZE]; struct hostent *he; struct sockaddr_in their_addr;// server's address information if (argc != 2) { fprintf(stderr,"usage: client hostname\n"); exit(1); } if ((he=gethostbyname(argv[1])) == NULL) { // get the host info perror("gethostbyname"); exit(1); } if ((sockfd = socket(AF_INET, SOCK_STREAM, 0)) == -1) { perror("socket"); exit(1); }

CEN4500C32 their_addr.sin_family = AF_INET; // host byte order their_addr.sin_port = htons(PORT); // short, network byte order their_addr.sin_addr = *((struct in_addr *)he->h_addr); // already network byte order memset(&(their_addr.sin_zero), '\0', 8); // zero the rest of the struct if (connect(sockfd, (struct sockaddr *)&their_addr, sizeof(struct sockaddr)) == -1){ perror("connect"); exit(1); } if ((numbytes=recv(sockfd, buf, MAXDATASIZE-1, 0)) == -1) { perror("recv"); exit(1); } buf[numbytes] = '\0'; printf("Received: %s",buf); close(sockfd); return 0; }

CEN4500C33 I/O Blocking socket() ; bind() ; listen(); while accept(); recv() ; send() ; Simple server has blocking problem Suppose 5 connections accepted. Suppose next accept() blocks. Other connections cannot send and receive. Cannot get keyboard input either.

CEN4500C34 select() :I/O Multiplexing waits on multiple file descriptors and timeout returns when any file descriptor is ready to be read or written or indicate an error, or timeout exceeded advantages simple application does not consume CPU cycles while waiting disadvantages does not scale to large number of file descriptors

CEN4500C35 Example: Server Programming create stream socket ( socket() ) Bind port to socket ( bind() ) Listen for new client ( listen() ) While Wait for (select() ) (depending on which file descriptors are ready) accept user connection and create a new socket (accept() ) data arrives from client (recv() ) data has to be send to client (send() )

CEN4500C36 Server: Alternative Ways of Handling Many Clients Forking a new process for each client: fork() But, creating new process is expensive. Multithreaded implementation: have one thread handling each client. Thread is like a process but light- weighted.

CEN4500C37 Network Programmer ’ s Mistakes byte ordering separating records in streams use of select() misinterpreting the project specification not knowing all available system calls

CEN4500C38 There are more System Calls Depends on communication type Datagram sockets use recvfrom() and sendto() for receiving and sending data Closing connection: close(), shutdown() Convenient functions (on UNIX) inet_aton, inet_ntoa inet_pton, inet_ntop

CEN4500C39 Literature short tutorial: Beej's Guide to Network Programming Unix Network Programming, volumes 1 and 2 by W. Richard Stevens. Published by Prentice Hall; ISBNs for volumes 1 and 2: X, Advanced Programming in the Unix Environment by W. Richard Stevens. Published by Addison Wesley. ISBN man pages on a Unix computer