Introduction to Socket Programming

Advisor: Quincy Wu Speaker: Kuan-Ta Lu Date: Nov. 25, 2010 Introduction to Socket Programming

Socket Interface • 是一種應用程式介面(API)，介於application層與transport層之間，並且提供標準的函式以符合不同的網路傳輸規格。 • 最早的Socket Interface於1982年由柏克萊大學為支援UNIX作業系統上的TCP/IP應用所開發的Socket介面，稱為Berkeley Socket Interface，而其軟體則稱為Berkeley Software Distribution(BSD)。 • Windows Sockets，簡稱Winsock，是以UNIX系統上Berkeley Sockets的函式為基礎，並加上了一些符合視窗環境特性的函式。

Socket Interface (con.)

Connection-oriented socket (TCP)

socket() • Create an endpoint for communication • int socket( int domain, int type, int protocol ); • domain • PF_INET for IPv4 • PF_INET6 for IPv6 • type • SOCK_STREAM for reliable TCP sockets • SOCK_DGRAM for unreliable fast UDP sockets • protocol • default: 0

bind() • Bind a name to a socket • int bind( int s, const struct sockaddr * name, socklen_t namelen ); • s • The file descriptor to be bound. • name • A pointer to the sockaddr structure that holds the address to be bound to the socket. • namelen • The length of the sockaddr structure pointed to by name.

bind() (con.) • struct sockaddr { sa_family_t sa_family; // address family, AF_xxx char sa_data[14]; // 14 bytes of protocol address }; • sa_family • represents address family • sa_data • contains data about address

bind() (con.) • struct in_addr { uint32_t s_addr; // 32bit IPv4 address (4 bytes) // network byte ordered }; struct sockaddr_in { sa_family_t sin_family; // Address family (2 bytes) in_port_t sin_port; // Port number (2 bytes) struct in_addr sin_addr; // Internet address (4 bytes) char sin_zero[8]; // Empty (for padding) (8 bytes) }

bind() (con.) • Example: struct sockaddr_in servaddr; servaddr.sin_family = AF_INET; servaddr.sin_addr.s_addr = htonl(INADDR_ANY); servaddr.sin_port = htons(13); Bind (sockfd, (struct sockaddr *) & servaddr, sizeof(servaddr) );

connect() • Initiate a connection on a socket • int connect( int s, const struct sockaddr * name, socklen_t namelen ); • s • The descriptor of the socket on which to initiate the connection. • name • The name of the socket to connect to for a SOCK_STREAM connection. • namelen • The length of the name, in bytes.

listen() • Listen for connections on a socket • int listen( int s, int backlog ); • s • The descriptor for the socket that you want to listen on. You can create a socket by calling socket(). • backlog • The maximum length that the queue of pending connections may grow to. • “backlog” = max # of ”pending connections” = handshake in progress or complete

accept() • Accept a connection on a socket • int accept( int s, struct sockaddr * addr, socklen_t * addrlen ); • s • A socket that's been created with socket(). • addr • A result parameter that's filled in with the address of the connecting entity, as known to the communications layer. • addrlen • It should initially contain the amount of space pointed to by addr; on return it contains the actual length (in bytes) of the address returned.

send() • Send a message to a connected socket • ssize_t send( int s, const void * msg, size_t len, int flags ); • msg • A pointer to the message that you want to send. • len • The length of the message. • flags • MSG_OOB • MSG_DONTROUTE

recv() • Receive a message from a socket • ssize_t recv( int s, void * buf, size_t len, int flags ); • buf • A pointer to a buffer where the function can store the message. • len • The size of the buffer. • flags • MSG_OOB • MSG_PEEK • MSG_WAITALL

close() • Close a socket descriptor • int close(int s); • Windows users: the function you need to use is called closesocket(), not close().

Connectionless socket (UDP)

sendto() • Send a message to a socket at a specific address • ssize_t sendto( int s, const void * msg, size_t len, int flags, const struct sockaddr * to, socklen_t tolen ); • to • A pointer to a sockaddr object that specifies the address of the target. • tolen • A socklen_t object that specifies the size of the to address.

recvfrom() • Receive a message from the socket at a specified address • ssize_t recvfrom( int s, void * buff, size_t len, int flags, struct sockaddr * from, socklen_t * fromlen ); • from • NULL, or a pointer to a sockaddr object where the function can store the source address of the message. • fromlen • A pointer to a socklen_t object that specifies the size of the from buffer. The function stores the actual size of the address in this object.

htons(), htonl(), ntohs(), ntohl() • Convert multi-byte integer types from host byte order to network byte order • uint32_t htonl(uint32_t hostlong); uint16_t htons(uint16_t hostshort); uint32_t ntohl(uint32_t netlong); uint16_t ntohs(uint16_t netshort); • htons() host to network short • htonl() host to network long • ntohs() network to host short • ntohl() network to host long

Demo • Enviroment • OS: CentOS 5.5 • Client • Server • Socket TCP (IPv4) • Socket UDP (IPv4) • Socket TCP (IPv6)

Reference • http://beej.us/guide/bgnet/output/html/multipage/index.html • http://www.qnx.com/developers/docs/6.3.0SP3/neutrino/sys_arch/tcpip.html • http://ms11.voip.edu.tw/~jryan/ref/20090326-chenglin-socket_programming.ppt • http://ms11.voip.edu.tw/~jryan/ref/SocketIntro.ppt • http://ms11.voip.edu.tw/~jryan/ref/Introduction_to_IPv6_programming.pdf • http://ms11.voip.edu.tw/~jryan/ref/Windows_Socket Programming_&_IPv6_Translation Middleware.pdf

Introduction to Socket Programming