![int connect_ socket( char *hostname, int port) {
int sock;
struct sockaddr_in sin;
struct hostent *host;
sock = socket( AF_ INET, SOCK_ STREAM, 0);
if (sock == -1)
return sock;
host = gethostbyname( hostname);
if (host == NULL) {
close( sock);
return -1;
}
memset (& sin, 0, sizeof( sin));
sin. sin_ family = AF_ INET;
sin. sin_ port = htons( port);
sin. sin_ addr. s_ addr = *( unsigned long *) host-> h_ addr_ list[ 0];
if (connect( sock, (struct sockaddr *) &sin, sizeof( sin)) != 0) {
close (sock);
return -1;
}
return sock;
}
Resolve the host
struct hostent *gethostbyname( const char *hostname);
/*Return nonnull pointer if OK, NULL on error */
Setup up the struct
unit16_t htons(unit16_t host16bitvaule)
/*Change the port number from host byte order to
network byte order */
Connect
connect(int socketfd, const struct sockaddr * servaddr,
socket_t addrlen)
/*Perform the TCP three way handshaking*/
Hostent structure
struct hostent{
char * h_name /*official name of host*/
char ** h_aliases; /* pointer ot array of
pointers to alias name*/
int h_addrtype /* host address type*/
int h_length /* length of address */
char ** h_addr_list /*prt to array of ptrs with
IPv4 or IPv6 address*/
}
Ipv4 socket address structure
struct socketaddr_in{
uint8_t sin_len; /*length of the structure (16)*/
sa_falimily_t sin_family /* AF_INT*/
in_port_t sin_port /* 16 bit TCP or UDP port number*/
struct in_addr sin_addr /* 32 bit Ipv4 address */
char sin_zero(8)/* unused*/
}
Make the socket
Socket(int family , int type, in t protocol);
return nonnegative value for OK, -1 for error](https://image.slidesharecdn.com/socketsintro-231221162616-b6f1f3ad/85/INTRODUCTION-TO-SOCKETS-IN-COMPUTER-NETWORKS-DEPT-OF-CSE-ppt-9-320.jpg)
![Sending / Receiving Data
• With a connection (SOCK_STREAM):
– int count = send(sock, &buf, len, flags);
• count: # bytes transmitted (-1 if error)
• buf: char[], buffer to be transmitted
• len: integer, length of buffer (in bytes) to transmit
• flags: integer, special options, usually just 0
– int count = recv(sock, &buf, len, flags);
• count: # bytes received (-1 if error)
• buf: void[], stores received bytes
• len: # bytes received
• flags: integer, special options, usually just 0
– Calls are blocking [returns only after data is sent (to
socket buf) / received]](https://image.slidesharecdn.com/socketsintro-231221162616-b6f1f3ad/85/INTRODUCTION-TO-SOCKETS-IN-COMPUTER-NETWORKS-DEPT-OF-CSE-ppt-13-320.jpg)
INTRODUCTION TO SOCKETS IN COMPUTER NETWORKS DEPT OF CSE.ppt
- 2. Why do we need sockets? Provides an abstraction for interprocess communication
- 3. • The services provided (often by the operating system) that provide the interface between application and protocol software. Application Network API Protocol A Protocol B Protocol C Definition
- 4. Functions –Define an “end- point” for communication –Initiate and accept a connection –Send and receive data –Terminate a connection gracefully Examples File transfer apps (FTP), Web browsers (HTTP), Email (SMTP/ POP3), etc…
- 5. Types of Sockets • Two different types of sockets : – stream vs. datagram • Stream socket :( a. k. a. connection- oriented socket) – It provides reliable, connected networking service – Error free; no out- of- order packets (uses TCP) – applications: telnet/ ssh, http, … • Datagram socket :( a. k. a. connectionless socket) – It provides unreliable, best- effort networking service – Packets may be lost; may arrive out of order (uses UDP) – applications: streaming audio/ video (realplayer), …
- 7. Addresses, Ports and Sockets • Like apartments and mailboxes – You are the application – Your apartment building address is the address – Your mailbox is the port – The post-office is the network – The socket is the key that gives you access to the right mailbox
- 8. Client – high level view Create a socket Setup the server address Connect to the server Read/write data Shutdown connection
- 9. int connect_ socket( char *hostname, int port) { int sock; struct sockaddr_in sin; struct hostent *host; sock = socket( AF_ INET, SOCK_ STREAM, 0); if (sock == -1) return sock; host = gethostbyname( hostname); if (host == NULL) { close( sock); return -1; } memset (& sin, 0, sizeof( sin)); sin. sin_ family = AF_ INET; sin. sin_ port = htons( port); sin. sin_ addr. s_ addr = *( unsigned long *) host-> h_ addr_ list[ 0]; if (connect( sock, (struct sockaddr *) &sin, sizeof( sin)) != 0) { close (sock); return -1; } return sock; } Resolve the host struct hostent *gethostbyname( const char *hostname); /*Return nonnull pointer if OK, NULL on error */ Setup up the struct unit16_t htons(unit16_t host16bitvaule) /*Change the port number from host byte order to network byte order */ Connect connect(int socketfd, const struct sockaddr * servaddr, socket_t addrlen) /*Perform the TCP three way handshaking*/ Hostent structure struct hostent{ char * h_name /*official name of host*/ char ** h_aliases; /* pointer ot array of pointers to alias name*/ int h_addrtype /* host address type*/ int h_length /* length of address */ char ** h_addr_list /*prt to array of ptrs with IPv4 or IPv6 address*/ } Ipv4 socket address structure struct socketaddr_in{ uint8_t sin_len; /*length of the structure (16)*/ sa_falimily_t sin_family /* AF_INT*/ in_port_t sin_port /* 16 bit TCP or UDP port number*/ struct in_addr sin_addr /* 32 bit Ipv4 address */ char sin_zero(8)/* unused*/ } Make the socket Socket(int family , int type, in t protocol); return nonnegative value for OK, -1 for error
- 10. Server – high level view Create a socket Bind the socket Listen for connections Accept new client connections Read/write to client connections Shutdown connection
- 11. Listening on a port (TCP) int make_ listen_ socket( int port) { struct sockaddr_ in sin; int sock; sock = socket( AF_ INET, SOCK_ STREAM, 0); if (sock < 0) return -1; memset(& sin, 0, sizeof( sin)); sin. sin_ family = AF_ INET; sin. sin_ addr. s_ addr = htonl( INADDR_ ANY); sin. sin_ port = htons( port); if (bind( sock, (struct sockaddr *) &sin, sizeof( sin)) < 0) return -1; return sock; } Make the socket Setup up the struct Bind bind(int sockfd, const struct sockaddr * myaddr, socklen_t addrlen); /* return 0 if OK, -1 on error assigns a local protocol adress to a socket*/
- 12. accepting a client connection (TCP) int get_ client_ socket( int listen_ socket) { struct sockaddr_ in sin; int sock; int sin_ len; memset(& sin, 0, sizeof( sin)); sin_ len = sizeof( sin); sock = accept( listen_ socket, (struct sockaddr *) &sin, &sin_ len); return sock; } Setup up the struct Accept the client connection accept(int sockefd, struct sockaddr * claddr, socklen_t * addrlen) /* return nonnegative descriptor if OK, -1 on error return the next completed connection from the front of the completed connection queue. if the queue is empty, the process is put to sleep(assuming blocking socket)*/
- 13. Sending / Receiving Data • With a connection (SOCK_STREAM): – int count = send(sock, &buf, len, flags); • count: # bytes transmitted (-1 if error) • buf: char[], buffer to be transmitted • len: integer, length of buffer (in bytes) to transmit • flags: integer, special options, usually just 0 – int count = recv(sock, &buf, len, flags); • count: # bytes received (-1 if error) • buf: void[], stores received bytes • len: # bytes received • flags: integer, special options, usually just 0 – Calls are blocking [returns only after data is sent (to socket buf) / received]
- 14. socket() bind() listen() accept() read() write() read() close() Socket() connect() write() read() close() TCP Client TCP Server Well-known port blocks until connection from client process request Connection establishment
- 15. Dealing with blocking calls • Many functions block – accept(), connect(), – All recv() • For simple programs this is fine • What about complex connection routines – Multiple connections – Simultaneous sends and receives – Simultaneously doing non-networking processing
- 16. Dealing with blocking (cont..) • Options – Create multi-process or multi-threaded code – Turn off blocking feature (fcntl() system call) – Use the select() function • What does select() do? – Can be permanent blocking, time-limited blocking or non- blocking – Input: a set of file descriptors – Output: info on the file-descriptors’ status – Therefore, can identify sockets that are “ready for use”: calls involving that socket will return immediately
- 17. select function call • int status = select() – Status: # of ready objects, -1 if error – nfds: 1 +largest file descriptor to check – readfds: list of descriptors to check if read-ready – writefds: list of descriptors to check if write-ready – exceptfds: list of descriptors to check if an exception is registered – Timeout: time after which select returns