Sockets

• Socket = abstraction of the port concept:– Application programs request that the operating

system create a socket when one is needed– O.S. returns a small integer (socket descriptor)

that the program uses to reference the socket– Application program can then use read and

write system calls on the socket– Program closes the socket when finished using

it

Creating a Socket

• The socket system call:int sd; // socket descriptorint pf; // protocol family (one of PF_INET,

// PF_PUP, PF_APPLETALK, // PF_UNIX, etc.)

int type; // type of service (one of SOCK_RAW, // SOCK_DGRAM,

SOCK_STREAM)int protocol; // specific protocol in pf

sd = socket(pf, type, protocol) // create a new socket

Binding a Socket to an Internet Source Address (cont)

• The bind system call:int sd; // socket descriptor

struct sockaddr_in addr; // structure specifying source // address

int len; // length (in bytes) of

// addr struct

bind(sd,addr,len) // bind socket to source IP // address

Connecting a Socket to a Destination Address

• The connect system call:int sd; // socket descriptor

struct sockaddr addr; // structure specifying dest addr

int len; // length (in bytes) of

// addr struct

connect(sd,addr,len) // connect socket to// dest address

• Can also use a sockaddr_in struct for dest address

Sending Data Through a Socket

• The write system call:

int sd; // socket descriptor

void *buffer; // address of the data to be sent

int len; // number of bytes to send

write(sd,buffer,len) // send data through socket

Receiving Data Through a Socket

• The read system call:

int sd; // socket descriptor

void *buffer; // address in memory at which // to store the data

int len; // number of bytes to receive

read(sd,buffer,len) // receive data through socket

Closing a Socket

• The close system call:

int sd; // socket descriptor

close(sd) // close socket

Datagram Socket Example: Receiver

#include <sys/types.h>#include <sys/socket.h>#include <netinet/in.h>#include <stdio.h>main() { int sock, len; struct sockaddr_in name; char buf [1024]; sock = socket(AF_INET, SOCK_DGRAM, 0); if (sock < 0) exit(-1); name.sin_family = AF_INET; name.sin_addr.s_addr = INADDR_ANY; name.sin_port = 0; if (bind(sock, (struct sockaddr *) &name, sizeof(name))) exit(-1); len = sizeof(name); if (getsockname(sock, (struct sockaddr *) &name, &len)) exit(-1); printf("Receiver listening on port %d\n",ntohs(name.sin_port)); if (read(sock, buf, 1024) < 0) exit(-1); printf("%s\n",buf); close(sock);}

Datagram Socket Example: Sender

#include <sys/types.h>#include <sys/socket.h>#include <netinet/in.h>#include <netdb.h>#include <stdio.h>#include <string.h>main(int argc, char **argv) { int sock; struct sockaddr_in name; struct hostent *hp, *gethostbyname(); if (argc != 3) exit(-1); sock = socket(AF_INET, SOCK_DGRAM, 0); if (sock < 0) exit(-1); hp = gethostbyname("prime.cs.ohiou.edu"); if (hp == 0) exit(-1); bcopy(hp->h_addr, &name.sin_addr, hp->h_length); name.sin_family = AF_INET; name.sin_port = htons(atoi(argv[1])); if (connect(sock, (struct sockaddr *) &name, sizeof(name))) exit(-1); if (write(sock, argv[2], (strlen(argv[2])+1)) < 0) exit(-1); close(sock);}

Obtaining Local Socket Addresses

• The getsockname system call:int sd; // socket descriptor

struct sockaddr *addr; // address structure to be filled

int *len; // pointer to integer that will // contain the length of

the // address

getsockname(sd, addr, len); // obtain local socket address

Obtaining and SettingSocket Options

• The getsockopt system call:int sd; // socket descriptorint level; // option for socket or protocol?int optionid; // which specific option?void *optionval; // where to place the requested

// valueint *len; // length of the optionval

getsockopt(sd, level, optionid, optionval, len); // obtain

// socket opt

Obtaining and SettingSocket Options (cont)

• Values for level (from <netinet/in.h>):

– SOL_SOCKET option for the socket

– IPPROTO_IP option for IP

– IPPROTO_ICMP option for ICMP

– IPPROTO_TCP option for TCP

– IPPROTO_UDP option for UDP

Obtaining and SettingSocket Options (cont)

• Values for optionid (from <sys/socket.h>):– SO_TYPE socket type

– SO_SNDBUF send buffer size

– SO_RCVBUF receive buffer size

– SO_DEBUG debugging info available?

– SO_ACCEPTCONN socket listening enabled?

– SO_BROADCAST broadcast supported?

– SO_REUSEADDR address reuse allowed?

– SO_KEEPALIVE keep alive after close?

Obtaining and SettingSocket Options (cont)

• The setsockopt system call:int sd; // socket descriptor

int level; // option for socket or protocol?

int optionid; // which specific option?

void *optionval; // option value

int *len; // length of the option value

setsockopt(sd, level, optionid, optionval, len); // set option // value

Socket Options for Servers

• The listen system call:int sd; // socket descriptor

int length; // length of request queue

listen(sd, length) // set socket request queue

// length

• Can only be used for SOCK_STREAM sockets

Servers: Accepting Connections

• The accept system call:int sd; // socket descriptorstruct sockaddr *name; // address of clientint *len; // length of address struct

newsock = accept(sd, addr, len) // accept connection

• A new socket is created that connects to the client• Server handles request, sends reply, closes newsock

Servers That ProvideMultiple Services

• The select system call:int ndesc; // check descriptors 0...ndesc-1void *indesc; // descriptors to check for inputvoid *outdesc; // descriptors to check for outputvoid *excdesc; // descriptors to check for exceptionsint *timeout; // how long to wait for a connection

nready = select(ndesc, indesc, outdesc, excdesc, timeout)// determine which descriptors

are // ready for I/O

Servers That ProvideMultiple Services (cont)

• The select system call:

nready = select(ndesc, indesc, outdesc, excdesc, timeout)

• Returns the number of descriptors from the specified set that are ready for I/O

• A process can use select to communicate over more than one socket at a time

• Typically each socket provides a distinct service

Miscellaneous (Useful)System Calls

• The gethostname system call:

char *name; // buffer to store nameint length; // size of buffer in

bytes

gethostname(name, length) // get name of host

• Defined in <netdb.h> include file• Process can learn host it’s running on

Miscellaneous (Useful)System Calls (cont)

• The network byte order conversion routines:– Network-to-host (short), ntohs, convert a short

int from network byte order to host byte order– Network-to-host (long), ntohl, convert a long

int from network byte order to host byte order– Host-to-network (short), htons, convert a short

int from network byte order to host byte order– Host-to-network (long), htonl, convert a long

int from network byte order to host byte order

Miscellaneous (Useful)System Calls (cont)

• Testing htons:#include <stdio.h>#include <string.h>#include <netdb.h>

main(int argc, char **argv) { if (argc != 2) exit(-1); printf("%d\n",atoi(argv[1])); printf("%d\n",htons(atoi(argv[1])));

}

Miscellaneous (Useful)System Calls (cont)

• The gethostbyname system call:

struct hostent *h; // hostent structure

char *name; // host name

h = gethostbyname(name); // fill in hostent with // info about

name

Miscellaneous (Useful)System Calls (cont)

• The hostent structure (defined in <netdb.h>):

struct hostent {

char *h_name; /* official name of host */

char **h_aliases; /* alias list */

int h_addrtype; /* host address type */

int h_length; /* length of address */

char **h_addr_list; /* list of addresses from

name server */

OHCE Server Example

• Check that program is running on the proper host• Create a socket on which the server will receive

requests• Bind the socket to the well-known port for the

ohce service• Loop forever

– Receive request and process it– Send reply to port and machine specified in the client’s

request

OHCE Client Example

• Create a socket on which to receive the server’s reply

• Bind it to some port on the local machine

• Create a socket on which to send a request to the server

• Bind it to the port the server’s listening on

• Create a request message and send it to the server

• Wait for a reply and print out the result

Summary

• The client-server model is widely-used for application interaction over a TCP/IP internet– Server: a program that offers a service that can be reached over a

network• Examples: web server, file server

– Client: a program that requests a service from a server• Examples: ping, browser

• Application programs typically interact with the networking software by using sockets and system calls:– Socket – create a socket– Bind – bind a socket to a port– Read/write – get/put data in a socket– Etc.