Sockets And Socket Address Structure

Sockets And Socket Address Structure

Mohit Mittal1

1Assistant Professor,

Anand College of Engineering & Management, Kapurthala.

Tarun Bhalla2

2Assistant Professor,

Anand College of Engineering & Management, Kapurthala.

ISSN: 2278-0181

Vol. 1 Issue 8, October – 2012

Abstract:-In this paper specifies the concept of socket and socket address structures. In this, we have discussed how communication has been performed between two hosts and discussed the role of the sockets. Sockets address of IPv4 and IPv6 is defined. The socket function call is also included which consists of various main functions like socket, connect, listen, bind, accept and close. It consists of two types of servers which uses socket function.

Keywords:Introduction, socket and its address, socket function calls, TCP socket call, and servers.

I. Introduction: Java's socket model is derived from BSD (UNIX) sockets, introduced in the early 1980s for inter-process communication using IP, the Internet Protocol. The Internet Protocol breaks all communications into packets, finite-sized chunks of data which are separately and individually routed from source to destination. IP allows routers, bridges, etc. to drop packets–there is no delivery guarantee. Packet size is limited by the IP protocol to 65535 bytes. Of this, a minimum of 20 bytes is needed for the IP packet header, so there is a maximum of 65515 bytes available for user data in each packet.

Sockets are a means of using IP to communicate between machines, so sockets are one major feature that allows Java to interoperate with legacy systems by simply talking to existing servers using their pre- defined protocol. [1]

1. API: The application interface is the interface available to the programmer for using the communication protocols. The API is depends to the OS the programming language.

   We discuss the socket API. With sockets, the network connection can be used as a file. Network I/O is, however, more complicated than file I/O because:

   • Asymmetric. The connection requires the program to know which process it is, the client or the server.

   • A network connection that is connection- oriented is somewhat like opening a file. A connectionless protocol doesnt have anything like an open.

   • A network application needs additional information to maintain protections, for example, of the other process.

   • There are more parameters required to specify network connection than the file Input/Output.

     The parameters have different formats for different protocols.

   • The network interface must support different protocols. These protocols may use different- size variable for addresses and other fields. [3]

     Figure 1: Socket Interface. [2]

2. Socket Address Structure:A Character Recognition deal with the problem of reading offline handwritten character i.e. at some point in time (in mins, sec, hrs.) after it has been written. However recognition of unconstrained handwritten text can be very difficult because characters cannot be reliably isolated especially when the text is cursive handwriting. [2]

   /* Generic Socket Address Structure, length=16*/

   <sys/socket.h> Structsocketaddr

   {

   unit8_t sa_len;

   sa_family_tsa_family; /* address family:

   AF_XXX value */

   char sa_data[14] /*up to 14 types of protocol-

   specific address */

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   Vol. 1 Issue 8, October – 2012

   }; [3]

   /* Ipv4 Socket Address Structure, length=16*/

   <netinet/in.h> structin_addr

   {

   in_addr_ts_addr; /* 32-bit IPv4 address, network byte ordered */

   };

   structsockaddr_in

   {

   unit8_t sin_len; /* length of structure (16 byte) */

   sa_family_tsin_family; /*AF_INET*/ in_port_tsin_port; /* 16-bit TCP or UDP port number, network byte ordered */

   structin_addrsin_addr; /*32-bit Ipv4 address, network byte ordered */

   charsin_zero[8]; /* unused initialize to all zeroes */

   };

   /* Ipv6 Socket Address Structure, length=24*/

   <netinet/in.h> struct in6_addr {

   unit8_t s6_addr[16]; /* 128-bit Ipv6 address, network byte ordered */

   };

   #define SIN6_LEN /* required for compile- time tests */

   struct sockaddr_in6

   {

   unit8_t sin6_len; /* length of this structure (24byte) */

   sa_family_t sin6_family; /*AF_INET6*/ in_port_t sin6_port; /* 16-bit TCP or UDP port number, network byte ordered */

   }; [3]

3. Procedure of Socket Programming

   In order to communicate between two processes, the two processes must provide the formation used by ICP/IP (or UDP/IP) to exchange data. This information is the 5-tupe: {protocol, local-addr, local-process, foreign-addr and foreign-process}.

   Several network systems calls are used to specify this information and use the socket. [3]

   Figure 2: Difference Between TCP and UDP [3]

4. SOCKET FUNCTION CALLING

   #include <sys/types.h>

   #include <sys/socket.h>

   Socket Function

   int socket (int family, int type, int protocol);

   Family: specifies the protocol family {AF_INET for TCP/IP}

   Type: indicates communications semantics SOCK_STREAM stream socket TCP SOCK_DGRAM datagram socket UDP SOCK_RAW raw socket

   Protocol: set to 0 except for raw sockets

   Returns on success: socket descriptor {a small nonnegative integer}

   On error: -1

   if (( sd= socket (AF_INET, SOCK_STREAM, 0)) < 0) err_sys(socket call error);

   Connect Function

   intconnect(intsockfd,conststructsockaddr*servaddr, socklen_taddrlen);

   sockfd: a socket descriptor returned by the socket function.

   *servaddr: a pointer to a socket address structure

   addrlen: the size of the socket address structure

   The socket address structure must contain the IP address and the port number for the connection wanted. In TCP connect initiates a three-way handshake. Connect returns only when the connection is established or when an error occurs.

   Returns on success: 0 on error: -1

   Example:

   if ( connect (sd, (structsockaddr*) &servaddr, sizeof(servaddr)) != 0)

   err_sys(connectcall error);

5. TCP SOCKET CALLS:

Bind Function

intbind (intsockfd, conststructsockaddr* myaddr, socklen_taddrlen);

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Bind assigns a local protocol address to aVsool.c1kIests.ue 8, October – 2012

Protocol address: a 32 bit IPv4 address and a 16 bit TCP or UDP port number.

sockfd: a socket descriptor returned by the socket function.

*myaddr: a pointer to a protocol-specific address.[2]

addrlen: the size of the socket address structure.

Servers bind their well-known port when they start. Returns on success: 0

On error : -1

Example:

If (bind (sd, (structsockaddr *) &servaddr ,sizeof (servaddr)) != 0)

errsys(bind call error);

Figure 3: TCP socket calls [2]

Listen Function

intlisten ( intsockfd, intbacklog );[2]

Listen is called only by a TCP server and performs two actions:

1. Converts an unconnected socket (sockfd) into a passive socket.

2. Specifies the maximum number of connections (backlog) that the kernel should queue for this socket.

Listen is normally called before the accept function.

Returns on success: 0 on error: -1 Example:

if (listen (sd, 2) != 0)

errsys(listen call error);

7. TWO TYPES OF SERVER

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Accept Function

intaccept ( intsockfd , structsockaddr*cliaddr, socklen_t*addrlen);

Accept is called by the TCP server to return the next completed connection from the front of the completed connection queue.

sockfd: This is the same socket descriptor as in listencall.

*cliaddr: used to return the protocol address of the connected peer process (i.e., the client process).[2]

*addrlen: {this is a value-result argument}

Before the accept call:We set the integer value pointed to by *addrlento the size of the socket address structure pointed to by *cliaddr;

on return from the accept call: This integer value contains the actual number of bytes stored in the socket address structure.

Returns on success: a new socket descriptor on error : -1

For accept the first argument sockfdis the listening socketand the returned value is the connected socket.

The server will have one connected socket for each client connection accepted.

When the server is finished with a client, the connected socket must be closed.

Example:

sfd= accept (sd, NULL, NULL);

if (sfd== -1) err_sys (accept error);[2]

Close Function

intclose (intsockfd);

Close marks the socket as closed and returns to the process immediately.

sockfd: This socket descriptor is no longer useable.

Note TCP will try to send any data already queued to the other end before the normal connection termination sequence.

Returns on success: 0 on error : -1

Example:

close (sd); [2]

Concurrent server forks a new process, so multiple clients can be handled at the same time.

Iterative server the server processes one request before accepting the next.

Concurrent Server listenfd = socket(); bind(listenfd,); listen(listenfd,)

for ( ; ; ) {

connfd = accept(listenfd, );

If (( pid = fork()) == 0) { /* child*/ close(listenfd);

/* process the request */ close(connfd);

exit(0);

}

close(connfd); /* parent*/

}[3]

Iterative Server listenfd = socket(); bind(listenfd,); listen(listenfd,)

for ( ; ; ) {

connfd = accept(listenfd, );

/* process the request */ close(connfd);

}[3]

Client

sockfd = socket(); connect(sockfd, )

/* process the request */ close(sockfd);[3]

CONCLUSION

In this paper ,we have concluded that the socket interface generally holds the communication between the user and the kernel. Also the two different applications can be interfaced through the communication network. In TCP socket calls, the client sends the request to the server and the server performs all the functions i.e. socket(), bind(), listen() and accept(). In concurrent servers, multiple clients can be handledat the same time, whereas in iterative server, the server processes only one request before accepting the next one. The Internet Protocol breaks all communications into packets, finite-sized chunks of data which are separately and individually routed from source to destination.

REFERENCES

1. Elementary TCP Sockets UNIX Network Programming Vol. 1, Second Ed.

   Stevens Chapter 4

   http://www.cs.usfca.edu/~parrt/doc/java/Sockets- notes.pdf

2. UNIX Network Programming by W. Richard Stevens, Prentice Hall,

Englewood Cliffs, NJ, 1997.

http://web.cs.wpi.edu/~rek/Undergrad_Nets/B 06/TCP_Sockets.pdf

4.http://www.ece.eng.wayne.edu/~gchen/ece56 50/lecture7.pdf

ISSN: 2278-0181

Vol. 1 Issue 8, October – 2012

Mohit Mittal received his B.Tech and M.Tech degree in Computer Science from Guru Nanak Dev University, in 2011. He is

working as Assistant Professor in Anand college of Engineering and Management, Kapurthala. His research areas include image processing, computer networks and Network Security.

Tarun Bhalla received his B.Tech degree in Computer Science from Punjab Technical University. He is currently working as a Assistant Professor in Anand College of

Engineering and Management, Kapurthala. His research interest area includes Database, Network Security, Mobile Computing and adhoc network .