Akn CN 6 CS 8 Socket Programming

8/9/2019 Akn CN 6 CS 8 Socket Programming

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Computer networking/socket/naj/1

Network ProgrammingTCP/IP Socket Programming

BSD SocketsAjit K Nayak

Department of CSEA,

SILICON, Bhubaneswar


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What is network program

Communicating among processes

The processes may reside

in a single computer or in computers connected to a network or

Computers across a network Client server computing

Request

Response

ServerClient


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Examples ClientExamples Client--ServerServer

Web client Web server

(iexplorer, netscape

Navigator, mozzila, . . . ) (IIS, Apache) FTP Client FTP server

Telnet Client Telnet server . . .

These are all precompiled programs andmay be used to communicate across a

network.


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Defining a Socket?

It is a two way, full duplex communication channel

An interface between application and the network

i.e.The application can send/receive data to/fromthe network via sockets

In UNIX terminology you can say that it is also a

file, Like file descriptor it is also recognized by asocket descriptor( a 16 bit integer)

Sockets are always created in pairs and they areused to communicate with other sockets

There are many kinds of sockets like Internet

sockets, UNIX sockets, . . .


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SocketsSockets

The socket is the BSD method foraccomplishing interprocess communication

(IPC) in a network. Thats why also called BSD sockets.

There are two important types of internetsockets

Stream socket, -- Datagram socket

Other type of internet sockets are

Raw socket, -- Sequence packet

We will concentrate on stream and datagram soc.


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Essential types of sockets

SOCK_DGRAM UDP

unreliable delivery

no order guarantees

no notion of connection

can send or receive

SOCK_STREAM

TCP

reliable delivery

in-order guaranteed

connection-oriented

bidirectional

App

socket3 2 1

Dest.

App

socket3 2 1 Dest


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Stream Oriented Comm.Stream Oriented Comm.

Server Client

Socket

Write

bind

listen

Read

Connect

Write

Accept

Read ..

Create a Socket

Bind it to a local port

Listen for a incoming

connection request

Connect from a client

Accept the request

Talk to each other

Close the socket

Close Close.


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How to ProgramHow to Program

We will use Socket API with C programmingin Linux environment

Socket API contains many data structures,functions and system calls which will help

us to write good and efficient network

programs

Requirement is any PC loaded with Linux, C

and Socket API

It is also possible to program in other

environments like windows and other UNIXsystems?


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Byte ordersByte orders

Address A+1 Address A

Increasing memoryaddress

Address A Address A+1Increasing memory

address

MSB 16-bit value LSB

High-order byte Low-order byteLittle-endian byte order

High-order byte Low-order byteBig-endian byte order


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Byte-ordering

Problem: different machines / OSs use different word

orderings

little-endian: lower bytes first

big-endian: higher bytes first

these machines may communicate with one anotherover the network

128.119.40.12

128 119 40 12

Big-Endianmachine 12.40.119.128

128 119 40 12

Little-Endianmachine


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IPV4 Socket Add StructureIPV4 Socket Add Structure

struct sockaddr_in{

uint8_t sin_len;

sa_family_t sin_family;in_port_t sin_port;struct in_addr sin_addr;

char sin_zero[8];}; //datatypes are as ofposix.1gsin_len: length of structure?

included in 4.3 BSD-RenoSa_family_t: address family ofsocket address structure

k dd


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sin_family: socket address familyAF_INET IPv4 family

AF_INET6 IPv6AF_LOCAL UNIX domain

AF_ROUTE routing socket

AF_KEY key socket

in_port_t: 16 bit unsigned integer

sin_port: port no of TCP or UDP

This number is used to identify a particular

server program

P


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Ports

Each host has 65,536ports

Some ports arereserved for specificapps called as IANAports or reserved ports,(0 1024) 20,21: FTP

23: Telnet

80: HTTP

1024 49151 arecalled IANA registeredports

Port 0

Port 1

Port 65535

Others are calledephemeral ports( IANA is

silent about it)A socket provides an

interface to address

IP:port pair

P 4 k ddIPV4 S k Add S


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The sin_zero member is unused, but it is setto zero ?

The length member was added in 4.3 BSD-Reno, when support for OSI was added

Length of structure 1+1+2+4+8 = 16 bytes

Socket address structures are used only on

a given host, the structure itself is not

communicated

But some fields like IP and port are used for

communication

C llS t C ll


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System CallsSystem Calls

Creating a socket#include

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

Family Description

AF_INET IPv4 protocols

AF_INET6 IPv6 protocols

AF_LOCALS Unix domain protocols

AF_ROUTE Routing sockets

AF_KEY Key socket

S C ll (2)S t C ll (2)


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System Calls(2)System Calls(2)

type: communication typetype Description

SOCK_STREAM stream socketSOCK_DGRAM datagram socket

SOCK_RAW raw socket

protocol: specifies protocol, usually set to 0except for raw sockets (so that socket choose

correct protocol based on type) Note:

not all combination of socket family and type are valid.

S t C ll (3)S t C ll (3)


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YES are valid but do not have acronyms

There are also various other values for family andtype arguments

On success the socket function returns a non-

negative integer, called socket descriptor This call is used by both server and client

sockfd=socket(AF_INET, SOCK_STREAM,0);

AF_INET AF_LOCAL AF_LOCAL AF_ROUTE AF_KEY

SOCK_STREAM TCP TCP YES

SOCK_DGRAM UDP UDP YES

SOCK_RAW IPv4 IPv6 YES YES

S t C ll (4)S t C ll (4)


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#includeint bind(int sockfd, const struct sockaddr *myaddr,

socklen_t addrlen);

bind assigns a local protocol address to asocket

Local protocol address consists of IPaddress along with a port number

Second argument is a pointer(?) to addressstructure

Third argument is the length(size) of

address structure (32-bit integer)

S t C ll (5)S st C lls(5)


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With TCP, calling bind lets us specify a portnumber, an IP address, both or neither.

Process specifies Result

IP address Port

wildcard 0 Kernel chooses IP address andport

wildcard nonzero Kernel chooses IP address,

process specifies port

Local IP 0 Process specifies IP address,

Kernel chooses port

Local IP nonzero Process specifies IP address,and port

S t C ll (5)S st m C lls(5)


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serv_addr.sin_addr.s_addr=htonl(INADDR_ANY)/* wild card */

serv_addr.sin_port=0;bind(sockfd, (struct sockaddr *)&serv_addr,

sizeof(serv_addr);

It returns 0 if OK or 1 on error

Normally a TCP client does not bind an IP

address to its socket. The Kernel choosesthe source IP when socket is connected,

based on outgoing interface.

B t d i f ti sByte ordering functions


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Byte ordering functionsByte ordering functions

#include

uint32_t htonl(uint32_t hostlong); it converts the long

integerhostlong from host byte order to network byte

order.

uint16_t htons(uint16_t hostshort); it converts the short

integerhostshort from host byte order to network byte

order.

Both returns value in network byte order

uint32_t ntohl(uint32_t netlong); it converts the longintegernetlong from network byte order to host byte order.

uint16_t ntohs(uint16_t netshort); it converts the short

integernetshort from network byte order to host byte order. Both returns values in host byte order



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#includeint listen(int sockfd, int backlog);

The listen function converts an unconnectedsocket into a passive socket, indicating thatthe kernel sould accept incoming connection

requests directed to this socket The call to listen moves the socket from the

CLOSED state to LISTEN state

The second argument specifies themaximum number of connection that the

kernel should queue for this socketlisten(sockfd,5);



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#includeint connect(int sockfd, const struct sockaddr

*servaddr, socklen_t addrlen);

Used by a client to connect to a server.

The connect function initiates TCPs three-

way handshake(?). The function returns only when the

connection is established(0) or when anerror occurs(-1)

If the connect fails, the socket is no longer

usable and must be closed.


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Errors with connectionErrors with connection


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Errors with connectionErrors with connection

If client receives no response to its SYN segment,ETIMEDOUT is returned. (SYN is sent in every 6sec and another 24 sec) If no response is received

after a total of 75 secs, the error isreturned.(4.4BSD)

if server response is RST, it indicates no

process(port) is waiting for connection andECONNREFUSED is returned (hard error)

If clients SYN elicits an ICMP destination

unreachable from intermediate router, kernelsaves the message but keeps sending SYNs till 75sec. After which kernel returns EHOSTUNREACH

orENETUNEECH.(soft error)



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#includeint accept(int sockfd, struct sockaddr *cliaddr,

socklen_t *addrlen);

It is called by TCP server to return the next completedconnection from the front of the connection queue.

If the completed connection queue is empty, the

process is put to sleep The cliaddr and addrlen arguments are used to return

the protocol address of the connected peer process.

The addrlen contains the sizeof client addressstructure and on return it contains the actual no ofbytes stored by the kernel in the socket address

structure



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If accept is successful, it returns a new descriptorthat was automatically created by the kernel.

This new descriptor refers to the TCP connection

with the client Now first one is called the listening socket and the

second one is called the connected socket

A given server normally creates only one listeningsocket, which exists for the life time of the server.

The kernel creates one connected socket for eachclient connection that is accepted.

When server is finished serving a client,connected socket for that client is closed



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addrlen = sizeof(cliaddr);connfd=accept(sockfd, (struct sockaddr *)

&cliaddr, &addrlen);

This function gives up to three values

An integer return, that is either a new socket

descriptor or an error indication(-1) The protocol address of the client process

(through cliaddr) and

Size of this address (through addrlen)

If protocol address is not required then both

cliaddr and addrlen is set to NULL;



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#includeint close(int sockfd);

The default action is to mark the socket as closed

and return to process

The socket descriptor is no 0longer usable by the

process; It cannot be used as an argument to reador write

But TCP will try to send any data that is already

queued to be sent to other end, and after this thenormal TCP connection termination sequence

takes place.( 4-way handshaking?)

Network I/ONetwork I/O-- ReadingReading


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Network I/ONetwork I/O- ReadingReading

Connection orientedint read(int sockfd, char *buf, int nbytes);

It reads nbytes(max) from socket to the buffer

Returns no of bytes read successfully from the socket

and 1 on error

Connection lessint recvfrom(int sockfd, char *buf, int nbytes, int flag,

struct sockaddr *from, int addrlen);

It receives nbytes(max) from a socket, whose address isgiven by the from address structure to the buffer

Returns no of bytes read successfully from the socket

and 1 on error

Network I/ONetwork I/O-- WritingWriting


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Network I/ONetwork I/O WritingWriting

Connection orientedint write(int sockfd, char *buf, int nbytes);

It writes n bytes(max) to the socket from buffer

Returns no of bytes written successfully to the socket

and 1 on error

Connection lessint sendto(int sockfd, char *buf, int nbytes, int flag,

struct sockaddr *to, int addrlen);

It sends nbytes(max) to a socket, whose address isgiven by the to address structure from the buffer

Returns no of bytes sent successfully from the socket

and 1 on error

ReferencesReferences


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ReferencesReferences

W. Richard Stevens, UNIX network programming,vol 1, PE

Beejs Guide to Network Programming using

internet socketswww.ecst.csuchioco.edu/~beej/guide/net

Sockets Progrmming,

http://www.seit.wlv.ac.uk/~jphb/comm/sockets.html Introduction to network functions in C

http://shoe.bocks.com/net

Unix-socket-faq for network programmingftp://rtfm.mit.edu/pub/usenet/news.answers/unix-faq
http://www.ecst.csuchioco.edu/~beej/guide/nethttp://www.seit.wlv.ac.uk/~jphb/comm/sockets.htmlhttp://shoe.bocks.com/netftp://rtfm.mit.edu/pub/usenet/news.answers/unix-faqftp://rtfm.mit.edu/pub/usenet/news.answers/unix-faqhttp://shoe.bocks.com/nethttp://www.seit.wlv.ac.uk/~jphb/comm/sockets.htmlhttp://www.seit.wlv.ac.uk/~jphb/comm/sockets.htmlhttp://www.ecst.csuchioco.edu/~beej/guide/nethttp://www.ecst.csuchioco.edu/~beej/guide/net


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Akn CN 6 CS 8 Socket Programming

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