tcp ip sockets

8/2/2019 TCP IP Sockets

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University of Cyprus

Department ofComputer Science

By: Dr. Vasos VassiliouSpring 2009

EPL375: Advanced NetworksTCP/IP Socket Programming

in C


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Overview

Introduction

TCP/IP Basic

UNIX/C Sockets

UDP Sockets

TCP Sockets

Utility Functions

Other Socket API

Summary


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Internet Protocol (IP)

Datagram (packet) protocol

Best-effort service

Loss

Reordering

Duplication Delay

Host-to-host delivery(not application-to-application)


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IP Address

32-bit identifier

Identifies a host interface (not a host)


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Transport Protocols

Best-effort is not sufficient !!!

Add services on top of IP

User Datagram Protocol (UDP) Data checksum

Best-effort

Transmission Control Protocol (TCP) Data checksum

Reliable byte-stream delivery

Flow and congestion control


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Identifying the ultimate destination

IP addresses identify hosts

Host has many applications

Ports (16-bit identifier)

194.42.16.25

Port 80 25 23

Application WWW E-mail Telnet

Ports


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TCP/IP Byte Transport

TCP/IP protocols transports bytes

Application protocol provides semantics

Application

TCP/IP

byte stream

Application

TCP/IP

byte stream

Here are somebytes. I dont

know whatthey mean.

Ill passthese tothe app.It knowswhat to

do.


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Socket

How does one speak TCP/IP?

Sockets provides interface to TCP/IP

Generic interface for many protocols


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Socket Types


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Socket

Identified by protocol and local/remote address/port

Applications may refer to many sockets

Socket abstraction Berkeley Sockets (traditional)

Generic

multiple families address representation independence)

Uses existing I/O programming interface as much as possible

Sockets work with Unix I/O services just like files,pipes

Sockets have special needs: establishing a connection specifying communication endpoint addresses


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TCP/IP Sockets

Family Type Protocol

TCPPF_INET

SOCK_STREAM IPPROTO_TCP

UDP SOCK_DGRAM IPPROTO_UDP

mySock = socket(family, type, protocol);

TCP/IP-specific sockets

Socket reference

File (socket) descriptor in UNIX

Socket handle in WinSock


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struct sockaddr

{

unsigned short sa_family; /* Address family (e.g., AF_INET) */

char sa_data[14]; /* Protocol-specific address information */

};

struct sockaddr_in

{

unsigned short sin_family; /* Internet protocol (AF_INET) */

unsigned short sin_port; /* Port (16-bits) */

struct in_addr sin_addr; /* Internet address (32-bits) */

char sin_zero[8]; /* Not used */

};

struct in_addr

{

unsigned long s_addr; /* Internet address (32-bits) */

};

Generic

IP

Specific

sockaddr

sockaddr_in

Family

Family Port

Blob

Internet address Not used

2 bytes 2 bytes 4 bytes 8 bytes


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Client: Initiates the connection

Server: Passively waits to respond

Clients and Servers

Client: Bob

Hi. Im Bob.

Nice to meet you, Jane.

Server: Jane

Hi, Bob. Im Jane


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Functions needed

Specify local and remote communication endpoints

Initiate a connection

Wait for incoming connection

Send and receive data

Terminate a connection gracefully Error handling


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TCP Client/Server Interaction

Client

1. Create a TCP socket

2. Establish connection

3. Communicate

4. Close the connection

Server

1. Create a TCP socket2. Assign a port to socket

3. Set socket to listen

4. Repeatedly:

a. Accept new connectionb. Communicate

c. Close the connection

Server starts by getting ready to receive client connections


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Creating a Socket

int socket(int family, int type, int proto)

system call returns a socket descriptor(small integer); -1on error

allocates resources needed for a communicationendpoint;

does not deal with endpoint addressing family specifies the protocol family

AF_INET for TCP/IP

type specifies the type of service(communication)

SOCK_STREAM, SOCK_DGRAM

protocol specifies the specific protocol

usually 0 which means the default


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Declaration for socket function


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Specifying an Endpoint Address

Remember that the sockets API is generic

There must be a generic way to specify endpoint addresses

TCP/IP requires an IP address and a port number for eachendpoint address.

Other protocol suites(families) may use other schemes.

Generic socket addresses(The C function that make up the sockets APIexpect structures of type sockaddr.) :struct sockaddr {

unsigned short sa_family; //specifies the address type

char sa_data[14]; //specifies the address value

};


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Assigning an address to a socket

int bind(int socket, (struct sockaddr *) address,

int address_length); The bind() system call is used to assign an address to an

existing socket.

bind returns 0 if successful or -1 on error

Example:struct sockaddr_in sin;int s;s = socket(AF_INET, SOCK_DGRAM, 0);

sin.sin_family = AF_INET;sin.sin_port = htons(9999);sin.sin_addr.s_addr = INADDR_ANY;bind(s, (struct sockaddr *)&sin, sizeof(sin));


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listen() connect() -- TCP Sockets

int listen(int socket, int qlength)

allows servers to prepare a socket for incoming connections puts the socket in a passive mode ready to accept connections

informs the OS that the protocol software should enqueue multiplesimultaneous requests that arrive at the socket

applies only to sockets that have selected reliable stream delivery

service

int connect(int, struct sockaddr *dest_addr, int);

binds a permanent destination to a socket

changes the socket state from unconnected stateto connected state

The semantics of connect depend on the underlaying protocol TCP connection (AF_INET, reliabe stream delivery service)

just store the destination address locally (connectionless)


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accept() -- TCP Sockets

int accept(int, struct sockaddr * addr, int *

addrlen) needs to wait for a connection

blocks until a connection request arrives

addrlen is a pointer to an integer; when a request arrives , the system fills in argument addr withthe address of the client that has placed the request and setsaddrlen to the length of the address.

system creates a new socket, returns the new socket descriptor


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send() and recv() -- TCP Sockets

int send(int s, const char *msg, int len, int flags)

connected socket

argument flags controls the transmission.

allows the sender to specify that the message should be sentout-of- band messages correspond to TCPs urgent data

allows the caller to request that the message be sent withoutusing local routine tables (take control of routine)

int recv(int s, char *buf, int len, int flags)

connected socket

argument flags allow the caller to control the reception

look ahead by extracting a copy of the next incoming messagewithout removing the message from the socket


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close() and shutdown()

close(int socket)

For UDP sockets, this will release the ownership on thelocal port that is bound to this socket

For TCP, this will initiate a two-way shutdown betweenboth hosts before giving up port ownership.

shutdown(int socket, int how)

f the how field is 0, this will disallow further reading(recv) from the socket.

If the how field is 1, subsequent writes (send) will bedisallowed. The socket will still need to be passed toclose.

Relationship Between Sockets and


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Relationship Between Sockets andFile Descriptors Socket handles are integer values. In UNIX, socket handles

can be passed to most of the low-level POSIX I/Ofunctions.

read(s, buffer, buff_length); //s could be a file descriptor too

write(s, buffer, buff_length) ;

Calling read on an open socket is equivalent to recv if the socket is UDP, then information about the sender of the

datagram will not be returned

Similarly the write function call is equivalent to send

UDP sockets may call connect to use send and write

use the socket library functions instead of the file I/Oequivalents.

Socket interface for Connectionless


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Socket interface for ConnectionlessIterative Server


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Client1. Create a TCP socket


3. Communicate


Server1. Create a TCP socket

2. Bind socket to a port


4. Repeatedly:

a. Accept new connection

b. Communicate


/* Create socket for incoming connections */if ((servSock = socket(PF_INET, SOCK_STREAM, IPPROTO_TCP)) < 0)

DieWithError("socket() failed");


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echoServAddr.sin_family = AF_INET; /* Internet address family */

echoServAddr.sin_addr.s_addr = htonl(INADDR_ANY);/* Any incoming interface */echoServAddr.sin_port = htons(echoServPort); /* Local port */

if (bind(servSock, (struct sockaddr *) &echoServAddr, sizeof(echoServAddr)) < 0)DieWithError("bind() failed");



3. Communicate





4. Repeatedly:


b. Communicate



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/* Mark the socket so it will listen for incoming connections */if (listen(servSock, MAXPENDING) < 0)

DieWithError("listen() failed");



3. Communicate





4. Repeatedly:


b. Communicate



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for (;;) /* Run forever */

{clntLen = sizeof(echoClntAddr);

if ((clntSock=accept(servSock,(struct sockaddr *)&echoClntAddr,&clntLen)) < 0)DieWithError("accept() failed");



3. Communicate





4. Repeatedly:


b. Communicate



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Server is now blocked waiting for connection from a client

Later, a client decides to talk to the server



3. Communicate





4. Repeatedly:


b. Communicate



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/* Create a reliable, stream socket using TCP */if ((sock = socket(PF_INET, SOCK_STREAM, IPPROTO_TCP)) < 0)

DieWithError("socket() failed");



3. Communicate





4. Repeatedly:


b. Communicate



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echoServAddr.sin_family = AF_INET; /* Internet address family */echoServAddr.sin_addr.s_addr = inet_addr(servIP); /* Server IP address */echoServAddr.sin_port = htons(echoServPort); /* Server port */

if (connect(sock, (struct sockaddr *) &echoServAddr, sizeof(echoServAddr)) < 0)DieWithError("connect() failed");



3. Communicate





4. Repeatedly:


b. Communicate



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if ((clntSock=accept(servSock,(struct sockaddr *)&echoClntAddr,&clntLen)) < 0)DieWithError("accept() failed");



3. Communicate





4. Repeatedly:


b. Communicate



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echoStringLen = strlen(echoString); /* Determine input length */

/* Send the string to the server */if (send(sock, echoString, echoStringLen, 0) != echoStringLen)

DieWithError("send() sent a different number of bytes than expected");



3. Communicate





4. Repeatedly:


b. Communicate



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/* Receive message from client */if ((recvMsgSize = recv(clntSocket, echoBuffer, RCVBUFSIZE, 0)) < 0)

DieWithError("recv() failed");



3. Communicate





4. Repeatedly:


b. Communicate



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close(sock); close(clntSocket)



3. Communicate





4. Repeatedly:


b. Communicate



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TCP Tidbits

Clientsend(Hello Bob)

recv() -> Hi Jane

Server

recv() -> Hello

recv() -> Bob

send(Hi )

send(Jane)

Client must know the servers address and port

Server only needs to know its own port

No correlation between send() and recv()


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Closing a Connection

close() used to delimit communication

Analogous to EOF

Echo Clientsend(string)

while (not received entire string)

recv(buffer)

print(buffer)

close(socket)

Echo Server

recv(buffer)

while(client has not closed connection

send(buffer)

recv(buffer)

close(client socket)


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Byte-order Transformations

Byte ordering is a function of machine architecture

Intel: little-endian Sparc, PowerPC: big-endian

Network order: big-endian

The byte order for the TCP/IP protocol suite is bigendian.


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Big-endian byte order


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Little-endian byte order


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Network Byte Order Functions


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Network Byte Order Functions

Functions:

u_long m =ntohl(u_long m) network-to-host byte order, 32 bit

u_long m =htonl(u_long m)

host-to-network byte order, 32 bit

ntohs(),htons() short (16 bit)

Example:

struct sockaddr_in sin;sin.sin_family = AF_INET;

sin.sin_port = htons(9999);

sin.sin_addr.s_addr = inet_addr;


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Address transformation


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unsigned int inet_addr(char *str)

str represents an IP address(dotted-quad notation); inet_addr willreturn it's equivalent 32-bit value in network byte order.

This value can be passed into the sin_addr.s_addr field of asocketaddr_in structure

-1 is returned if the string can not be interpreted

char *inet_ntoa(struct in_addr ip) Converts the 32-bit value which is assumed to be in network byte

order and contained in ip to a string

The pointer returned by inet_ntoa contains this string. However,subsequent calls to inet_ntoa will always return the same pointer, socopying the string to another buffer is recommended before callingagain.

Address transformation


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Obtaining Information About Hosts, etc.

struct hostent *hptr; /*includes host address in

binary*/hptr=gethostbyname(char *name);

Ex.:gethostbyname(www.csc.ncsu.edu);

Struct hostent *hptr;

hptr=gethostbyaddr(char *addr,int addrlen, intaddrtype);

Ex: gethostbyaddr(&addr, 4, AF_INET);
http://www.csc.ncsu.edu/http://www.csc.ncsu.edu/


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Obtaining Information About Hosts, etc.

int inet_addr(char *dotdecimal);

Ex.: sin_addr = inet_addr(152.14.51.129);

struct servent *sptr; /* includes port and protocol */

sptr=getservbyname(char *name, char *proto);

Ex.: getservbyname(smtp, tcp);

struct protoent *pptr; /* includes protocol number */

pptr=getprotobyname(char *name);Ex.: getprotobyname(tcp);


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Others Include files

#include ;

#include ;

#include ;

#include ;

#include ;

#include ;

#include ;

#include ;

#include ;

#include


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Summary

TCP/IP basic

UNIX/C Sockets

socket() ; bind() ; connect() ; listen() ; accept() ;sendto() ; recvfrom(); send() ; recv() ; read() ; write();

some utility functions

tcp ip sockets

Documents