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Socket

programming

Socket programming

What is a socket?

goal: learn how to build client/server applications that communicate using

sockets

socket: door between application process and end-end-transport protocol

2-2

Internet

controlled

by OS

controlled byapp developer

transport

application

physical

link

network

process

transport

application

physical

link

network

processsocket

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What is a socket?

Socket is an interface between application and network (the lower levels of the protocol stack) The application creates a socket

The socket type dictates the style of communication

reliable vs. best effort

connection-oriented vs. connectionless

Once a socket is setup the application can pass data to the socket for network transmission

receive data from the socket (transmitted through the network, received from some other host)

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Most popular types of sockets

• TCP socket

• Type: SOCK_STREAM

• reliable delivery

• in-order guaranteed

• connection-oriented

• bidirectional

UDP socket

Type: SOCK_DGRAM

unreliable delivery

no order guarantees

no notion of “connection” –app indicates destination

for each packet

can send or receive

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Ports

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• Each host machine has an IP address (or more!)

• Each host has 65,536 ports (2?)

• As you know some ports are reserved for specific apps

• 20,21: FTP

• 23: Telnet

• 80: HTTP

• see RFC 1700 (about 2000 ports are reserved)

Port 0

Port 1

Port 65535

A socket provides an interface to send data to/from the network through a port

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 (one difference: assume outgoing mail is placed by

you in your mailbox)

Q: How do you choose which port a socket connects to?

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Socket programming

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Application Example:1. Client reads a line of characters (data) from its

keyboard and sends the data to the server.2. The server receives the data and converts

characters to uppercase.3. The server sends the modified data to the client.4. The client receives the modified data and displays

the line on its screen.

Ports

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• Each host machine has an IP address (or more!)

• Each host has 65,536 ports (2?)

• As you know some ports are reserved for specific apps

• 20,21: FTP

• 23: Telnet

• 80: HTTP

• see RFC 1700 (about 2000 ports are reserved)

Port 0

Port 1

Port 65535

A socket provides an interface to send data to/from the network through a port

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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 (one difference: assume outgoing mail is placed by

you in your mailbox)

Q: How do you choose which port a socket connects to?

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Socket programming with UDP

UDP: no “connection” between client & server

no handshaking before sending data

sender explicitly attaches IP destination address and port # to each packet

rcvr extracts sender IP address and port# from received packet

UDP: transmitted data may be lost or received out-of-order

Application viewpoint:

UDP provides unreliable transfer of groups of bytes (“datagrams”) between

client and server

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Client/server socket interaction: UDP

close

clientSocket

read datagram from

clientSocket

create socket:

clientSocket =socket(AF_INET,SOCK_DGRAM)

Create datagram with server IP and

port=x; send datagram via

clientSocket

create socket, port= x:

serverSocket =socket(AF_INET,SOCK_DGRAM)

read datagram from

serverSocket

write reply to

serverSocket

specifying

client address,

port number

Application 2-11

server (running on serverIP) client

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Example app: UDP client

from socket import *

serverName = ‘hostname’

serverPort = 12000

clientSocket = socket(socket.AF_INET,

socket.SOCK_DGRAM)

message = raw_input(’Input lowercase sentence:’)

clientSocket.sendto(message,(serverName, serverPort))

modifiedMessage, serverAddress =

clientSocket.recvfrom(2048)

print modifiedMessage

clientSocket.close()

Python UDPClientinclude Python’s socket library

create UDP socket for

server

get user keyboardinput

Attach server name, port to

message; send into socket

print out received string

and close socket

read reply characters from

socket into string

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2-13

Example app: UDP server

from socket import *

serverPort = 12000

serverSocket = socket(AF_INET, SOCK_DGRAM)

serverSocket.bind(('', serverPort))

print “The server is ready to receive”

while 1:

message, clientAddress = serverSocket.recvfrom(2048)

modifiedMessage = message.upper()

serverSocket.sendto(modifiedMessage, clientAddress)

Python UDPServer

create UDP socket

bind socket to local port

number 12000

loop forever

Read from UDP socket into message, getting client’s address (client IP and port)

send upper case string

back to this client

Socket programming with TCP

client must contact server

server process must first be running

server must have created socket (door) that welcomes client’s contact

client contacts server by:

Creating TCP socket, specifying IP address, port number of server process

when client creates socket: client TCP establishes connection to server TCP

when contacted by client, server TCP creates new socket for server process to communicate with that particular client

allows server to talk with multiple clients

source port numbers used to distinguish clients

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TCP provides reliable, in-orderbyte-stream transfer (“pipe”) between client and server

application viewpoint:

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Client/server socket interaction: TCP

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wait for incoming

connection requestconnectionSocket =

serverSocket.accept()

create socket,port=x, for incoming

request:serverSocket = socket()

create socket,connect to hostid, port=x

clientSocket = socket()

server (running on hostid) client

send request using

clientSocketread request from

connectionSocket

write reply to

connectionSocket

TCP connection setup

close

connectionSocket

read reply from

clientSocket

close

clientSocket

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Example app: TCP client

from socket import *

serverName = ’servername’

serverPort = 12000

clientSocket = socket(AF_INET, SOCK_STREAM)

clientSocket.connect((serverName,serverPort))

sentence = raw_input(‘Input lowercase sentence:’)

clientSocket.send(sentence)

modifiedSentence = clientSocket.recv(1024)

print ‘From Server:’, modifiedSentence

clientSocket.close()

Python TCPClient

create TCP socket for

server, remote port 12000

No need to attach server

name, port

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2-17

Example app: TCP server

from socket import *

serverPort = 12000

serverSocket = socket(AF_INET,SOCK_STREAM)

serverSocket.bind((‘’,serverPort))

serverSocket.listen(1)

print ‘The server is ready to receive’

while 1:

connectionSocket, addr = serverSocket.accept()

sentence = connectionSocket.recv(1024)

capitalizedSentence = sentence.upper()

connectionSocket.send(capitalizedSentence)

connectionSocket.close()

Python TCPServer

create TCP welcoming

socket

server begins listening for

incoming TCP requests

loop forever

server waits on accept()for incoming requests, new socket created on return

read bytes from socket (but

not address as in UDP)

close connection to this

client (but not welcoming

socket)

The bind function

The bind function associates and (can exclusively) reserves a

port for use by the socket

int status = bind(sockid, &addrport, size);

status: error status, = -1 if bind failed

sockid: integer, socket descriptor

addrport: struct sockaddr, the (IP) address and port of the machine

(address usually set to INADDR_ANY – chooses a local address)

size: the size (in bytes) of the addrport structure

bind can be skipped for both types of sockets. When and

why?

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Skipping the bind

SOCK_DGRAM:

if only sending, no need to bind. The OS finds a port each

time the socket sends a pkt

if receiving, need to bind

SOCK_STREAM:

destination determined during conn. setup

don’t need to know port sending from (during connection

setup, receiving end is informed of port)

On the connecting end

• When connecting to another host (i.e., connecting end is the client and the receiving end is the server), the OS automatically assigns a free port for the outgoing connection.

• During connection setup, receiving end is informed of port)

• You can however bind to a specific port if need be.

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Connection Setup

A connection occurs between two ends

Server: waits for an active participant to request connection

Client: initiates connection request to passive side

Once connection is established, server and client ends

are “similar”

both can send & receive data

either can terminate the connection

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Server and clients

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socket()

bind()

listen()

accept()

write()

read()

read()

TCP Server

close()

socket()

TCP Client

connect()

write()

read()

close()

connection establishment

data request

data reply

end-of-file notification

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Connection setup steps

Server end:

step 1: listen (for incoming requests)

step 3: accept (a request)

step 4: send/recv

The accepted connection is on a new socket

The old socket continues to listen for other active participants

Client end:

step 2: request &

establish connection

step 4: send/recv

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Server

l-socka-sock-1 a-sock-2

Client1

socket

Client2

socket