Socket programming. What is a socket? goal: learn how to build client/server applications that communicate using sockets socket: door between application.

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

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Internet

controlledby OS

controlled byapp developer

transport

application

physical

link

network

process

transport

application

physical

link

network

processsocket

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

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

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

closeclientSocket

read datagram fromclientSocket

create socket:clientSocket =socket(AF_INET,SOCK_DGRAM)

Create datagram with server IP andport=x; send datagram viaclientSocket

create socket, port= x:serverSocket =socket(AF_INET,SOCK_DGRAM)

read datagram fromserverSocket

write reply toserverSocketspecifying 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 fromsocket into string

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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:

Client/server socket interaction: TCP

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wait for incomingconnection requestconnectionSocket =serverSocket.accept()

create socket,port=x, for incoming request:serverSocket = socket()

create socket,connect to hostid, port=xclientSocket = socket()

server (running on hostid) client

send request usingclientSocketread request from

connectionSocket

write reply toconnectionSocket

TCP connection setup

closeconnectionSocket

read reply fromclientSocket

closeclientSocket

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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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Example app: TCP server

from socket import *serverPort = 12000serverSocket = 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 welcomingsocket

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)

Socket Creation in C int s = socket(domain, type, protocol);

s: socket descriptor, an integer (like a file-handle) domain: integer, communication domain

e.g., PF_INET (IPv4 protocol) – typically used type: communication type

SOCK_STREAM: reliable, 2-way, connection-based service SOCK_DGRAM: unreliable, connectionless, other values: need root permission, rarely used, or obsolete

protocol: specifies protocol (see file /etc/protocols for a list of options) - usually set to 0

NOTE: socket call does not specify where data will be coming from, nor where it will be going to - it just creates the interface. 18

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

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

Client1socket

Client2socket

From: Dan Rubenstein’s slides: http://www.cs.columbia.edu/~danr/courses/6761/Summer03/intro/6761-1b-sockets.ppt

Server Socket: listen & accept

Called on server side:

int status = listen(sock, queuelen); status: 0 if listening, -1 if error sock: integer, socket descriptor queuelen: integer, # of active participants that can “wait”

for a connection listen is non-blocking: returns immediately

int s = accept(sock, &addr, &addrlen); s: integer, the new socket (used for data-transfer) sock: integer, the orig. socket (being listened on) addr: struct sockaddr, address of the active participant addrlen: sizeof(addr): value/result parameter

must be set appropriately before call adjusted by OS upon return

accept is blocking: waits for connection before returning 25

connect

int status = connect(sock, &addr, addrlen); status: 0 if successful connect, -1 otherwise sock: integer, socket to be used in connection addr: struct sockaddr: address of passive participant addrlen: integer, sizeof(addr)

connect is blocking

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Sending / Receiving Data int count = send(sock, &buf, len, flags);

count: # bytes transmitted (-1 if error) buf: void*, buffer to be transmitted len: integer, length of buffer (in bytes) to transmit flags: integer, special options, usually just 0

int count = recv(sock, &buf, len, flags); count: # bytes received (-1 if error) buf: void*, stores received bytes len: # bytes received flags: integer, special options, usually just 0

Calls are blocking [returns only after data is sent (to socket buf) / received] 27

close

When finished using a socket, the socket should be closed:

status = close(s); status: 0 if successful, -1 if error s: the file descriptor (socket being closed)

Closing a socket closes a connection frees up the port used by the socket

28From: Dan Rubenstein’s slides: http://www.cs.columbia.edu/~danr/courses/6761/Summer03/intro/6761-1b-sockets.ppt

The struct sockaddr

The struct to store the Internet address of a host:

struct sockaddr_in {short sin_family;u_short sin_port;struct in_addr sin_addr;char sin_zero[8];

};

sin_family = AF_INET // Specifies the address family sin_port: // Specifies the port #(0-65535) sin_addr: // Specifies the IP address sin_zero: unused // unused!

29From: Dan Rubenstein’s slides: http://www.cs.columbia.edu/~danr/courses/6761/Summer03/intro/6761-1b-sockets.ppt

SOCKADDR Example

struct sockaddr_in server; // Definition

memset(&server, 0, sizeof(server)); // Initilize to 0

server.sin_family = AF_INET; // Set address family

server.sin_port = htons(MYPORTNUM); // Set port

server.sin_addr.s_addr = htonl(INADDR_ANY);// Set address

Host Byte-Ordering: the byte ordering used by a host (big-endian or little-endian)

Network Byte-Ordering: the byte ordering used by the network – always big-endian

Any words sent through the network should be converted to Network Byte-Order prior to transmission (and back to Host Byte-Order once received) 30

Network Byte-Ordering

u_long htonl(u_long x);

u_short htons(u_short x);

u_long ntohl(u_long x);

u_short ntohs(u_short x);

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• On big-endian machines, these routines do nothing• On little-endian machines, they reverse the byte order

128.119.40.12

128

119

40 12

128.119.40.12

128

119

40 12

Big-Endianmachine

Little-Endianmachine

hton

l nto

hl

128

119

40 12 128

119

4012

From: Dan Rubenstein’s slides: http://www.cs.columbia.edu/~danr/courses/6761/Summer03/intro/6761-1b-sockets.ppt

TIPS 1• Sometimes, an ungraceful exit from a program (e.g., ctrl-c) does

not properly free up a port

• Eventually (after a few minutes), the port will be freed

• You can kill the process, or

• To reduce the likelihood of this problem, include the following code:• In header include:

#include <signal.h>

void cleanExit(){exit(0);}

• In socket code:

signal(SIGTERM, cleanExit);

signal(SIGINT, cleanExit); 32

Tips 2

Check Beej's Guide to Network Programming Using Internet Sockets http://beej.us/guide/bgnet/output/html/multipage/index.html

Search the specification for the function you need to use for more info, or check the man pages.

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Tips 3

How to find the IP address of the machine my server program is running on?

Use 127.0.0.1 or localhost for accessing a server running on your local machine.

For a remote server running linux use the bash shell command: “$ /sbin/ifconfig”

For windows, use ipconfig in cmd

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summary

application architectures client-server P2P

application service requirements: reliability, bandwidth, delay

Internet transport service model connection-oriented, reliable: TCP unreliable, datagrams: UDP

our study of network apps now complete!

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specific protocols: HTTP FTP SMTP, POP, IMAP DNS P2P: BitTorrent,

DHT socket programming:

TCP, UDP sockets

typical request/reply message exchange: client requests info or

service server responds with data,

status code

message formats: headers: fields giving info

about data data: info being

communicated

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important themes: control vs. data msgs

in-band, out-of-band centralized vs.

decentralized stateless vs. stateful reliable vs. unreliable

msg transfer “complexity at

network edge”

summarymost importantly: learned about

protocols!