CSCI 4311 COMPUTER NETWORKS

02: Sockets

Vassil Roussevvassil@cs.uno.edu

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A socket is the basic remote communication abstraction provided by the OS to processes.

controlledby operating

system

controlled byapplicationdeveloper

controlledby operating

system

controlled byapplicationdeveloper

process

Host (end system)

Internet

process

Host (end system)

TCP/IPstack

TCP/IPstack

Network access is managed by the OS. OS provides a socket API to allow network communication

socket socket

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Socket Programming using TCP service

processwrite

readbytes

bytes

processread

writesocket socket

Application Viewpoint TCP provides reliable, in-order transfer of

a stream of bytes between two end points (processes).

Internet

Same interface as other stream I/O

(files, pipes, etc.).

Pair of pipes abstraction

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read

write

TCP communication is based on the client/server concept.

write

readbytes

bytes

Welcomingsocket

TCP Client

3-way handshake

Connectionsocket

Clientsocket

TCP Server

Server is started first &waits for connection

requests.

Client initiates connection via 3-way handshake. Once established, connection

is completely symmetrical.

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A port number is a 16-bit number that uniquely identifies a network process on a host.

Server binds to adesired port number

(welcoming socket).Client must know port

number to reach server.

Port PID

0000 ---

0021

0022

0025

0080

0631

…

…

…

…

Most standard services

have a default port:File Transfer

Secure Shell

Simple Mail Transfer

World Wide Web

Print Service

Client sockets use any available port >1023.

ftpd

sshd

smtpd

httpd

cupsd

bind()

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Network Addressing for Sockets

A B socket connection: <IP-hostA, portA, IP-hostB, portB>

DNSHost name:cook.cs.uno.edu

Mnemonic, logical ID

IP address:137.30.120.32

Unique, routable ID

process

socket

Host (end system)

Internet

process

socket

Host (end system)

TCP/IPstack

TCP/IPstack

processprocess

socket

Host (end system)

Internet

processprocess

socket

Host (end system)

TCP/IPstack

TCP/IPstack

Local process IDshostA hostB

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The CapsLock example service converts strings sent by clients to upper case.Protocol:

Client: Gets a line of text from the user via the system’s standard input

Client: Sends the line to the server over the network

Server: Receives the line from the clientServer: Converts it to uppercaseServer: Sends back the uppercase line

Client: Receives uppercase line from serverClient: Displays it via the system’s standard output

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Stream View of the Problem (Client)

Flash reviewInput stream:

– Sequence of bytes going into a process– E.g., from keyboard, file, socket

Output stream: – Sequence of bytes coming out of a process– E.g., to display, file, socket

Clientprocess

Std input

Clientsocket

inFromServer

outToServer

Standard output

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Client/Server Socket Interaction: TCP

Wait for incomingconnection requestconnectionSocket =welcomeSocket.accept()

Create socket for incoming requests, bind to port 6789

welcomeSocket = ServerSocket(6789, 5)

Create socket,connect to 137.30.120.32, port = 6789

clientSocket = new Socket(137.30.120.32, 6789)

CloseconnectionSocket

Read reply from clientSocket

Close clientSocket

Server (running on cook.cs.uno.edu) Client

Send request using clientSocketRead request fromconnectionSocket

Write reply toconnectionSocket

TCP connection setup

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processsend

recv

processrecv

sendsocket socket

Internet

Socket Programming using UDP service

Application Viewpoint UDP provides unreliable transfer of

groups of bytes—datagrams—between two end points (processes).

Not a stream interface

Each datagram addressed explicitly

(no connection).

No delivery guarantee!

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Client/Server Socket Interaction: UDP

close clientSocket

Server (running on cook.cs.uno.edu)

read reply from clientSocket

Create socket for sending requests:

clientSocket = DatagramSocket()

Client

Create, address (137.30.120.40,9876),send datagram request using clientSocket

Create socket for incoming request:

serverSocket = DatagramSocket(9876)

read request fromserverSocket

write reply toserverSocketspecifying clienthost address,port number

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TCP/UDP CapsLock Example Demos

gitlab: vroussev/csci4311-f14/code