client-server model and socket programmingmqhuang/courses/3613/s2019/... · client-server model...

Client-Server ModelSocket Programming

Client-Server Modeland Socket Programming

Miaoqing HuangUniversity of Arkansas

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Outline

1 Client-Server Model

2 Socket Programming

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Outline



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Client-Server Model

A distributed application structurePartition tasks between the providers of a resource or service,called servers, and service requesters, called clients

One of the central ideas of network computingExamples: email exchange, web access, and database access

Server

Client Client

Request

Reply

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Asymmetric Relationship

SpecializationClients specialize in user interfaceServers specialize in managing data and application logic

SharingMany clients can be supported by few serversClient predominately makes requests, server makes replies

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Email Example

ClientServer

Client

Email client sends message to server

Message is stored on POP server

Later, recipient’s email client retrieves message from server

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Chat Room Example

Client

Server

Client

Chat clients send user’s typing to server

Chat server receives typing from all users and sends to all clients

Other user’s clients display aggregated typing from chat server

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Outline



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What is a socket? (socket←→ telephone)

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Socket vs. Telephone

Socket (aka, Internet Socket)An endpoint of a bidirectional inter-process communication flowacross an Internet Protocol-based computer networkA socket address consists of

An IP address: the location of the computerA port: mapped to the application program process

Communication protocol: TCP or UDP

TelephoneAn endpoint of a bidirectional inter-person communication flowacross a telephone networkAn address of a telephone consists of

A telephone number(Possibly) an extension

Mobile phone standard: GSM or CDMA

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IP Address and Port Number

Internet Protocol addressA numerical label assigned to each device (e.g., computer,printer) participating in a computer network that uses the InternetProtocol for communicationServe two principal functions

Host or network interface identificationLocation addressing

Current IP version: IPv4Port number

A port is an application-specific or process-specific softwareconstruct serving as a communications endpointA specific port is identified by its number, i.e., the port numberA port number is a 16-bit unsigned integer, [0, 65535]

Well-known ports: 0-1,023Registered ports: 1,024-49,151Dynamic, private or ephemeral ports: 49,152-65,535

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IPv4 Address Example

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Two Types of Sockets

Stream socket, aka, connection-oriented socketEstablish a connection before transferring data

Reliable, in-order

Use Transmission Control Protocol (TCP)Datagram socket, aka, connectionless socket

Each packet sent or received on a Datagram socket is individuallyaddressed and routed

May arrive in any orderMay get lost

Use User Datagram Protocol (UDP)

TCP UDP

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Inter-process Communication using Datagram Socket (UDP)

agreed port

any port

sendingprocess receiving

process

computercomputer

s1 s2message

Client1 Create a socket (s1)2 Bind the socket to any available

port of the local computerThis step may be skippedwhen programming in C

3 Send message to a knownsocket address (s2)

Receive message from s1

Server1 Create a socket (s2)2 Bind the socket to an

agreed socket address3 Receive message from

the socket (s2)Send message to s1

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Inter-process Communication using Stream Socket (TCP)

agred port

any port

clientprocess

serverprocess

computer

s1

s2

s3connection

Client1 Create a socket (s1)2 Send a connection request

to a known socket address(s2)

3 Start writing and readingthrough the connectionbetween s1 and s3

Server1 Create a socket (s2)2 Bind the socket to an

agreed socket address3 Listen to and accept the

connection request from s2Create a new socket s3for the connection to s1

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System Calls to Send/Receive Datagrams (in C language)

Following system calls are used:socket: to create a socket and get a file descriptor for itbind: to bind a socket address to the file descriptor of a socketsendto: send a message through a bound socket to a socketaddressrecvfrom: to receive a message through a socketclose: to destroy the socket when it is no longer needed

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System Calls for Stream Communication (in C language)

Following system calls are used:socket: to create a socket and get a file descriptor for itbind: to bind a socket address to the file descriptor of a socketconnect: to make a connect requestlisten: to specify how many requests should be queuedaccept: to accept a connect requestwrite: send information via connected socketsread: receive information via connected socketsclose: to destroy the socket when it is no longer needed

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Create a Socket

int socket (int domain, int type, int protocol); // prototype

int s;if(( s = socket(AF_INET, SOCK_DGRAM, 0))<0) {

perror("socket failed");return;

}

domainMust be AF_INET

typeSOCK_DGRAM: Datagram socketSOCK_STREAM: Stream socket

protocolSpecify the protocol0: system to select a suitable protocol

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Bind a Socket

int bind (int s, struct sockaddr * socketAddress, int addrlength);

struct sockaddr_in {short sin_family; // Must be AF_INETu_short sin_port; // Port numberstruct in_addr sin_addr; // IP address; INADDR_ANY: local IPchar sin_zero[8];

}

sSocket descriptor

socketAddressSpecify the socket addressUse struct sockaddr_in

addrlengthThe size of the structure in the second argument

Server has to bind the socket to an known socket address

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Close a Socket

int close (int s);

sSocket descriptor

A socket has to be closed so that it can be re-used by anotherprogram

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Send a Message using a Datagram Socket

int sendto(int s, char * msg, int len, int flags, struct sockaddr *to, int tolen);

sSocket descriptor

msgPointer to the message string

lenLength of the message

flagsNormally zero

toDestination Socket

tolenSize of the structure sockaddr

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Receive a Message using a Datagram Socket

int recvfrom(int s, char *buf, int len, int flags, struct sockaddr * from, int *fromlen);

sSocket descriptor

buf and lenThe buffer to save the received message

flagsNormally zero

from and fromlenThe structure to save the sender’s socket address

recvfrom is a blocking call

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Listen to a Socket and Accept a Connection RequestServer functions of a Stream Socket

int listen(int s, int backlog);

Listen on its socket for requests from clients for connectionsbacklog: the number of requests for connection that can bequeued at that socket

int accept(int s, struct sockaddr * clientAddress,int * clientLength);

Accepts the first connection in the queue at socket sThe result is a descriptor for a new socket that has been createdfor use as one end of the stream

How to deal with multiple connections simultaneously?Spawn a thread for each connection

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How to deal with multiple connections simultaneously?

Spawn a thread for each connection

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How to deal with multiple connections simultaneously?Spawn a thread for each connection

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Request a Connection by a Client Process

int connect(int s, struct sockaddr *server, int addrLen);

Request a connection via the socket address of the listeningprocess

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Read and Write via a Connection

int write(int s, char * message, int msglen);

Write a message into the connection

int read(int s, char * buffer, int bufsize);

Read a message from the connection

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client-server model and socket programmingmqhuang/courses/3613/s2019/... · client-server model...

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