Client Software Design

Objectives:Understand principles of C/S design, with

focus on clients

Review Windows implementations of Socket functions

CS431-cotter 2

Architectural Objectives for C/S

• Partition applications across multiple machines– better utilization of computing resources

• Place user interaction portion of application closer to the user (client)– Better utilization of network bandwidth.

• Share server resources among many clients– Improve efficiency / utilization of resources– Synchronize use of shared resources– Control / manage access to shared resources

CS431-cotter 3

C/S Architecture

Application

Client Server

CS431-cotter 4

Example C/S Applications

• File Servers– C/S application used to locate and retrieve remote

files

• Database Applications– C/S used to present database queries to remote db

• Groupware– C/S used to share messages among group members

• Web Server– C/S used to query web server for web site

information, pages.

CS431-cotter 5

2-Tier vs. 3-Tier C/S Design

• 3-Tier design extends distribution of application across more levels / platforms.

• ClientLocal Server Enterprise ServerLocal Database Enterprise

DatabaseApp. Server Database Server

CS431-cotter 6

Client Software Design Issues

• Identifying Server Location– Command Line Argument– User query – Embedded (fixed) server ID

• Parsing Address Argument– domain name vs. IP address format

• Look up domain name– hptr = gethostbyname( example_name)– IP addr in hptr --> h_addr;

• Network byte order vs. local byte order

CS431-cotter 7

TCP Client Algorithm

• Find IP address and port number of server

• Allocate a socket

• Allow TCP to choose an arbitrary, unused local port

• Connect the socket to the server

• Communicate with server (application level)

• Close connection

CS431-cotter 8

TCP Client Algorithm Issues

• Chose a local Port number– allow connect to select port– [could use bind ( ) if needed]

• Identify local IP address– allow connect to specify local IP address– use gethostname ( )– use gethostbyname ( )

CS431-cotter 9

TCP Client Algorithm Issues

• Client / Server Communications– request / response interaction– write / read (send / recv)

• Single write may require multiple reads– response may be segmented– continue appending reads until return length =

0

CS431-cotter 10

TCP Client Algorithm Issues

• Connection may not be certain when all information or requests have been transferred

• Partial close allows graceful termination– shutdown ( s, direction)– 0 = no input, 1 = no further output, 2 = both directions– Sends an EOF to Server– When server has completed transmission, it shuts

down and then closes socket.

CS431-cotter 11

TCP Receive Completion

• Protocol may fragment transmitted packet, but does not directly support any way for the application to determine when the full packet has been received.

• Therefore, application needs to provide a mechanism to identify when all segments of the packet have been received.

CS431-cotter 12

Reception Complete Methods

• Predetermine packet size. Reception is complete when the predetermined number of bytes have been received.– All messages are a fixed size– Each message is of a predetermined size

• Terminate each packet with a sentinel character• Follow each packet with a packet of 0 bytes• etc.

CS431-cotter 13

UDP Client Algorithm

• Find IP addr and port number of server• Allocate a socket• Allow UDP to choose an arbitrary, unused local

port• Specify Server for messages• Communicate with server (application level)• Close socket

CS431-cotter 14

UDP Client Algorithm Issues

• UDP Basic communication Modes– Connected: use aconnect call to specify remote

server. then use read/write to communicate– Unconnected: specify server address with each

message

• UDP transfers entire message in a single call. – (assumes buffer space and transport are sufficient)– partial close provides no intersocket communications– UDP is UNRELIABLE! App must be able to deal with

inconsistent results.

CS431-cotter 15

Windows Sockets Introduction

• History

• Shared Functions

• Protocol Specific Functions

• Socket Database Routines

CS431-cotter 16

Windows SocketsWinsock 1.1

• First formal specification developed in 1992 through an industry task force

• Winsock 1.1 finalized 1993.

• Intended to facilitate porting BSD socket code to Windows environment

• Supports both 16 bit and 32 bit operating environments (Windows 3.0 and up).

CS431-cotter 17

Windows Sockets

• Actual implementations are system or machine dependent– All implementations should support winsock.h– Implementation support provided through

winsock.dll

CS431-cotter 18

Windows Sockets

• Negotiate for appropriate winsock support

• retvalue = WSAStartup (WORD version, WSADATA SocketImp)– version = The highest version of sockets that

the app can use.– SocketImp = data structure containing info on

the available socket implementations– retvalue = 0 or error

CS431-cotter 19

Windows Sockets

• Close down access to socket implementation

• retvalue = WSACleanup ( void);– retvalue = 0 or SOCKET_ERROR

CS431-cotter 20

Windows SocketsCommon Socket Routines

• retval = inet_addr ( dotted )– dotted = string with IP address in dotted

decimal form– retval = unsigned long with IP addr in binary

form or INADDR_NONE if not valid

• retval = inet_ntoa ( ipaddr )– ipaddr = struct in_addr with IP addr info– retval = string with IP address indotted

decimal form

CS431-cotter 21

Windows Sockets Common Socket Routines

• htonl ( ); htons ( );

• ntohl ( ); ntohs ( );

• socket ( );

• bind ( );

• connect ( );

• select ( );

• closesocket ( );

• shutdown ( );

CS431-cotter 22

Windows Sockets Common Socket Routines

• retvalue = ntohl(netlong);– converts unsigned long from network format to local– (IP Address)

• retvalue = ntohs(netshort);– converts unsigned short from network format to local– (Port Number)

• retvalue = htonl(hostlong);– converts unsigned long from local format to network

• retvalue = htons(hostshort);– converts unsigned short from local format to network

CS431-cotter 23

Windows Sockets Common Socket Routines

• retvalue = socket (family, type, protocol);– retvalue = socket descriptor or INVALID_SOCKET

(use WSAGetLastError() to retrieve error code)– family = protocol family (PF_INET for IP)– type = (service type: SOCK_STREAM for TCP,

SOCK_DGRAM for UDP)– protocol = protocol number or use 0 to match type

CS431-cotter 24

Windows Sockets Common Socket Routines

• retvalue = bind (socket, localaddr, addrlen);– retvalue = 0 for success or SOCKET_ERROR– socket = socket to be bound to a port– localaddr = sockaddr struct for local binding address– addrlen = length of localaddr

• retvalue = connect (socket, addr, addrlen)– retvalue = 0 for success or SOCKET_ERROR– socket = local socket to be used for connection– addr = sockaddr struct for remote binding address– addrlen = length of addr

CS431-cotter 25

Windows Sockets Common Socket Routines

• retvalue = select (ignore, refds, wefds, exfds, time);– Will be explained later…….

• retvalue = closesocket (socket);– retvalue = 0 or SOCKET_ERROR

– socket = socket to be closed;

• retvalue = shutdown ( socket, how);– retvalue = 0 or SOCKET_ERROR;

– socket = socket to be shutdown;

– how = 0 (incoming), 1 (outgoing), 2 (both directions)

CS431-cotter 26

Windows Sockets Protocol Specific Functions

TCP UDPaccept ( ); recvfrom( );

listen ( ); sendto ( );

recv ( );

send ( );

CS431-cotter 27

TCP Connection-OrientedCommunications

• retvalue = listen (socket, queuelen);– retvalue = 0 / SOCKETT_ERROR– socket = socket being monitored– queuelen = incoming request queue size

• retvalue = accept (socket, addr, addrlen);– retvalue = socket descriptor assigned to new connection– socket = incoming socket being monitored– addr = struct sockaddr that is filled in with incoming

address information– addrlen = length of address structure

CS431-cotter 28

TCP Connection-OrientedCommunications

• retvalue = recv (socket, buffer, length, flags);

– retvalue = # of bytes received, or 0 if connection is closed, or SOCKET_ERROR if an error occurred

– socket = socket used for incoming message– buffer = place where incoming message is stored– length = length of buffer– flags = control type info....

CS431-cotter 29

TCP Connection-OrientedCommunications

• retvalue = send (socket, msg, msglen, flags);

– retvalue = # of bytes sent, or SOCKET_ERROR if an error occurred

– socket = socket used for outgoing message– msg = pointer to outgoing message– msglen = length of message– flags = control type info....

CS431-cotter 30

UDP Connectionless Communications

• ret = recvfrom (socket, buf, buflen, flags, from,

fromlen);

SOCKET socket = client socket

char FAR* buf = buffer to receive incoming msg

int buflen = size of buffer

int flags = control bits (OOB data, etc.)

struct sockaddr FAR* from = server address

int fromlen = size of address structure

CS431-cotter 31

UDP Connectionless Communications

• ret = sendto ( socket, msg, msglen, flags, to, tolen);

SOCKET socket = client socket descriptor

const char FAR* msg = message for server

int mesglen = length of message

int flags = control flags (OOB data, etc.)

const struct sockaddr FAR* to = server address

int tolen = length of address structure

CS431-cotter 32

Windows Socketsdatabase routines

• gethostbyaddr ( );– Returns primary Domain Name for given IP address

• gethostbyname ( );– Returns primary IP address for given Domain Name

• gethostname ( );– Returns localhost domain name

• getprotobyname ( );– Returns protocol number for given protocol name

• getservbyname ( );– Returns well-known port number for given well-known

port name

CS431-cotter 33

Summary

• Foundations for Client / Server Programming

• Client Side Design Principles

• Windows Sockets Introduction

• Windows Sockets Functions