Design of Client Software

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Chapter 7

Design of Client Software(Recommended reading: Comer, Vol 3, Chapters 6, 7)

In the previous chapter, we studied a

simple TCP client.

In this chapter, we study more issues about

TCP clients, and then study UDP clients.

Review of C Programming

Memory Copy Function

The function memcpy() copies the value of

one field to another:

The first argument is a pointer to the

destination field.

The second argument is a pointer to the

source field.

The third argument is the number of

bytes to be copied.

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or

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Example

Copy the value of the y field to the x

field:

memcpy ( &x , &y , sizeof(x) ) ;

Pointer to Structure

If the pointer variable pv stores the address

of a structure, we can access any member of

this structure as follows:

pv -> member_name

(*pv) . member_name

Example

struct info {

} ;

int

int

ID ;

age ;

struct info student, *pv ;

student . ID = 1234 ;

student . age = 18 ;

pv -> ID = 5678 ;

(*pv) . age = 19 ;

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Pointer Pointer

A pointer pointer is a memory location that

holds the address of a pointer.

Example:

char **x ;

Design of TCP Clients

A TCP client adopts TCP for connection-oriented communication. Recall its major steps

for communication:

Initialize sockets.

Communicate between sockets.

Close sockets.

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Also recall the sequence of execution of

WinSock functions:

In this section, we study more issues in the

design of client programs.

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Specifying the Endpoint Address of the Server

The client has to specify the endpoint

address of the server, so that it can take

initiative to connect to the server. An endpoint address contains three items:

Address family

IP address

Port number

For TCP/IP, the first item is alwaysAF_INET.

Then the client has to specify the IP

address and port number of the server.

Problem

How can a client program know the IP

address and port number of the server?

Some possible solutions

1. The programmer specifies the server's

endpoint address in the client program.

When the server is moved to

another computer, the clientprogram must be modified andrecompiled!

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2. When a user invokes a client program,

he specifies the endpoint address.

The client program is general and

flexible. This is a common approach.

Examples

The user specifies the IP address

and port number:

158.182.7.1 2000

The user specifies the domain name

(a symbolic representation of an IPaddress) and the port number:

cs8888.comp.hkbu.edu.hk 2000

Web browsing uses the well-known

port number 80. When a userinvokes a client program (i.e., abrowser), he just inputs the domain

name or IP address of the webserver.

3. The client program obtains the endpoint

address from stable storage (such as afile on a local disk).

The client program cannot be

executed without the file.

Example: bookmarks in web

browsing.

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4. The client program uses a separate

protocol to find the endpoint address

(e.g., multicast or broadcast a

message to which all serversrespond).

Format Conversion for IP Address

Very often, the IP address of the server is

given in various formats:

dotted decimal notation (e.g.,158.10.2.3) or

domain name (e.g.,

cs8888.comp.hkbu.edu.hk).

It is necessary to convert these

formats to 32-bit format. Conversion for Dotted Decimal Notation

Call inet_addr() to convert an IP

address in dotted decimal notation to

32-bit IP address.

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Conversion for Domain Name

Call gethostbyname() to convert the

domain name to 32-bit IP address. This

function

takes a string of domain name as an

input, and

returns the address of a hostnet

structure:

The above structure contains several

items, and the item h_addr contains

the pointer to the IP address.

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Example

char *ServerName =

"cs8888.comp.hkbu.edu.hk" ;

struct hostent *h ;

struct sockaddr_in ServerAddr ;

ServerAddr.sin_family = AF_INET ;

ServerAddr.sin_port = 2000 ;

h = gethostbyname (ServerName) ;

memcpy ( &ServerAddr.sin_addr ,

h->h_addr , h->h_length );

Looking Up a Well-Known Port Number

TCP/IP defines some standard applications

(e.g., email, www), and the server of each

application has a well-known port number. Examples

Email server uses port number 25.

Web server uses port number 80.

Problem

When a client program is given thename of a standard server application,

how can it look up the well-known port

number for this application?

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Solution

The client program calls getservbyname().

This function has two arguments:

a string that specifies the application

(e.g., smtp);

a string that specifies the protocol to be

used (e.g., tcp).

This function returns a pointer to a structure

of type servent:

where s_portcontains the port number innetwork byte order.

Example

To look up the port number for SMTP

(simple mail transfer protocol), a client

program calls getservbyname() as follows:struct servent *s ;

s = getservbyname ( "smtp" , "tcp" )

;

ServerAddr . sin_port = s->s_port ;

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Choosing a Local Port Number

A client has a local port number to form a

local endpoint address.

A client does not have a preassigned port

number, and it has to select a local port

number when it is executed.

The client can select any port number within

the range 49152 65535, as long as this

number is not being used by the otherapplications on the same computer.

A TCP client usually allows the TCP/IP

software to select an unused local port

number automatically.

Send and Receive Data A client calls send() once to send a block of

data.

A client may have to call recv() multiple

times to receive a complete block of data,

because it may receive a portion at a time(why?).

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Example

The client sends a message to a server,

and expects a reply with not more than

120 characters.

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The Design of UDP Clients

A UDP client is similar to a TCP client, except that

it needs not establish a connection to the server. Warning about UDP unreliability

Recall that UDP does not guarantee reliable

datagram delivery.

A UDP client may have to tackle reliability

itself (e.g., through retransmission, packetresequencing, ...)

This may be complicated.

Even if a UDP client works well in local

networks, it may not work across a wide area

internet (why?)

It may be difficult to debug a UDP client

program.

When a UDP client calls socket() to create a

new socket, it must specify that it needs a

datagram socket. Example:

s = socket( AF_INET , SOCK_DGRAM , 0 );

A UDP client uses eitherconnected mode or

unconnected mode for communication.

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Unconnected Mode

Each time a client sends a message, it has to

specify the endpoint address of the server.

sendto()

Call sendto() to send a block of data to

a specified endpoint address.

The function sendto() is similar to

send(), except that sendto() needs the

destination endpoint address as anadditional argument.

recvfrom()

Call recvfrom() to receive

data contained in one segment, and

the sender's endpoint address.

The function recvfrom() is similar to

recv(), except that recvfrom() extracts

and records the sender's endpoint

address.

It is more convenient to send a

reply.

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Example

charSOCKET

buf[100] ;s;

struct sockaddr_in ServerAddr ;

sendto ( s , buf , sizeof(buf) , 0 ,

&ServerAddr , sizeof(ServerAddr) );

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Examplechar

SOCKET

buf[100] ;

s ;

struct sockaddr_in

int len;

RemoteAddr ;

len = sizeof ( RemoteAddr ) ;

recvfrom ( s , buf , sizeof(buf) , 0

, &RemoteAddr , &len) ) ;

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Connected Mode

Using the connected mode, the client calls

connect() to specify a remote endpoint

address, and then it can send and receivemessages without specifying the remote

endpoint address again.

The connected mode is convenient when the

client interacts with only one server at a

time, because the client only needs tospecify the remote endpoint address once no

matter how many datagrams it sends.

Communication in Connected Mode

A UDP client calls connect() with

SOCK_DGRAMto record the remoteendpoint address in the socket's data

structure (not to establish a connection

to the server.)

Then the UDP client can call send() to

send a message and recv() to receive a

message contained in one segment.

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Remarks

In network programming, we have to handle

many details.

A complete program is quite complicated.

Many programs may contain the same

segments of code (e.g., the code for allocating

and connecting a socket).

We can code these segments as procedures.

The program is more structured, and the

procedures can be re-used in the other

programs.

Network programming becomes simpler.

A programmer may construct a library of

procedures for his/her own use.

Example

A programmer may write a procedure for

allocating and connecting a TCP socket:

sock_descriptor = connectTCP ( machine, service )

Whenever he writes a network program, he

can call this procedure without concerning

their details.

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Tutorial Problems

What is the main difference between the usage

ofsend() for TCP communication and UDP

communication?

What is the main difference between the usage

ofrecv() for TCP communication and UDP

communication?

A student argues that a UDP client in

unconnected mode can call recv() instead of

recvfrom() to receive data, because the client

must know the endpoint address of the server

and therefore it is not necessary to use

recvfrom() to extract this endpoint address. Do

you agree? If a client wants to communicate with three

servers, does this client require three sockets?

List the sequence of execution of WinSock for

a UDP client using the

unconnected mode,

connected mode.

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