1. 1
Chapter 9: Clients and
Servers Interactions

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The Client Server Architecture
• Is a network architecture
– Which separates a client (often an application that
uses a GUI) from a server. Each instance of the client
software can send requests to a server.
• Specific Types of servers include:
• application servers
• file servers
• terminal servers
• mail servers

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The Client Server Architecture
• Client/server is network architecture
– Which separates a client (often an application that uses
a GUI) from a server. Each instance of the client
software can send requests to a server.
–
– Specific Types of servers include:
• application servers
• file servers
• terminal servers
• mail servers
– Properties of a server:
• Passive (Slave)
• Waiting for requests
• On requests serves them and send a reply

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• Properties of a client:
– Active (Master)
– Sending requests
– Waits until reply arrives
• A popular client in widespread use today is
the web browser which communicates with
web servers over the internet to fetch and
display web page content.

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5
EXTENDING BROWSER
• The designer of a particular browser may not be able to
sustain the development effort required to support all of the
possible document formats that the browser might be used
to view
• In the case of proprietary document formats (e.g., Adobe
PDF, Real Audio and others), the owners of a format may
not want to expose its encoding.
• To accommodate these requirements, browsers typically
incorporate features that allow new functionality to be
added to them without recompilation. We say that
applications that can be extended in this manner are
extensible
(Helper Applications, Plug-ins and Java Applets)

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HELPER APPLICATIONS
• The response to a HTTP request contains MIME headers
describing the type of document being fetched.
• We simulate the effect of getting the headers we would see
when fetching the URL
http://eftmk.kutkm.edu.my/bits2513/lectures/Lecture7.pdf
– The Content-Type of the document is application/pdf
• A browser can use this type to index into a table and locate
a helper application that can be used to display the
contents of the document. This table can be configured by
the user to associate helper applications with MIME types.
• This provides a simple mechanism that allows the browser
functionality to be extended by allowing it to invoke external
applications (Adobe Reader) to display documents.

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PLUGIN
• A plugin is a piece of code that can be loaded into same
address space as the browser process.
• Because plugins run in the same address space as the
browser, they have to be carefully constructed. A faulty
plugin could corrupt some of the browser’s internal data
structures. In general, writing plugins requires a great deal
of care.
• To use plugins, the browser configured with:
– The pathname of a directory that contains he available plugins
– A table that describes which plugins to use with which MIME types.

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Understanding How Clients and
Servers Interact
• Client/server is a computational architecture that involves
client processes requesting service from server processes
• A server can be software application that provides a
service to other software or the computer on which the
server software is running.
• A server (software application) must be installed and
managed by someone with the title of system
administrator, network administrator or for Web servers-
Webmaster.

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All-in-One Clients
• As the number of applications that use the Internet
grows, the need for client software on computers to use
these applications also arises.
• One solution to the growing number of Internet
applications is an all-in-one client or universal client.
– can handle several applications and adjust appropriately
– changing buttons and functions to accommodate each
application.
• Microsoft Internet Explorer is a good example of an all-
in-one client.

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Client Client
host host
Server
host

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Client/Server Applications
Some of the most popular applications on the Internet
follow the client/server design:
– Email clients
– FTP (File transfer) clients
– Web browsers

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“I want to
collaborate
with my
“I want to access colleague”
some information”
Client
Server Peer-to-peer
Client/server

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Distinctions
• Client-server
– Asymmetric relationship
– Client predominately makes requests, server
makes replies
• Peer-to-peer
– Symmetric relationship

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Client/Server Sessions
• As shown in Figure 7.1,
TCP (Transmission
Control Protocol)
manages the three-way
handshake that
establishes a session to
be used by application
protocols, such as HTTP
or FTP.

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• Like most application programs, a client and a server use a transport
protocol to communicate.
• Figure 7.2 illustrates a client and a server using the TCP/IP protocol
stack.
Figure 7.2: A client and a server

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• The client and server each interact with a protocol in the transport
layer of the stack. A sufficiently powerful computer can run multiple
servers and clients at the same time. Such a computer must have
the necessary hardware resources (e.g. a fast CPU and sufficient
memory) and have an operating system capable of running multiple
applications concurrently (e.g. UNIX or Windows).
Figure 7.3 illustrates such a setup.
• The computer in the middle might be running an FTP server and a
WWW server. Modern computers are often capable of running many
servers at the same time.

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Tiered Architecture
• Two-tier architecture:
– A generic Client/Server architecture has two types of
nodes on the network: clients and servers.
• Problems with a two-tier system:
– Because the relative power of clients has grown
considerably, we could shift processing to the client,
but then maintaining data integrity is difficult

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Three-tier client/server
Local-area network
Application
logic
Presentation
Note: many clients per Shared
application server, several data
application servers per data
server

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3-TIER ARCHITECTURE
C lient tie r
M id dle tier App lic a tion
Inform ation tie r
Da ta ba se

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3-TIER ARCHITECTURE
• Information tier
– Referred to as data tier or bottom tier
– Maintains data for application
– Stores data in relational database management system
• Middle tier
– Implements business logic and presentation logic
– Controls interactions between application clients and
application data
– Acts as intermediary between data in information tier
and application clients

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3-TIER ARCHITECTURE
• Client tier
– Referred to as top tier
– Application’s user interface
– Users interact with application through user interface
– Interacts with middle tier to make requests and to
retrieve data from information tier
– Displays data to user

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Servers
• Web Server
• DNS Server
• DHCP Server
• WINS Server
• RAS Server
• Printer Server
• Domain Controller Server

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Web Server (cont)
• Running program
• Stores set of Web documents
• Responds to request from browser by sending
copy of document

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Basic Common Features
A Web server programs may differ in detail among them,
But they all share some basic common features:
• HTTP responses to HTTP requests:
– every Web server program operates by accepting HTTP requests from
the network and providing an HTTP response to the requester.
– The HTTP response typically consists of an HTML document, but can
also be a raw text file, an image, or some other type of document.
– if something bad is found in client request or while trying to serve the
request,
• a Web server has to send an error response which may include some
custom HTML or text messages to better explain the problem to end users.
• Logging:
– usually Web servers have also the capability of logging some detailed
information about client requests and server responses to log files.
– this allows the Webmaster to collect statistics by running log analyzers
on log files.

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Extra features
• Configurability of available features by configuration files or even by an external
user interface.
• Authentication - optional authorization request (request of user name and
password) before allowing access to some or all kind of resources.
• Handling of static content and dynamic content
• Module support, in order to allow the extension of server capabilities by adding or
modifying software modules which are linked to the server software or that are
dynamically loaded (on demand) by the core server.
• HTTPS - support (by SSL or TLS) in order to allow secure (encrypted) connections to
the server on the standard port 443 instead of usual port 80.
• Content compression (i.e. by gzip encoding) to reduce the size of the responses (to
lower bandwidth usage, etc.).
• Virtual Host to serve many web sites using one IP address
• Large file support to be able to serve files whose size is greater than 2 GB on 32 bit
OS
• Bandwidth throttling - limit the speed of responses in order to not saturate the
network and to be able to serve more clients.

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Exploring the Features and
Functions of Web Servers
Some popular Web servers which it is free for download
include the following:
– Apache Web Server
– Microsoft Internet Information Services (IIS)
– Tomcat
– Netscape Enterprise Server (NES)

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Ability to Support Virtual Servers
and Virtual Hosts
• A Web server should be able to support virtual servers
and virtual hosting.
• Most virtual hosts handle multiple domain names on the
same server by having the Uniform Resource Locator
(URL) serve as a path to a file.
• Figure 7.5 illustrates the difference between virtual
servers and virtual hosting.

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Ability to Support Virtual Servers
and Virtual Hosts (Continued)

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Access Control Method
Based on the user’s IP address or user ID
1. access control allows the Web server to limit to which
files a user can read or write. User Ids are associated
with passwords to verify a user’s identity.
2. It can changing the port at which a server is listening.
– Port 80 is the default port for Web servers.
– Apache Web Server controls access to its
resources via a process known as authentication,
which requires a user to enter a valid user ID and
password to access a Web site.

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Access Control (Continued)

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Encrypting Protocols
• A secure protocol used by Web servers is SSL (Secure
Sockets Layer).
• When you see a URL with https: at the beginning instead
of http:, you know that this Web server is using the SSL
protocol for security.

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Database Interfaces
• Before selecting a Web server or a virtual hosting
service, find out what databases the server supports and
what tools can exchange information with the database.
• Popular databases are MS Access, MySQL, Oracle, and
SQL Server.

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Ability to Monitor Performance
• Microsoft IIS uses Performance Monitor, a program that
comes with Windows Server 2003 and Windows 2000
Server, to monitor performance.
• Other utility programs are Microsoft Web Capacity
Analysis Tool (WCAT) and Server Check Pro by
NetMechanic.

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Web Server Protocols
• HTTP and TCP/IP are the two main protocols used with
Web servers.
• HTTP methods used for browser requests are GET,
POST, HEAD, PUT, and DELETE.
• The most frequently used method is GET, which
requests files from the Web server.
• A dialog is a series of commands from the sender to the
receiver and replies from the receiver to the sender.

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COMMON HTTP REQUEST TYPES
• Also known as request methods
• Most popular are get and post
– Retrieve and send client form data to Web server
– get request
• Sends form content as part of URL
• Retrieves appropriate resource from Web server
• Limits query to 1024 characters
– post request
• Updates contents of Web server (posting new messages to
forum)
• Has no limit for length of query
• Not part of URL and cannot be seen by user

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IIS
• Internet Information Services is a set of
Internet-based services for servers using
Microsoft Windows.
• It is the world's second most popular web server
in terms of overall websites behind the Apache
HTTP Server
• The servers currently include FTP, SMTP, NNTP
and HTTP/HTTPS.