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COMPUTER NETWORK
(18EC71)
Dr. Shivashankar
Professor
Department of Electronics & Communication
Engineering
RRIT, Bangalore
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Course Outcomes
After Completion of the course, student will be able to
Understand the concepts of networking thoroughly.
Describe the various network architectures
Identify the protocols and services of different layers
Distinguish the basic network configurations and
standards associated with each network models.
Analyze a simple network and measurements of its
parameters.
Text Book:
Data Communications and Networking , Forouzan, 5th
Edition, McGraw Hill, 2016 ISBN: 1-25-906475-3
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Dr. Shivashankar, E&CE, RRIT
Module 5
Application Layer
• The application layer provides services to the user.
• Communication is provided using a logical connection, which
means that the two application layers assume that there is an
imaginary direct connection through which they can send and
receive the messages.
• Application Layer provides a facility by which users can forward
several emails and it also provides a storage facility.
• This layer allows users to access, retrieve and manage files in a
remote computer.
• It allows users to log on as a remote host.
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Providing Services
• The Internet was originally designed for the same purpose: to provide service
to users around the world.
• New protocols can be added or some protocols can be removed or replaced
by the Internet authorities.
• Since the application layer is the only layer that provides services to the
Internet user, it allows new application protocols to be easily added to the
Internet.
Standard Application-Layer Protocols
• There are several application-layer protocols that have been standardized and
documented by the Internet authority.
• Each standard protocol is a pair of computer programs that interact with the
user and the transport layer to provide a specific service to the user.
• Ex: Telnet, FTP, TFTP, SMTP, SNMP, DNS, DHCP.
Nonstandard Application-Layer Protocols
• A programmer can create a nonstandard application-layer program.
• It is the creation of a nonstandard (proprietary) protocol, which does not even
need the approval of the Internet authorities if privately used.
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Application-Layer Paradigms
Two paradigms have been developed during the lifetime of the Internet:
1. the client-server paradigm
2. the peer-to-peer paradigm.
Traditional Paradigm: Client-Server
• The traditional paradigm is called the client-server paradigm.
• Service provider is an application program, called the server process; it runs continuously, waiting
for another application program, called the client process, to make a connection through the
Internet and ask for service.
• The server process must be running all the time; the client process is started when the client
needs to receive service.
• Several traditional services are still using this paradigm, including the World Wide Web (WWW)
and its vehicle HyperText Transfer Protocol (HTTP), file transfer proto- col (FTP), secure shell (SSH),
e-mail, and so on.
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Dr. Shivashankar, E&CE, RRIT
Figure 25.2 Example of a
client-server paradigm
Peer-to-Peer
• A new paradigm, called the peer-to-peer paradigm (P2P paradigm) has emerged to
respond to the needs of some new applications.
• In this paradigm, there is no need for a server process to be running all the time and
waiting for the client processes to connect.
• The responsibility is shared between peers.
• A computer connected to the Internet can provide service at one time and receive
service at another time.
• A computer can even provide and receive services at the same time.
• There are some new applications, such as BitTorrent, Skype, IPTV, and Internet
telephony.
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Figure 25.3 Example of
a peer-to-peer paradigm
Mixed Paradigm
• An application may choose to use a mixture of the two
paradigms by combining the advantages of both.
• For example, a light-load client-server communication can be
used to find the address of the peer that can offer a service.
• When the address of the peer is found, the actual service can be
received from the peer by using the peer-to- peer paradigm.
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Dr. Shivashankar, E&CE, RRIT
WORLD WIDE WEB
• The idea of the Web was first proposed by Tim Berners-Lee in 1989 at CERN,
the European Organization for Nuclear Research, to allow several researchers
at different locations throughout Europe to access each others’ researches.
• The commercial Web started in the early 1990s.
• The Web today is a repository of information in which the documents, called
web pages, are distributed all over the world and related documents are
linked together.
• The popularity and growth of the Web can be related to two terms in the
above statement: distributed and linked.
• Linking allows one web page to refer to another web page stored in another
server somewhere else in the world.
• The linking of web pages was achieved using a concept called hypertext,
• The WWW today is a distributed client-server service, in which a client using a browser
can access a service using a server. However, the service provided is distributed over
many locations called sites.
• Each site holds one or more web pages.
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Dr. Shivashankar, E&CE, RRIT
Web Client (Browser)
• A variety of vendors offer commercial browsers that interpret and display a
web page, and all of them use nearly the same architecture.
• Each browser usually consists of three parts: a controller, client protocols, and
interpreters.
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Dr. Shivashankar, E&CE, RRIT
Figure 26.2 Browser
Web Server
• A web server is software and hardware that uses HTTP and other protocols to
respond to client requests made over the World Wide Web.
• The main job of a web server is to display website content through storing,
processing and delivering webpages to users.
• Besides HTTP, web servers also support SMTP and FTP, used for email, file
transfer and storage.
• Web server hardware is connected to the internet and allows data to be
exchanged with other connected devices, while web server software controls
how a user accesses hosted files.
• All computers that host websites must have web server software.
Uniform Resource Locator (URL):
A URL (Uniform Resource Locator) is a unique identifier used to locate a resource
on the Internet.
It is also referred to as a web address.
Ex: https://www.rrit.ac.in
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Dr. Shivashankar, E&CE, RRIT
Components of URL”
• Protocol. The first identifier is the abbreviation for the client-server program that we
need in order to access the web page. Although most of the time the protocol is HTTP
(HyperText Transfer Protocol) and FTP (File Transfer Protocol).
• Host. The host identifier can be the IP address of the server or the unique name given
to the server
• Port. The port, a 16-bit integer, is normally predefined for the client-server appli-
cation. For example, if the HTTP protocol is used for accessing the web page, the well-
known port number is 80. However, if a different port is used, the number can be
explicitly given.
• Path. The path identifies the location and the name of the file in the underlying
operating system.
• The format of this identifier normally depends on the operat- ing system. In UNIX, a
path is a set of directory names followed by the file name, all separated by a slash. For
example, /top/next/last/myfile is a path
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HyperText Transfer Protocol (HTTP)
• It is a protocol used to access the data on the World Wide Web (www).
• The HTTP protocol can be used to transfer the data in the form of plain text,
hypertext, audio, video, and so on.
• HTTP is similar to the FTP as it also transfers the files from one host to another
host. But, HTTP is simpler than FTP as HTTP uses only one connection, i.e., no
control connection to transfer the files.
Nonpersistent versus Persistent Connections
• Nonpersistent Connections
• In a nonpersistent connection, one TCP connection is made for each
request/response. The following lists the steps in this strategy:
• The client opens a TCP connection and sends a request.
• The server sends the response and closes the connection.
• The client reads the data until it encounters an end-of-file
marker
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Persistent Connections
• Persistent Connection is one of the basic HTTP connections.
• It establishes the connection between the users and the servers.
• The Persistent connection can also be called as the HTTP/1.1.
Message Formats
• The HTTP protocol defines the format of the request and response messages.
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Message Formats
• The HTTP protocol defines the format of the request
and response messages.
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HTTP Security
• HTTP does not provide security.
• HTTP can be run over the Secure Socket Layer (SSL).
• In this case, HTTP is referred to as HTTPS. HTTPS
provides confidentiality, client and server
authentication, and data integrity.
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Web Caching: Proxy Servers
• HTTP supports proxy servers.
• A proxy server is a computer that keeps copies of responses to recent
requests.
• The HTTP client sends a request to the proxy server. The proxy server checks
its cache.
• If the response is not stored in the cache, the proxy server sends the request
to the corresponding server. Incoming responses are sent to the proxy server
and stored for future requests from other clients.
• The proxy server reduces the load on the original server, decreases traffic, and
improves latency.
• To use the proxy server, the client must be configured to access the proxy
instead of the target server.
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Dr. Shivashankar, E&CE, RRIT
FTP
• File Transfer Protocol (FTP) is the standard protocol provided by TCP/IP for
copying a file from one host to another.
• Although transferring files from one system to another seems simple and
straightforward, some problems must be dealt with first.
• For example, two systems may use different file name conventions.
• Two systems may have different ways to represent data.
• Two systems may have different directory structures.
• Although we can transfer files using HTTP, FTP is a better choice to transfer
large files or to transfer files using different formats.
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Dr. Shivashankar, E&CE, RRIT
Figure 26.10 FTP
FTP: Two Connections
Control Connection:
• It remains connected during the entire interactive FTP session.
• When a user starts an FTP session, the control connection opens.
• For control communication, FTP uses the same approach as TELNET.
• During this control connection, commands are sent from the client to the server and
responses are sent from the server to the client.
• Commands, which are sent from the FTP client control process, are in the form of ASCII
uppercase. Table 26.4 Some FTP commands
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Dr. Shivashankar, E&CE, RRIT
Command Argument(s) Description
ABOR Abort the previous command
CDUP Change to parent directory
CWD Directory name Change to another directory
DELE File name Delete a file
LIST Directory name List subdirectories or files
MKD Directory name Create a new directory
PASS User password Password
PASV Server chooses a port
PORT Port identifier Client chooses a port
PWD Display name of current directory
QUIT Log out of the system
RETR File name(s) Retrieve files; files are transferred from server to client
RMD Directory name Delete a directory
RNFR File name (old) Identify a file to be renamed
RNTO File name (new) Rename the file
STOR File name(s) Store files; file(s) are transferred from client to server
STRU F, R, or P Define data organization (F: file, R: record, or P: page)
TYPE A, E, I Default file type (A: ASCII, E: EBCDIC, I: image)
USER User ID User information
MODE S, B, or C Define transmission mode (S: stream, B: block, or C:
compressed
Data Connection
• The data connection is opened and then closed for each file transfer activity.
• It opens each time commands that involve transferring files are used, and it
closes when the file is transferred.
• The data connection uses the well-known port 20 at the server site.
• The following shows the steps to create data connection:
 A connection data source stores information about how a data
connection should access an external system. This includes details such
as server information, database name and authentication method.
 Using the PORT command the client sends this port number to the
server.
 The server receives the port number and issues an active open using
the well- known port 20 and the received ephemeral port number.
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Dr. Shivashankar, E&CE, RRIT
ELECTRONIC MAIL
• Electronic Mail (e-mail) is one of most widely used services of Internet.
• In an application such as HTTP or FTP, the server program is running all the
time, waiting for a request from a client.
• When the request arrives, the server provides the service.
• There is a request and there is a response.
• Message in mail not only contain text, but it also contains images, audio and
videos data.
• The person who is sending mail is called sender and person who receives mail
is called recipient.
• Architecture
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Dr. Shivashankar, E&CE, RRIT
E-mail architecture
1. a user agent (UA),
2. a message transfer agent (MTA),
3. and a message access agent (MAA).
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Dr. Shivashankar, E&CE, RRIT
Figure 26.12 Common scenario
Web-Based Mail
• Webmail is a more advanced technology that allows the user to manage emails
without having to install fancy and huge software on their computer machine, they
achieve this by hosting their service on the cloud.
• Webmail is provided by many companies and ISPs.
• Since they are hosted on the cloud they require an internet connection, also the emails
cannot be downloaded or modified in the offline mode.
• This necessity for an internet connection constitutes the disadvantage of using
webmail.
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Dr. Shivashankar, E&CE, RRIT
Figure 26.22 Web-based e-mail, cases I and II
E-Mail Security
• e-mail exchanges can be secured using two
application-layer securities designed in particular for e-
mail systems.
• Two of these protocols, Pretty Good Privacy (PGP) and
Secure/Multipurpose Internet Mail Extensions
(S/MIME)
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Dr. Shivashankar, E&CE, RRIT
TELNET
• TELNET stands for Teletype Network.
• It is a type of protocol that enables one computer to connect to local
computer.
• It is a used as a standard TCP/IP protocol for virtual terminal service which is
given by ISO.
• Computer which starts connection known as the local computer. Computer
which is being connected to i.e. which accepts the connection known
as remote computer. When the connection is established between local and
remote computer.
• Telnet operates on client/server principle.
• Local computer uses telnet client program and the remote computers uses
telnet server program.
• Telnet can be used for a variety of activities on a server, including editing files,
running various programs and checking email.
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Dr. Shivashankar, E&CE, RRIT
Local versus Remote Logging
2/11/2023 25
Dr. Shivashankar, E&CE, RRIT
Figure 26.23 Local versus remote logging
Domain Name system (DNS)
• Since the Internet is so huge today, a central directory system cannot hold all
the mapping.
• In addition, if the central computer fails, the whole communication network
will collapse.
• A better solution is to distribute the information among many computers in
the world.
• In this method, the host that needs mapping can contact the closest computer
holding the needed information.
• This method is used by the Domain Name System (DNS).
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Dr. Shivashankar, E&CE, RRIT
Figure 26.28 Purpose of DNS
• DNS is a directory service that provides a mapping between the name of a
host on the network and its numerical address.
• DNS is required for the functioning of the internet.
• Each node in a tree has a domain name, and a full domain name is a sequence
of symbols specified by dots.
• DNS is a service that translates the domain name into IP addresses. This
allows the users of networks to utilize user-friendly names when looking for
other hosts instead of remembering the IP addresses.
• For example, suppose the FTP site at EduSoft had an IP address of
132.147.165.50, most people would reach this site by specifying
ftp.EduSoft.com.
• Therefore, the domain name is more reliable than IP address.
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Dr. Shivashankar, E&CE, RRIT
Name Space
• The names assigned to machines must be carefully selected from a name
space with complete control over the binding between the names and IP
addresses.
• In other words, the names must be unique because the addresses are unique.
• A name space that maps each address to a unique name can be organized in
two ways:
 flat or
 hierarchical.
• In a flat name space, a name is assigned to an address. A name in this space is
a sequence of characters without structure.
• In a hierarchical name space, each name is made of several parts. The first
part can define the nature of the organization, the second part can define the
name of an organization, the third part can define departments in the
organization, and so on.
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Dr. Shivashankar, E&CE, RRIT
DNS in the Internet
• DNS is a protocol that can be used in different platforms. In the Internet, the domain
name space (tree) was originally divided into three different sections: generic domains,
country domains, and the inverse domains.
• Generic Domains
• It defines the registered hosts according to their generic behavior.
• Each node in a tree defines the domain name, which is an index to the DNS database.
• It uses three-character labels, and these labels describe the organization type.
• Ex: edu, .com, .org etc.
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Dr. Shivashankar, E&CE, RRIT
Resolution
• Mapping a name to an address is called name-address resolution.
• DNS is designed as a client-server application.
• A host that needs to map an address to a name or a name to an address calls
a DNS client called a resolver.
DNS Messages
• To retrieve information about hosts, DNS uses two types of messages: query
and response.
2/11/2023 30
Dr. Shivashankar, E&CE, RRIT Figure 26.38 DNS message
Registrars
• How are new domains added to DNS? This is done through a registrar, a
commercial entity accredited by ICANN.
• A registrar first verifies that the requested domain name is unique and then
enters it into the DNS database. A fee is charged.
• Today, there are many registrars; their names and addresses can be found at
• http://www.intenic.net
• To register, the organization needs to give the name of its server and the IP
address of the server.
• For example, a new commercial organization named wonderful with a server
named ws and IP address 200.200.200.5 needs to give the following
information to one of the registrars:
• Domain name: ws.wonderful.com IP address: 200.200.200.5
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Dr. Shivashankar, E&CE, RRIT
Dynamic Domain Name System (DDNS)
• The DNS master file must be updated dynamically. In DDNS, when a binding
between a name and an address is determined, the information is sent,
usually by DHCP.
• To provide security and prevent unauthorized changes in the DNS records,
DDNS can use an authentication mechanism.
Security of DNS
• DNS is one of the most important systems in the Internet infrastructure; it
provides cru- cial services to Internet users.
• Applications such as Web access or e-mail are heavily dependent on the
proper operation of DNS.
• To protect DNS, IETF has devised a technology named DNS Security (DNSSEC)
that provides message origin authentication and message integrity using a
security service called digital signature.
2/11/2023 32
Dr. Shivashankar, E&CE, RRIT
Thanks
2/11/2023 33
Dr. Shivashankar, E&CE, RRIT

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MODULE-5_CCN.pptx

  • 1. COMPUTER NETWORK (18EC71) Dr. Shivashankar Professor Department of Electronics & Communication Engineering RRIT, Bangalore 2/11/2023 1 Dr. Shivashankar, E&CE, RRIT
  • 2. Course Outcomes After Completion of the course, student will be able to Understand the concepts of networking thoroughly. Describe the various network architectures Identify the protocols and services of different layers Distinguish the basic network configurations and standards associated with each network models. Analyze a simple network and measurements of its parameters. Text Book: Data Communications and Networking , Forouzan, 5th Edition, McGraw Hill, 2016 ISBN: 1-25-906475-3 2/11/2023 2 Dr. Shivashankar, E&CE, RRIT
  • 3. Module 5 Application Layer • The application layer provides services to the user. • Communication is provided using a logical connection, which means that the two application layers assume that there is an imaginary direct connection through which they can send and receive the messages. • Application Layer provides a facility by which users can forward several emails and it also provides a storage facility. • This layer allows users to access, retrieve and manage files in a remote computer. • It allows users to log on as a remote host. 2/11/2023 3 Dr. Shivashankar, E&CE, RRIT
  • 4. Providing Services • The Internet was originally designed for the same purpose: to provide service to users around the world. • New protocols can be added or some protocols can be removed or replaced by the Internet authorities. • Since the application layer is the only layer that provides services to the Internet user, it allows new application protocols to be easily added to the Internet. Standard Application-Layer Protocols • There are several application-layer protocols that have been standardized and documented by the Internet authority. • Each standard protocol is a pair of computer programs that interact with the user and the transport layer to provide a specific service to the user. • Ex: Telnet, FTP, TFTP, SMTP, SNMP, DNS, DHCP. Nonstandard Application-Layer Protocols • A programmer can create a nonstandard application-layer program. • It is the creation of a nonstandard (proprietary) protocol, which does not even need the approval of the Internet authorities if privately used. 2/11/2023 4 Dr. Shivashankar, E&CE, RRIT
  • 5. Application-Layer Paradigms Two paradigms have been developed during the lifetime of the Internet: 1. the client-server paradigm 2. the peer-to-peer paradigm. Traditional Paradigm: Client-Server • The traditional paradigm is called the client-server paradigm. • Service provider is an application program, called the server process; it runs continuously, waiting for another application program, called the client process, to make a connection through the Internet and ask for service. • The server process must be running all the time; the client process is started when the client needs to receive service. • Several traditional services are still using this paradigm, including the World Wide Web (WWW) and its vehicle HyperText Transfer Protocol (HTTP), file transfer proto- col (FTP), secure shell (SSH), e-mail, and so on. 2/11/2023 5 Dr. Shivashankar, E&CE, RRIT Figure 25.2 Example of a client-server paradigm
  • 6. Peer-to-Peer • A new paradigm, called the peer-to-peer paradigm (P2P paradigm) has emerged to respond to the needs of some new applications. • In this paradigm, there is no need for a server process to be running all the time and waiting for the client processes to connect. • The responsibility is shared between peers. • A computer connected to the Internet can provide service at one time and receive service at another time. • A computer can even provide and receive services at the same time. • There are some new applications, such as BitTorrent, Skype, IPTV, and Internet telephony. 2/11/2023 6 Dr. Shivashankar, E&CE, RRIT Figure 25.3 Example of a peer-to-peer paradigm
  • 7. Mixed Paradigm • An application may choose to use a mixture of the two paradigms by combining the advantages of both. • For example, a light-load client-server communication can be used to find the address of the peer that can offer a service. • When the address of the peer is found, the actual service can be received from the peer by using the peer-to- peer paradigm. 2/11/2023 7 Dr. Shivashankar, E&CE, RRIT
  • 8. WORLD WIDE WEB • The idea of the Web was first proposed by Tim Berners-Lee in 1989 at CERN, the European Organization for Nuclear Research, to allow several researchers at different locations throughout Europe to access each others’ researches. • The commercial Web started in the early 1990s. • The Web today is a repository of information in which the documents, called web pages, are distributed all over the world and related documents are linked together. • The popularity and growth of the Web can be related to two terms in the above statement: distributed and linked. • Linking allows one web page to refer to another web page stored in another server somewhere else in the world. • The linking of web pages was achieved using a concept called hypertext, • The WWW today is a distributed client-server service, in which a client using a browser can access a service using a server. However, the service provided is distributed over many locations called sites. • Each site holds one or more web pages. 2/11/2023 8 Dr. Shivashankar, E&CE, RRIT
  • 9. Web Client (Browser) • A variety of vendors offer commercial browsers that interpret and display a web page, and all of them use nearly the same architecture. • Each browser usually consists of three parts: a controller, client protocols, and interpreters. 2/11/2023 9 Dr. Shivashankar, E&CE, RRIT Figure 26.2 Browser
  • 10. Web Server • A web server is software and hardware that uses HTTP and other protocols to respond to client requests made over the World Wide Web. • The main job of a web server is to display website content through storing, processing and delivering webpages to users. • Besides HTTP, web servers also support SMTP and FTP, used for email, file transfer and storage. • Web server hardware is connected to the internet and allows data to be exchanged with other connected devices, while web server software controls how a user accesses hosted files. • All computers that host websites must have web server software. Uniform Resource Locator (URL): A URL (Uniform Resource Locator) is a unique identifier used to locate a resource on the Internet. It is also referred to as a web address. Ex: https://www.rrit.ac.in 2/11/2023 10 Dr. Shivashankar, E&CE, RRIT
  • 11. Components of URL” • Protocol. The first identifier is the abbreviation for the client-server program that we need in order to access the web page. Although most of the time the protocol is HTTP (HyperText Transfer Protocol) and FTP (File Transfer Protocol). • Host. The host identifier can be the IP address of the server or the unique name given to the server • Port. The port, a 16-bit integer, is normally predefined for the client-server appli- cation. For example, if the HTTP protocol is used for accessing the web page, the well- known port number is 80. However, if a different port is used, the number can be explicitly given. • Path. The path identifies the location and the name of the file in the underlying operating system. • The format of this identifier normally depends on the operat- ing system. In UNIX, a path is a set of directory names followed by the file name, all separated by a slash. For example, /top/next/last/myfile is a path 2/11/2023 11 Dr. Shivashankar, E&CE, RRIT
  • 12. HyperText Transfer Protocol (HTTP) • It is a protocol used to access the data on the World Wide Web (www). • The HTTP protocol can be used to transfer the data in the form of plain text, hypertext, audio, video, and so on. • HTTP is similar to the FTP as it also transfers the files from one host to another host. But, HTTP is simpler than FTP as HTTP uses only one connection, i.e., no control connection to transfer the files. Nonpersistent versus Persistent Connections • Nonpersistent Connections • In a nonpersistent connection, one TCP connection is made for each request/response. The following lists the steps in this strategy: • The client opens a TCP connection and sends a request. • The server sends the response and closes the connection. • The client reads the data until it encounters an end-of-file marker 2/11/2023 12 Dr. Shivashankar, E&CE, RRIT
  • 13. Persistent Connections • Persistent Connection is one of the basic HTTP connections. • It establishes the connection between the users and the servers. • The Persistent connection can also be called as the HTTP/1.1. Message Formats • The HTTP protocol defines the format of the request and response messages. 2/11/2023 13 Dr. Shivashankar, E&CE, RRIT
  • 14. Message Formats • The HTTP protocol defines the format of the request and response messages. 2/11/2023 14 Dr. Shivashankar, E&CE, RRIT
  • 15. HTTP Security • HTTP does not provide security. • HTTP can be run over the Secure Socket Layer (SSL). • In this case, HTTP is referred to as HTTPS. HTTPS provides confidentiality, client and server authentication, and data integrity. 2/11/2023 15 Dr. Shivashankar, E&CE, RRIT
  • 16. Web Caching: Proxy Servers • HTTP supports proxy servers. • A proxy server is a computer that keeps copies of responses to recent requests. • The HTTP client sends a request to the proxy server. The proxy server checks its cache. • If the response is not stored in the cache, the proxy server sends the request to the corresponding server. Incoming responses are sent to the proxy server and stored for future requests from other clients. • The proxy server reduces the load on the original server, decreases traffic, and improves latency. • To use the proxy server, the client must be configured to access the proxy instead of the target server. 2/11/2023 16 Dr. Shivashankar, E&CE, RRIT
  • 17. FTP • File Transfer Protocol (FTP) is the standard protocol provided by TCP/IP for copying a file from one host to another. • Although transferring files from one system to another seems simple and straightforward, some problems must be dealt with first. • For example, two systems may use different file name conventions. • Two systems may have different ways to represent data. • Two systems may have different directory structures. • Although we can transfer files using HTTP, FTP is a better choice to transfer large files or to transfer files using different formats. 2/11/2023 17 Dr. Shivashankar, E&CE, RRIT Figure 26.10 FTP
  • 18. FTP: Two Connections Control Connection: • It remains connected during the entire interactive FTP session. • When a user starts an FTP session, the control connection opens. • For control communication, FTP uses the same approach as TELNET. • During this control connection, commands are sent from the client to the server and responses are sent from the server to the client. • Commands, which are sent from the FTP client control process, are in the form of ASCII uppercase. Table 26.4 Some FTP commands 2/11/2023 18 Dr. Shivashankar, E&CE, RRIT Command Argument(s) Description ABOR Abort the previous command CDUP Change to parent directory CWD Directory name Change to another directory DELE File name Delete a file LIST Directory name List subdirectories or files MKD Directory name Create a new directory PASS User password Password PASV Server chooses a port PORT Port identifier Client chooses a port PWD Display name of current directory QUIT Log out of the system RETR File name(s) Retrieve files; files are transferred from server to client RMD Directory name Delete a directory RNFR File name (old) Identify a file to be renamed RNTO File name (new) Rename the file STOR File name(s) Store files; file(s) are transferred from client to server STRU F, R, or P Define data organization (F: file, R: record, or P: page) TYPE A, E, I Default file type (A: ASCII, E: EBCDIC, I: image) USER User ID User information MODE S, B, or C Define transmission mode (S: stream, B: block, or C: compressed
  • 19. Data Connection • The data connection is opened and then closed for each file transfer activity. • It opens each time commands that involve transferring files are used, and it closes when the file is transferred. • The data connection uses the well-known port 20 at the server site. • The following shows the steps to create data connection:  A connection data source stores information about how a data connection should access an external system. This includes details such as server information, database name and authentication method.  Using the PORT command the client sends this port number to the server.  The server receives the port number and issues an active open using the well- known port 20 and the received ephemeral port number. 2/11/2023 19 Dr. Shivashankar, E&CE, RRIT
  • 20. ELECTRONIC MAIL • Electronic Mail (e-mail) is one of most widely used services of Internet. • In an application such as HTTP or FTP, the server program is running all the time, waiting for a request from a client. • When the request arrives, the server provides the service. • There is a request and there is a response. • Message in mail not only contain text, but it also contains images, audio and videos data. • The person who is sending mail is called sender and person who receives mail is called recipient. • Architecture 2/11/2023 20 Dr. Shivashankar, E&CE, RRIT
  • 21. E-mail architecture 1. a user agent (UA), 2. a message transfer agent (MTA), 3. and a message access agent (MAA). 2/11/2023 21 Dr. Shivashankar, E&CE, RRIT Figure 26.12 Common scenario
  • 22. Web-Based Mail • Webmail is a more advanced technology that allows the user to manage emails without having to install fancy and huge software on their computer machine, they achieve this by hosting their service on the cloud. • Webmail is provided by many companies and ISPs. • Since they are hosted on the cloud they require an internet connection, also the emails cannot be downloaded or modified in the offline mode. • This necessity for an internet connection constitutes the disadvantage of using webmail. 2/11/2023 22 Dr. Shivashankar, E&CE, RRIT Figure 26.22 Web-based e-mail, cases I and II
  • 23. E-Mail Security • e-mail exchanges can be secured using two application-layer securities designed in particular for e- mail systems. • Two of these protocols, Pretty Good Privacy (PGP) and Secure/Multipurpose Internet Mail Extensions (S/MIME) 2/11/2023 23 Dr. Shivashankar, E&CE, RRIT
  • 24. TELNET • TELNET stands for Teletype Network. • It is a type of protocol that enables one computer to connect to local computer. • It is a used as a standard TCP/IP protocol for virtual terminal service which is given by ISO. • Computer which starts connection known as the local computer. Computer which is being connected to i.e. which accepts the connection known as remote computer. When the connection is established between local and remote computer. • Telnet operates on client/server principle. • Local computer uses telnet client program and the remote computers uses telnet server program. • Telnet can be used for a variety of activities on a server, including editing files, running various programs and checking email. 2/11/2023 24 Dr. Shivashankar, E&CE, RRIT
  • 25. Local versus Remote Logging 2/11/2023 25 Dr. Shivashankar, E&CE, RRIT Figure 26.23 Local versus remote logging
  • 26. Domain Name system (DNS) • Since the Internet is so huge today, a central directory system cannot hold all the mapping. • In addition, if the central computer fails, the whole communication network will collapse. • A better solution is to distribute the information among many computers in the world. • In this method, the host that needs mapping can contact the closest computer holding the needed information. • This method is used by the Domain Name System (DNS). 2/11/2023 26 Dr. Shivashankar, E&CE, RRIT Figure 26.28 Purpose of DNS
  • 27. • DNS is a directory service that provides a mapping between the name of a host on the network and its numerical address. • DNS is required for the functioning of the internet. • Each node in a tree has a domain name, and a full domain name is a sequence of symbols specified by dots. • DNS is a service that translates the domain name into IP addresses. This allows the users of networks to utilize user-friendly names when looking for other hosts instead of remembering the IP addresses. • For example, suppose the FTP site at EduSoft had an IP address of 132.147.165.50, most people would reach this site by specifying ftp.EduSoft.com. • Therefore, the domain name is more reliable than IP address. 2/11/2023 27 Dr. Shivashankar, E&CE, RRIT
  • 28. Name Space • The names assigned to machines must be carefully selected from a name space with complete control over the binding between the names and IP addresses. • In other words, the names must be unique because the addresses are unique. • A name space that maps each address to a unique name can be organized in two ways:  flat or  hierarchical. • In a flat name space, a name is assigned to an address. A name in this space is a sequence of characters without structure. • In a hierarchical name space, each name is made of several parts. The first part can define the nature of the organization, the second part can define the name of an organization, the third part can define departments in the organization, and so on. 2/11/2023 28 Dr. Shivashankar, E&CE, RRIT
  • 29. DNS in the Internet • DNS is a protocol that can be used in different platforms. In the Internet, the domain name space (tree) was originally divided into three different sections: generic domains, country domains, and the inverse domains. • Generic Domains • It defines the registered hosts according to their generic behavior. • Each node in a tree defines the domain name, which is an index to the DNS database. • It uses three-character labels, and these labels describe the organization type. • Ex: edu, .com, .org etc. 2/11/2023 29 Dr. Shivashankar, E&CE, RRIT
  • 30. Resolution • Mapping a name to an address is called name-address resolution. • DNS is designed as a client-server application. • A host that needs to map an address to a name or a name to an address calls a DNS client called a resolver. DNS Messages • To retrieve information about hosts, DNS uses two types of messages: query and response. 2/11/2023 30 Dr. Shivashankar, E&CE, RRIT Figure 26.38 DNS message
  • 31. Registrars • How are new domains added to DNS? This is done through a registrar, a commercial entity accredited by ICANN. • A registrar first verifies that the requested domain name is unique and then enters it into the DNS database. A fee is charged. • Today, there are many registrars; their names and addresses can be found at • http://www.intenic.net • To register, the organization needs to give the name of its server and the IP address of the server. • For example, a new commercial organization named wonderful with a server named ws and IP address 200.200.200.5 needs to give the following information to one of the registrars: • Domain name: ws.wonderful.com IP address: 200.200.200.5 2/11/2023 31 Dr. Shivashankar, E&CE, RRIT
  • 32. Dynamic Domain Name System (DDNS) • The DNS master file must be updated dynamically. In DDNS, when a binding between a name and an address is determined, the information is sent, usually by DHCP. • To provide security and prevent unauthorized changes in the DNS records, DDNS can use an authentication mechanism. Security of DNS • DNS is one of the most important systems in the Internet infrastructure; it provides cru- cial services to Internet users. • Applications such as Web access or e-mail are heavily dependent on the proper operation of DNS. • To protect DNS, IETF has devised a technology named DNS Security (DNSSEC) that provides message origin authentication and message integrity using a security service called digital signature. 2/11/2023 32 Dr. Shivashankar, E&CE, RRIT