The document summarizes several key application layer protocols in computer networks. It discusses the Domain Name System (DNS) which maps domain names to IP addresses. It also describes remote login protocols like Telnet and SSH, electronic mail protocols like SMTP, file transfer protocols like FTP and SCP, and the Hypertext Transfer Protocol (HTTP) which underlies the World Wide Web. It provides an overview of Voice over IP (VoIP) technology and signaling protocols used for call setup and management in IP telephony systems.

1. C O M PUTER NETWO RKS
Applications
Mr. Shylesh B C

2. Text Book
Computer and Communication Networks
By Nader F. Mir
Pearson Edition, 2007
Chapters 9.1 to 9.6, 18.1,18.2

3. Contents
– Application-layer overview
– Domain Name System (DNS)
– Remote login protocols
– Electronic mail (e-mail)
– File transfer and FTP
– World Wide Web (WWW) and HTTP
– Overview of IP telephony
– VoIP signaling protocols

4. Application-layer overview
• The application layer is built on the transport
layer and provides network services to user
applications
• The application layer defines and performs such
applications as
– electronic mail (e-mail), remote access to computers,
file transfers, newsgroups, and the Web, as well as
streaming video, Internet radio and telephony, P2P
file sharing, multi user networked games, streaming
stored video clips, and real-time video conferencing.

5. Web communication between two
end systems

6. Domain Name System (DNS)
• One of the most important components of the application
layer is the Domain Name System (DNS) server.
• DNS is a distributed hierarchical and global directory that
translates machine or domain names to numerical IP
addresses.
• DNS can be thought as a distributed database system
used to map host names to IP addresses, and vice
versa.
• DNS is an application-layer protocol, and every Internet
service provider whether for an organization, a university
campus, or even a residence has a DNS server.

7. Hierarchy of domain name space,
labels, and domain names

8. Hierarchy of DNS domain name
servers

9. Name/Address Mapping
Recursive mapping
Iterative mapping

10. Remote login protocols
• A client/server model can create a
mechanism that allows a user to establish
a session on the remote machine and then
run its applications.
• This application is known as remote login.
• Two remote login protocols are
– TELNET
– SSH.

11. TELNET Protocol
• TELNET (terminal network) is a TCP/IP
standard for establishing a connection to a
remote system.
• TELNET allows a user to log in to a
remote machine across the Internet by
first making a TCP connection and then
pass the detail of the application from the
user to the remote machine.

12. Secure Shell (SSH) Protocol
• Secure Shell (SSH), another remote login
protocol, is based on UNIX programs.
SSH uses TCP for communications.
• It is more powerful and flexible than
TELNET and allows the user to more
easily execute a single command on a
remote client.

13. Electronic mail (e-mail)
• It is Simple Mail Transfer Protocol (SMTP) and
E-mail
• The Simple Mail Transfer Protocol (SMTP) plays
a major role in transferring Internet electronic
mail.
• This protocol transfers electronic mail (e-mail )
from the mail server of a source to the mail
servers of destinations.
• SMTP is older than the Hypertext Transfer
Protocol (HTTP), the Web communication
protocol, and imposes certain restrictions, such
as limits on the size of e-mail content.

14. Two users exchanging e-mail
through SMTP

15. File transfer and FTP
• File transfer is another computer
networking application.
• It is always essential that files and
information geographically distributed over
different locations be shared among the
members of a working group.
• Two file transfer protocols are
– FTP
– SCP.

16. File Transfer Protocol (FTP)
• File Transfer Protocol (FTP) is part of the
TCP/IP suite and is very similar to
TELNET.
• Both FTP and TELNET are built on the
client/server paradigm, and both allow a
user to establish a remote connection.
• However, TELNET provides a broader
access to a user, whereas FTP allows
access only to certain files.

17. Secure Copy Protocol (SCP)
• The Secure Copy Protocol (SCP) is similar to
TELNET but is secure.
• Incorporated in the SCP structure are a number of
encryption and authentication features that are
similar to those in SSH.
• Also similar is the exchange of commands
between local and remote hosts.
• SCP commands automatically prompt the user for
the password information when it is time to
access a remote machine.
• SCP cannot handle file transfer between
machines of significantly different architectures.

18. World Wide Web (WWW) & HTTP
• The World Wide Web (WWW), or simply
Web, is a global network of servers linked
by a common protocol allowing access to
all connected hypertext resources.
• When a client host requests an object, a
Web server responds by sending the
requested object through browsing tools.
• A browser is a user agent displaying the
requested Web page.
• The Hyper Text Transfer Protocol (HTTP)
transfers that page at the application layer.

19. • HTTP uses TCP rather than UDP, since
reliability of delivery is important for Web pages
with text.
• The TCP connection-establishment delay in
HTTP is one of the main contributing delay
factors associated with downloading Web
documents.
• HTTP is based on the client/server idea, having
a client and a server program, both of which can
be executed on different end systems.
• The communication is carried out through an
exchange of HTTP messages.

20. Web Caching (Proxy Server)

21. Overview of IP telephony
• An IP telephone can be used to make telephone calls
over IP networks.
• Voice over IP (VoIP), or IP telephony, uses packet-
switched networks to carry voice traffic in addition to data
traffic.
• The basic scheme of IP telephony starts with pulse code
modulation. The encoded data is transmitted as packets
over packet-switched networks.
• At a receiver, the data is decoded and converted back to
analog form.
• The packet size must be properly chosen to prevent large
delays.
• The IP telephone system must also be able to handle the
signaling function of the call setup, mapping of phone
number to IP address, and proper call termination.

22. Voice over IP system

23. VoIP signaling protocols
• The IP telephone system must be able to handle
signaling for call setup, conversion of phone
number to IP address mapping, and proper call
termination.
• Signaling is required for call setup, call
management, and call termination.
• In the standard telephone network, signaling
involves identifying the user's location given a
phone number, finding a route between a calling
and a called party, and handling the issue of call
forwarding and other call features.

24. THANK YOU
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