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ECE 4450:427/527 - Computer Networks
Spring 2017
Dr. Nghi Tran
Department of Electrical & Computer Engineering
Lecture 8: Application Layer
Dr. Nghi Tran (ECE-University of Akron)
ECE 4450:427/527
Computer Networks
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Network Applications
write programs that
– run on (different) end systems
– communicate over network
– e.g., web server software communicates with browser software
No need to write software for network-core devices
– network-core devices do not run user applications
– applications on end systems allows for rapid app development,
propagation
Dr. Nghi Tran (ECE-University of Akron)
ECE 4450:427/527
Computer Networks
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Processes
process: an instance of a
program running within a
host.
• within same host, two
processes communicate
using inter-process
communication (defined by
OS).
• processes in different hosts
communicate by
exchanging messages
Dr. Nghi Tran (ECE-University of Akron)
client process: process that
initiates communication
server process: process that
waits to be contacted
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Computer Networks
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Sockets
• process sends/receives
messages to/from its socket
• socket analogous to door
– sending process: shoves message
out its door
– sending process relies on
transport infrastructure on other
side of its door, which brings
message to socket at receiving
process
– When message arrives at the host,
it passes through receiving's
process door (socket); receiving
process will act on message
– A software interface: Interface
between application layer and
transport layer
Dr. Nghi Tran (ECE-University of Akron)
host or
server
host or
server
process
controlled by
app developer
socket
socket
TCP with
buffers,
variables
ECE 4450:427/527
process
Internet
TCP with
buffers,
variables
controlled
by OS
Computer Networks
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Addressing Processes: Ports
• Application processes communicate by exchanging messages
across the network
• Web: a client browser process exchanges messages with a web
server process.
• We need something to identify receiving and sending processes:
Port numbers
• A web-server is identified by port 80
• A mail server is identified by port 25
• Port number is needed, since a host might run many network
applications
Dr. Nghi Tran (ECE-University of Akron)
ECE 4450:427/527
Computer Networks
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Port Discovery
• Use well-publicized ports for different services
– DNS (Domain Name System) uses port 53
– Email uses port 25
– HTTP uses port 80
• Use one port as a “port-mapper” service
– Call 411 to learn the port of any other process
– Allows for dynamic allocation of ports to different
services
– Allows for the assignment of ports to newly created
services
Dr. Nghi Tran (ECE-University of Akron)
ECE 4450:427/527
Computer Networks
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Applications
• Two of the most popular—
– The World Wide Web and
– Email.
• Broadly speaking, both of these applications use the
request/reply paradigm—users send requests to
servers, which then respond accordingly.
Dr. Nghi Tran (ECE-University of Akron)
ECE 4450:427/527
Computer Networks
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Traditional Applications
• It is important to distinguish between application
programs and application protocols.
• For example, the HyperText Transport Protocol (HTTP)
is an application protocol that is used to retrieve Web
pages from remote servers.
• There can be many different application programs—
that is, Web clients like Internet Explorer, Chrome,
Firefox, and Safari—that provide users with a
different look and feel, but all of them use the same
HTTP protocol to communicate with Web servers
over the Internet.
Dr. Nghi Tran (ECE-University of Akron)
ECE 4450:427/527
Computer Networks
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Applications
• Two very widely-used, standardized application
protocols:
– SMTP: Simple Mail Transfer Protocol is used to exchange
electronic mail.
– HTTP: HyperText Transport Protocol is used to
communicate between Web browsers and Web servers.
Dr. Nghi Tran (ECE-University of Akron)
ECE 4450:427/527
Computer Networks
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Web Services
• So far, we just talked about traditional applications:
interactions between a human and machine
• There is an increasing demand for direct computerto-computer direction: Usually, come from business
world
Dr. Nghi Tran (ECE-University of Akron)
ECE 4450:427/527
Computer Networks
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Example
• We order a book from Amazon
• Once book is shipped, we are able to track via
Fedex.com
• But we are also able to track its directly from
Amazon:
– Amazon needs to send a query to Fedex in the format
Fedex understands, interpret the result, so that the result
can be displayed
– Amazon and Fedex needs a protocol to exchange
information – Package Tracking Protocol
Dr. Nghi Tran (ECE-University of Akron)
ECE 4450:427/527
Computer Networks
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Web Services
• There are so many potential protocols of this type: We will
have a new problem – the Scale.
• It is not scale in the size of the network, but in the number of
different kinds of applications
• We better to have some tools to simplify the task of specifying
protocols and building them
– Two architectures have been advocated as solutions, both referred to
as Web Services
– The name is so generic, many mistakenly assume it includes any sort of
services associated with the Web.
– In fact, name is taken the term for the individual applications that offer
a remotely-accessible service to client applications to form network
applications.
– SOAP (Simple Object Access Protocol) & REST (Representational State
Transfer)
Dr. Nghi Tran (ECE-University of Akron)
ECE 4450:427/527
Computer Networks
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More Materials
2.1 Principles of network
applications
2.2 Web and HTTP
2.3 FTP
2.4 Electronic Mail
2.6 P2P applications
2.7 Socket programming with
TCP
2.8 Socket programming with
UDP
– SMTP, POP3, IMAP
2.5 DNS
Computer Networking: A Top Down Approach, 6th edition, Chapter 2.
By Jim Kurose, Keith Ross Addison-Wesley, April 2012.
Dr. Nghi Tran (ECE-University of Akron)
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Computer Networks
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