1. 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

2. 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

3. Processes client process: process that initiates communication
server process: process that waits to be contacted
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)
ECE 4450:427/527

4. 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
process
TCP with
buffers,
variables
socket
host or
server
process
TCP with
buffers,
variables
socket
host or
server
controlled by
app developer
Internet
controlled
by OS
Dr. Nghi Tran (ECE-University of Akron)
ECE 4450:427/527

5. 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

6. Port Discovery Use well-publicized ports for different services
DNS (Domain Name System) uses port 53
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

7. Applications Two of the most popular—
The World Wide Web and .
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

8. 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

9. 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

10. Web Services
So far, we just talked about traditional applications: interactions between a human and machine
There is an increasing demand for direct computer-to-computer direction: Usually, come from business world
Dr. Nghi Tran (ECE-University of Akron)
ECE 4450:427/527

11. 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

12. 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

13. More Materials 2.1 Principles of network applications 2.2 Web and HTTP
2.3 FTP
2.4 Electronic Mail
SMTP, POP3, IMAP
2.5 DNS
2.6 P2P applications
2.7 Socket programming with TCP
2.8 Socket programming with UDP
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)
ECE 4450:427/527