1. ECE 4450:427/527 - Computer Networks Spring 2015 Dr. Nghi Tran Department of Electrical & Computer Engineering Lecture 8: Application Layer Dr. Nghi Tran (ECE-University of Akron) ECE 4450:427/527Computer Networks 1

2. Network Applications Dr. Nghi Tran (ECE-University of Akron) ECE 4450:427/527Computer Networks 2 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

3. Processes Dr. Nghi Tran (ECE-University of Akron) ECE 4450:427/527Computer Networks 3 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 client process: process that initiates communication server process: process that waits to be contacted

4. Sockets Dr. Nghi Tran (ECE-University of Akron) ECE 4450:427/527Computer Networks 4 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 Internet controlled by OS controlled by app developer

5. Addressing Processes: Ports Dr. Nghi Tran (ECE-University of Akron) ECE 4450:427/527Computer Networks 5 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

6. Port Discovery Dr. Nghi Tran (ECE-University of Akron) ECE 4450:427/527Computer Networks 6 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

7. Applications Dr. Nghi Tran (ECE-University of Akron) ECE 4450:427/527Computer Networks 7 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.

8. Traditional Applications Dr. Nghi Tran (ECE-University of Akron) ECE 4450:427/527Computer Networks 8 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.

9. Applications Dr. Nghi Tran (ECE-University of Akron) ECE 4450:427/527Computer Networks 9 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.

10. Web Services Dr. Nghi Tran (ECE-University of Akron) ECE 4450:427/527Computer Networks 10 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

11. Example Dr. Nghi Tran (ECE-University of Akron) ECE 4450:427/527Computer Networks 11 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

12. Web Services Dr. Nghi Tran (ECE-University of Akron) ECE 4450:427/527Computer Networks 12 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)

13. More Materials Dr. Nghi Tran (ECE-University of Akron) ECE 4450:427/527Computer Networks 13 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, 6 th edition, Chapter 2. By Jim Kurose, Keith Ross Addison-Wesley, April 2012.