Application Layer In Computer Layer, Study notes of Computer Networks

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Topic Name:- Introduction to Computer Network

 Contents of Topic: Behrouz A. Forouzan  Learning Outcome: Students get to know about basics of Computer Network

Figure 25.1: Logical connection at the application layer

25. 4

 1 Principles of network applications  (^) 2 Web and HTTP  3 FTP  4 Electronic Mail  5 DNS 2: Application Layer 5

Client-server architecture  (^) In client-server architecture there is an always-on host called server  (^) Which services requests from many other hosts called clients  (^) Clients do not directly communicate with each other  (^) Server has a fixed well-known address called an IP address  (^) A single server host is incapable of keeping up with all the requests from its clients Ex. News website server can quickly become overload if it has only one server handling all of its requests  (^) For this reason clusters of hosts sometime referred to as a server farm are often used to create a powerful virtual server in client server architecture

server: ◦ (^) always-on host (called server) ◦ (^) permanent IP address ◦ (^) server farms for scaling clients: ◦ (^) communicate with server ◦ (^) may be intermittently connected ◦ (^) may have dynamic IP addresses ◦ (^) do not communicate directly with each other 2: Application Layer 8 client/server

◦ (^) Client server and P2P are two common architectures for network applications ◦ (^) However many applications are organized as hybrids of the client server and P2P architecture 2: Application Layer 10

 Process: program running within a host.  Process can be thought of as a program that is running within end system  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 2: Application Layer 11  (^) Note: applications with P2P architectures have client processes & server processes

 identifier includes both IP address and port numbers associated with process on host.  (^) Example port numbers:  (^) HTTP server: 80  (^) Mail server: 25  (^) to send HTTP message to web server:  (^) IP address: 128.119.245.  (^) Port number: 80  to receive messages, process must have identifier  host device has unique 32-bit IP address  (^) Q: does IP address of host on which process runs be enough for identifying the process?  A: No, many processes can be running on same host 2: Application Layer 13

 (^) 2.1 Principles of network applications  (^) app architectures  (^) app requirements  (^) 2.2 Web and HTTP  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 2: Application Layer 14

Chapter 26: Objective

 (^) The first section introduces the World Wide Web. It then discusses the HyperText Transfer Protocol, the most common client-server application program used in relation to the World Wide Web.  (^) The second section discusses the File Transfer Protocol, which is the standard protocol provided by TCP/IP for copying a file from one host to another.  (^) The third section discusses electronic mail, which involves two protocols: SMPT and POP. As we will see, the nature of this application is different from the other two previous applications. We need two different protocols to handle electronic mail.

Chapter 26: Objective (continued)

 (^) The fourth section discusses TELNET, a general client-server program that allows users to log in to a remote machine and use any application available on the remote host.  (^) The fifth section discusses Secure Shell, which can be used as a secured TELNET, but it can also provide a secure tunnel for other applications.  (^) The sixth section talks about the Domain Name System, which acts as the directory system in the Internet. It maps the name of an entity to its IP address.

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26.26.1 World Wide Web 26.26.1 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.

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World Wide Web Architecture World Wide Web Architecture

The WWW today is a distributed client-server service, in which a client using a browser can access a service using a server. The service provided is distributed over many locations called sites. Each site holds one or more web pages. Each web page can contain some links to other web pages in the same or other sites.

• Simple web page has no links to other web pages.
• Composite web page has one or more links to other web pages. Each web page is a file with a name and address. Web Server The web page is stored at the server. Each time a