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ECE 4450:427/527 - Computer Networks Spring 2014

ECE 4450:427/527 - Computer Networks Spring 2014. Dr. Nghi Tran Department of Electrical & Computer Engineering. Lecture 8: Application Layer. Network Applications. write programs that run on (different) end systems communicate over network

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ECE 4450:427/527 - Computer Networks Spring 2014

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  1. ECE 4450:427/527 - Computer NetworksSpring 2014 Dr. Nghi Tran Department of Electrical & Computer Engineering Lecture 8: Application Layer 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 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 ECE 4450:427/527

  4. Sockets host or server host or server process process socket socket TCP with buffers, variables TCP with buffers, variables • 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 controlled by app developer Internet controlled by OS 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 ECE 4450:427/527

  6. 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 ECE 4450:427/527

  7. 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. 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. 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. 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 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 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 individualapplications that offer a remotely-accessible service to client applications to form network applications. • SOAP (Simple Object Access Protocol) & REST (Representational State Transfer) 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. ECE 4450:427/527

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