Chapter02_SG - CH01-02_p1-30 4:34 PM Page 13 CHAPTER 2 The Application Layer 13 CH01-02_p1-30 4:34 PM Page 14 14 STUDY COMPANION FOR COMPUTER

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CHAPTER 2 The Application Layer 13
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14 STUDY COMPANION FOR COMPUTER NETWORKING, THIRD EDITION Most Important Ideas and Concepts from Chapter 2 t Application-layer protocol. In Chapter 1 we noted that “ A protocol defines the format and the order of messages exchanged between two or more communicat- ing entities, as well as the actions taken on the transmission and/or receipt of a mes- sage or other event .” In chapter 2, we have seen how processes send and receive messages in an application-layer protocol. As a review, identify the messages ex- changed and actions taken by the following protocols: HTTP, FTP, DNS, SMTP. t Client/server versus peer-to peer. These are the two approaches that we studied structuring a network application. In the client/server paradigm (see page 75 of the textbook), a client process requests a service by sending one or more messages to a server process. The server process implements a service by reading the client request, performing some action (for example, in the case of an HTTP server, find- ing a Web page), and sending one or more messages in reply (in the case of HTTP, returning the requested object). In a peer-to-peer approach, the two ends of the protocol are equals (as in a telephone call). t Two services provided by the Internet’s transport layer: reliable, congestion- controlled data transfer (TCP), and unreliable data transfer (UDP). These are the only services available to an Internet application to transfer data from one process to another remote process. The Internet transport layer does not provide a minimum guaranteed transfer rate, or a bound on the delay from source to destination. t HTTP: request/response interaction. The HTTP protocol is a simple applica- tion-layer protocol. A client (Web browser) makes a request with a GET message, and a Web server provides a reply (see Figure 2.6 on page 89 in your textbook). This is a classical client/server approach. Since HTTP uses TCP to provide reliable transfer of the GET request from client-to-server, and the reply from server-to- client, a TCP connection must be set up. ATCP setup request is sent from the TCP in the client to the TCP in the server, with the TCP server replying to the TCP client. Following this exchange, the HTTP GET message can be sent over the TCP connection from client-to-server, and the reply received (see Figure 2.7 on page 92 in your textbook). With non-persistent HTTP, a new TCP connection must be set up each time the client wants to contact the server. With persistent HTTP, multi- ple HTTP GET messages can be sent over a single TCP connection, resulting in performance gains from not having to set up a new TCP for each of the HTTP re- quests beyond the first. t
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This note was uploaded on 09/19/2011 for the course CS 324 taught by Professor Mohammedhajali during the Spring '10 term at Amity University.

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Chapter02_SG - CH01-02_p1-30 4:34 PM Page 13 CHAPTER 2 The Application Layer 13 CH01-02_p1-30 4:34 PM Page 14 14 STUDY COMPANION FOR COMPUTER

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