Non-Persistent and Persistent Connections in Java

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222 Non-Persistent and Persistent Connections
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HTTP can use both non-persistent connections and persistent connections Non-persistent connections is the default mode for HTTP/10 Conversely, persistent connections is the default mode for HTTP/11
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Non-Persistent Connections
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Let us walk through the steps of transferring a Web page from server to client for the case of non-persistent connections Suppose the page consists of a base HTML file and 10 JPEG images, and that all 11 of these objects reside on the same server Suppose the URL for the base HTML file is wwwsomeSchooledu/someDepartment/homeindex
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file:///D|/Downloads/Livros/computa o/Computer%20NetDown%20Approach%20Featuring%20the%20Internet/httphtm (2 of 14)20/11/2004 15:51:52
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The HyperText Transfer Protocol
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Here is what happens: 1 The HTTP client initiates a TCP connection to the server wwwsomeSchooledu Port number 80 is used as the default port number at which the HTTP server will be listening for HTTP clients that want to retrieve documents using HTTP 2 The HTTP client sends a HTTP request message into the socket associated with the TCP connection that was established in step 1 The request message either includes the entire URL or simply the path name /someDepartment/homeindex (We will discuss the HTTP messages in some detail below) 3 The HTTP server receives the request message via the socket associated with the connection that was established in step 1, retrieves the object /someDepartment/homeindex from its storage (RAM or disk), encapsulates the object in a HTTP response message, and sends the response message into the TCP connection 4 The HTTP server tells TCP to close the TCP connection (But TCP doesn't actually terminate the connection until the client has received the response message in tact) 5 The HTTP client receives the response message The TCP connection terminates The message indicates that the encapsulated object is an HTML file The client extracts the file from the response message, parses the HTML file and finds references to the ten JPEG objects 6 The first four steps are then repeated for each of the referenced JPEG objects As the browser receives the Web page, it displays the page to the user Two different browsers may interpret (ie, display to the user) a Web page in somewhat different ways HTTP has nothing to do with how a Web page is interpreted by a client The HTTP specifications ([RFC 1945] and [RFC 2068]) only define the communication protocol between the client HTTP program and the server HTTP program The steps above use non-persistent connections because each TCP connection is closed after the server sends the object -- the connection does not persist for other objects Note that each TCP connection transports exactly one request message and one response message Thus, in this example, when a user requests the Web page, 11 TCP connections are generated In the steps described above, we were intentionally vague about whether the client obtains the 10 JPEGs over ten serial TCP connections, or whether some of the JPEGs are obtained over parallel TCP connections Indeed, users can configure modern browsers to control the degree of parallelism In their default modes, most browsers open five to ten parallel TCP connections, and each of these connections handles one request-response transaction If the user prefers, the maximum number of parallel connections can be set to one, in which case the ten connections are established serially As we shall see in the next chapter, the use of parallel connections shortens the response time since it cuts out some of the RTT and slow-start delays Parallel TCP connections can also allow the requesting browser to steal a larger share of its fair share of the end-to-end transmission bandwidth Before continuing, let's do a back of the envelope calculation to estimate the amount of time from when a client requests the base HTML file until the file is received by the client To this end we define the round-trip time RTT, which is the time it takes for a small packet to travel from client to server and then back to the client The RTT includes packet propagation delays, packet queuing delays in intermediate routers and switches, and packet processing delays (These delays were discussed in Section 16) Now consider what happens when a user clicks on a hyperlink This causes the browser to initiate a TCP connection between the browser and the Web server; this involves a "threeway handshake" -- the client sends a small TCP message to the server, the server acknowledges and responds with a small message, and finally the client acknowledges back to the server One RTT elapses after the first two parts of the three-way handshake After completing the first two parts of the handshake, the client sends the HTTP request message into the TCP connection, and TCP "piggybacks" the last acknowledgment (the third part of the three-way handshake) onto the request message Once the request message arrives at the server, the server sends the HTML file into the TCP connection This HTTP request/response eats up another RTT Thus, roughly, the total response time is 2RTT plus the transmission time at the server of the HTML file
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