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Table Of Contents  The TCP/IP Guide
 9  TCP/IP Application Layer Protocols, Services and Applications (OSI Layers 5, 6 and 7)
      9  TCP/IP Key Applications and Application Protocols
           9  TCP/IP File and Message Transfer Applications and Protocols (FTP, TFTP, Electronic Mail, USENET, HTTP/WWW, Gopher)
                9  TCP/IP World Wide Web (WWW, "The Web") and the Hypertext Transfer Protocol (HTTP)
                     9  TCP/IP Hypertext Transfer Protocol (HTTP)
                          9  HTTP General Operation and Connections

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HTTP Operational Model and Client/Server Communication
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HTTP Persistent Connection Establishment, Management and Termination
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HTTP Transitory and Persistent Connections and Pipelining
(Page 2 of 3)

Persistent Connections

The solution to the inefficiency problem of transitory connections came in HTTP/1.1, which allows an HTTP client and server to set up a persistent connection. The basic operation of HTTP is not changed; the main difference is that by default, the TCP connection is kept open after each request/response set, so that the next request and response can be exchanged immediately. The session is only closed when the client is done requesting all the documents it needs.

Keeping the TCP connection between an HTTP client and server alive between requests is probably the single most important way that HTTP/1.1 improves performance over version 1.0. Clients are able to get their files more quickly because they don't have to wait for a TCP connection before each resource is retrieved. Server load is reduced and memory use in busy servers is conserved. Network congestion is reduced through the elimination of unnecessary TCP handshaking segments.

Pipelining

Persistent connections offer another important performance-enhancing option to HTTP clients: the ability to pipeline requests. Suppose the client needs to send a request for files A, B and C to a server. Since the requests for all of these files will be sent in the same TCP session, there is no need for the client to wait for a response to its request for A before sending the request for B. The client can send requests “rapid-fire”, one after the other. This also improves the efficiency of the server, which will be able to fill the requests in the order in which they are received, as soon as it is able, without having to pause to wait for each new request to be sent.

Key Concept: HTTP/0.9 and HTTP/1.0 only supported transitory connections between an HTTP client and server, where just a single request and response could be exchanged on a TCP connection. This is very inefficient for the modern Web, where clients frequently need to make dozens of requests to a server. HTTP/1.1 operates by default using persistent connections: once a TCP connection is established, the client can send many requests to the server and receive replies to each in turn. This allows files to be retrieved more quickly, and conserves server resources and Internet bandwidth. The client can even pipeline its requests, sending the second one immediately, without having to first wait for a reply to the first one. HTTP/1.1 still supports transitory connections for backwards compatibility, when needed.



Previous Topic/Section
HTTP Operational Model and Client/Server Communication
Previous Page
Pages in Current Topic/Section
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2
3
Next Page
HTTP Persistent Connection Establishment, Management and Termination
Next Topic/Section

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Version 3.0 - Version Date: September 20, 2005

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