Please Whitelist This Site? I know everyone hates ads. But please understand that I am providing premium content for free that takes hundreds of hours of time to research and write. I don't want to go to a pay-only model like some sites, but when more and more people block ads, I end up working for free. And I have a family to support, just like you. :) If you like The TCP/IP Guide, please consider the download version. It's priced very economically and you can read all of it in a convenient format without ads. If you want to use this site for free, I'd be grateful if you could add the site to the whitelist for Adblock. To do so, just open the Adblock menu and select "Disable on tcpipguide.com". Or go to the Tools menu and select "Adblock Plus Preferences...". Then click "Add Filter..." at the bottom, and add this string: "@@||tcpipguide.com^$document". Then just click OK. Thanks for your understanding! Sincerely, Charles Kozierok Author and Publisher, The TCP/IP Guide

NOTE: Using software to mass-download the site degrades the server and is prohibited. If you want to read The TCP/IP Guide offline, please consider licensing it. Thank you.

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

HTTP Operational Model and Client/Server Communication 1 2 3 HTTP Persistent Connection Establishment, Management and Termination

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.

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.

HTTP Operational Model and Client/Server Communication 1 2 3 HTTP Persistent Connection Establishment, Management and Termination