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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 Features, Capabilities and Issues

HTTP Caching Features and Issues 1 2 3 HTTP Security and Privacy

In my overview of the HTTP operational model, I described how HTTP was designed to support not just communication between a client and server, but also the inclusion of intermediaries that may sit in the communication path between them. One of the most important types of intermediary is a device called a proxy server, or more simply, just a proxy.

A proxy is a “middleman” that acts as both a client and a server. It accepts requests from a client as if it were a server, then forwards them (possibly modifying them) to the real server, which sees the proxy as a client. The server responds back to the proxy, which forwards the reply back to the client. Proxies can be either transparent, meaning that they do not modify requests and responses, or non-transparent, if they do so in order to provide a particular service.

Since proxies have the ability to fully process all client requests and server responses, they can be extremely useful in a number of circumstances. They can be used to implement or enhance many important capabilities.

Proxies can be set up to examine both outgoing requests and incoming responses, to address various security concerns. For example, filtering can be set up to prevent users from requesting “objectionable” content, or to screen out harmful replies such as files containing hidden viruses.

As I mentioned in the previous topic, it can be advantageous to set up a “shared cache” that is implemented on an intermediary, so resources requested by one client can be made available to another. This can be done within a proxy server.

In some circumstances, the existence of a proxy server can significantly improve performance, particularly by reducing latency. An excellent example of this is the proxy server that is used by my own satellite Internet connection.

Due to the distance from the earth to the satellite, it takes over 500 milliseconds for a round trip request/response cycle between my PC and an Internet server. If I load a Web page containing images, I would have to wait 500+ milliseconds to get the HTML page, at which point my browser would then have to generate new requests for each graphical element, meaning another 500+ millisecond delay for each.

Instead, my ISP has a proxy server to which I send my requests for Web pages. It looks through the HTML of these pages and automatically requests any elements such as graphics for me. It then sends them straight back to my machine, cutting the time required to display a full Web page drastically.

HTTP Caching Features and Issues 1 2 3 HTTP Security and Privacy