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 General Operation and Connections 1 2 3 HTTP Transitory and Persistent Connections and Pipelining

Intermediaries and The HTTP Request/Response Chain

The simple request/response pair between a client and server becomes more complex when intermediaries are placed in the virtual communication path between the client and server. These are devices such as proxies, gateways or tunnels that are used to improve performance, provide security or perform other necessary functions for particular clients or servers. Proxies are particularly commonly used on the Web, because they can greatly improve response time for groups of related client computers.

When an intermediary is involved in HTTP communication, it acts as a “middleman”. Rather than the client speaking directly to the server and vice-versa, they each talk to the intermediary. This allows the intermediary to perform functions such as caching, translation, aggregation, or encapsulation. For example, consider an exchange through a single intermediary device. The two-step communication process above would become four steps:

1. Client Request: The HTTP client sends a request message to the intermediary device.
   
   
2. Intermediary Request: The intermediary processes the request, making changes to it if necessary. It then forwards the request to the actual server.
   
   
3. Server Response: The server reads and interprets the request, takes appropriate action and then sends a response. Since it received its request from the intermediary, its reply goes back to the intermediary.
   
   
4. Intermediary Response: The intermediary processes the request, again possibly making changes, and then forwards it back to the client.

As you can see, the intermediary acts as if it were a server from the client's perspective, and as a client from the server's viewpoint. Many intermediaries are designed to be able to “intercept” a variety of TCP/IP protocols, by “posing” as the server to a client and the client to a server. Most protocols are unaware of the existence of the interposition of an intermediary in this fashion. HTTP, however, includes special support for certain intermediaries such as proxy servers, providing headers that control how intermediaries handle HTTP requests and replies.

It is possible for two or more intermediaries to be linked together between the client and server. For example, the client might send a request to intermediary 1, which then forwards to intermediary 2, which then talks to the server; see Figure 316. The process is reversed for the reply. The HTTP standard uses the phrase request/response chain to refer collectively to the entire set of devices involved in an HTTP message exchange.

HTTP General Operation and Connections 1 2 3 HTTP Transitory and Persistent Connections and Pipelining