HTTP: The Definitive Guide

Persistent Connections

Web clients often open connections to the same site. For example, most of the embedded images in a web page often come from the same web site, and a significant number of hyperlinks to other objects often point to the same site. Thus, an application that initiates an HTTP request to a server likely will make more requests to that server in the near future (to fetch the inline images, for example). This property is called site locality.

For this reason, HTTP/1.1 (and enhanced versions of HTTP/1.0) allows HTTP devices to keep TCP connections open after transactions complete and to reuse the preexisting connections for future HTTP requests. TCP connections that are kept open after transactions complete are called persistent connections. Nonpersistent connections are closed after each transaction. Persistent connections stay open across transactions, until either the client or the server decides to close them.

By reusing an idle, persistent connection that is already open to the target server, you can avoid the slow connection setup. In addition, the already open connection can avoid the slow-start congestion adaptation phase, allowing faster data transfers.

Persistent Versus Parallel Connections

As we’ve seen, parallel connections can speed up the transfer of composite pages. But parallel connections have some disadvantages:

• Each transaction opens/closes a new connection, costing time and bandwidth.

• Each new connection has reduced performance because of TCP slow ...