High Performance Web Sites

Keep-Alive

HTTP is built on top of Transmission Control Protocol (TCP). In early implementations of HTTP, each HTTP request required opening a new socket connection. This is inefficient because many HTTP requests in a web page go to the same server. For example, most requests for images in a web page all go to a common image server. Persistent Connections (also known as Keep-Alive in HTTP/1.0) was introduced to solve the inefficiency of opening and closing multiple socket connections to the same server. It lets browsers make multiple requests over a single connection. Browsers and servers use the Connection header to indicate Keep-Alive support. The Connection header looks the same in the server's response.

GET /us.js.yimg.com/lib/common/utils/2/yahoo_2.0.0-b2.js HTTP/1.1
Host: us.js2.yimg.com
User-Agent: Mozilla/5.0 (...) Gecko/20061206 Firefox/1.5.0.9
Accept-Encoding: gzip,deflate
Connection: keep-alive

HTTP/1.1 200 OK
Content-Type: application/x-javascript
Last-Modified: Wed, 22 Feb 2006 04:15:54 GMT
Connection: keep-alive

The browser or server can close the connection by sending a Connection: close header. Technically, the Connection: keep-alive header is not required in HTTP/1.1, but most browsers and servers still include it.

Pipelining, defined in HTTP/1.1, allows for sending multiple requests over a single socket without waiting for a response. Pipelining has better performance than persistent connections. Unfortunately, pipelining is not supported in Internet Explorer (up ...