A feature of Hypertext Transfer Protocol (HTTP) version 1.1 that minimizes the number of connections that Web browsers need to make to access content on Web servers. Keepalives, which are also known as persistent connections, are supported by both Microsoft Internet Information Services (IIS) and Microsoft Internet Explorer.
When a Web browser that supports keepalives (such as Internet Explorer 4) makes an HTTP GET request to a Web server that supports keepalives (such as IIS 4), the Web browser includes a new “Connection:Keep-Alive” header in the list of HTTP headers that it sends to the Web server in the request. The Web server responds by giving the client the file it requested (usually an HTML page or an image file). After the server sends the file to the client, instead of closing the TCP/IP socket it keeps the socket open for a period of time in case the client wants to download additional files. A typical Web page might include a dozen images, and normally up to four sockets are kept open for transferring files between the client and the server.
The term “keepalives” also refers to special packets used to keep a TCP connection open on a TCP/IP internetwork.
Keepalives do not work unless they are supported by both the Web browser and the Web server.