HTTP: The Definitive Guide

The Mysteries of Connection Close

Connection management—particularly knowing when and how to close connections—is one of the practical black arts of HTTP. This issue is more subtle than many developers first realize, and little has been written on the subject.

“At Will” Disconnection

Any HTTP client, server, or proxy can close a TCP transport connection at any time. The connections normally are closed at the end of a message,^[18] but during error conditions, the connection may be closed in the middle of a header line or in other strange places.

This situation is common with pipelined persistent connections. HTTP applications are free to close persistent connections after any period of time. For example, after a persistent connection has been idle for a while, a server may decide to shut it down.

However, the server can never know for sure that the client on the other end of the line wasn’t about to send data at the same time that the “idle” connection was being shut down by the server. If this happens, the client sees a connection error in the middle of writing its request message.

Content-Length and Truncation

Each HTTP response should have an accurate Content-Length header to describe the size of the response body. Some older HTTP servers omit the Content-Length header or include an erroneous length, depending on a server connection close to signify the actual end of data.

When a client or proxy receives an HTTP response terminating in connection close, and the actual transferred ...