Some clients and servers may wish to be compatible with some previous implementations of persistent connections in HTTP/1.0 clients and servers. Persistent connections in HTTP/1.0 must be explicitly negotiated as they are not the default behavior. HTTP/1.0 experimental implementations of persistent connections are faulty, and the new facilities in HTTP/1.1 are designed to rectify these problems. The problem was that some existing 1.0 clients may be sending Keep-Alive to a proxy server that doesn't understand Connection, which would then erroneously forward it to the next inbound server, which would establish the Keep-Alive connection and result in a hung HTTP/1.0 proxy waiting for the close on the response. The result is that HTTP/1.0 clients must be prevented from using Keep-Alive when talking to proxies.
However, talking to proxies is the most important use of persistent connections, so that prohibition is clearly unacceptable. Therefore, we need some other mechanism for indicating a persistent connection is desired, which is safe to use even when talking to an old proxy that ignores Connection. Persistent connections are the default for HTTP/1.1 messages; we introduce a new keyword (Connection: close) for declaring non-persistence.
The following describes the original HTTP/1.0 form of persistent connections.
When it connects to an origin server, an HTTP client MAY send the Keep-Alive connection-token in addition to the Persist connection- token:
An HTTP/1.0 server would then respond with the Keep-Alive connection token and the client may proceed with an HTTP/1.0 (or Keep-Alive) persistent connection.
An HTTP/1.1 server may also establish persistent connections with HTTP/1.0 clients upon receipt of a Keep-Alive connection token. However, a persistent connection with an HTTP/1.0 client cannot make use of the chunked transfer-coding, and therefore MUST use a Content-Length for marking the ending boundary of each message.
A client MUST NOT send the Keep-Alive connection token to a proxy server as HTTP/1.0 proxy servers do not obey the rules of HTTP/1.1 for parsing the Connection header field.