19.7.1 Compatibility with HTTP/1.0 Persistent Connections
19.7.1 Compatibility with HTTP/1.0 Persistent Connections
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:
Connection: Keep-Alive
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.
