Multiple or unsolicited responses are forbidden

J.P. Martin-Flatin (martin-flatin@epfl.ch)
Sun, 14 Jun 1998 20:10:26 +0200 

I was wondering whether persistent HTTP/1.1 connections could be used as 
bi-directional sockets, where both the client and the server can send data 
asynchronously to the other side. In other words:

* Can an HTTP server send data asynchronously to an HTTP client (response 
without request)?

* Can an HTTP server send multiple responses to an HTTP client, without 
the client knowing in advance how many responses it will receive, or how 
long it will take for the server to send all its responses?

This would have allowed all socket-based client/server applications to 
tunnel traffic over HTTP, and therefore go across a firewall easily.

Implicitly, section 1.4 answers no to all these questions. I think this 
should be stated explicitly (HTTP is clearly not primarily geared towards 
that, but more and more people are using HTTP outside the Web context).

Here is the first paragraph of section 1.4:

  The HTTP protocol is a request/response protocol. A client sends a
  request to the server in the form of a request method, URI, and protocol
  version, followed by a MIME-like message containing request modifiers,
  client information, and possible body content over a connection with a
  server. The server responds with a status line, including the message's
  protocol version and a success or error code, followed by a MIME-like
  message containing server information, entity metainformation, and
  possible entity-body content. The relationship between HTTP and MIME is
  described in appendix 19.4.

I suggest to add the following right after:

  The server must not send more than one response message to a given
  request message (that is, multiple responses are forbidden). The server
  must not send unsolicited messages to a client: all messages sent by
  the server are responses to requests previously issued by this client.

Jean-Philippe Martin-Flatin