Invalid HTTP2 preface handling?

Section 3.5 says:

   Clients and servers MUST treat an invalid connection preface as a
   connection error (Section 5.4.1
<http://tools.ietf.org/html/draft-ietf-httpbis-http2-16#section-5.4.1>)
of type PROTOCOL_ERROR.  A GOAWAY
   frame (Section 6.8
<http://tools.ietf.org/html/draft-ietf-httpbis-http2-16#section-6.8>)
MAY be omitted in this case, since an invalid
   preface indicates that the peer is not using HTTP/2.

I'm wondering what would be the problem if on an invalid preface, the
server check if it is actually a valid HTTP/1 request and if so, then to
proceed on that basis?     This would allow a server to converse a HTTP/1
client that is pointed at a HTTP/2 port - it could be application specific
if that conversation was an error message or just a normal HTTP/1
conversation.

Is there some attack we would be enabling if we allowed such behaviour in
our server?  Anything else undesirable about doing this?

regards


-- 
Greg Wilkins <gregw@intalio.com>  @  Webtide - *an Intalio subsidiary*
http://eclipse.org/jetty HTTP, SPDY, Websocket server and client that scales
http://www.webtide.com  advice and support for jetty and cometd.