Re: 505 response a MUST?

I wrote:

> >>  ... and section 3.1 spells out various rules about version number
> >>  usage, but does not specify that a server MUST send a 505 response
> >>  if it receives a major version number higher than the highest
> >>  version it implements.

On Fri, 27 Feb 1998, Roy T. Fielding wrote:

> That isn't why 505 was created.  It allows a future server to deny
> service to older protocols.  Since we cannot know whether HTTP/2.0
> is incompatible with HTTP/1.1 (the major version change does not imply
> incompatibility, it just removes the requirement for compatibility),
> it is inappropriate for a server to be required to respond in error
> to a message it might be able to respond to normally.  That's why it
> is not a MUST, and why Apache responds normally to an HTTP-version of
> HTTP/2.0 if it can interpret the request as an HTTP/1.1 server.

  That logic does not seem sound to me - if changing the major version
number means that the protocol _may_ be not backward compatible, then a
recipient that does not implement 2.0 cannot know whether or not it can
_correctly_ interpret the message as a 1.1 message.  Your logic assumes
that the authors of 2.0 will not make any semantic changes to existing
protocol elements.

  The well-known problems with intermediate systems altering the response
version also, I think, argue in favor of the more restrictive usage.