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Expectation Failed MUST or SHOULD?

From: Scott Lawrence <lawrence@agranat.com>
Date: Tue, 17 Feb 1998 16:58:51 -0500
Message-Id: <199802172158.QAA02286@devnix.agranat.com>
To: HTTP Working Group <http-wg@cuckoo.hpl.hp.com>
Cc: Jim Gettys <jg@w3.org>

  I noticed this in the course of trying to work through documenting
  what features we implement (I guess that means that it's working
  :-).  I think this is just an editorial issue - there is a SHOULD
  here that may leave implementors with the idea that the Expect
  header can be ignored.

  draft-ietf-http-v11-spec-rev-01.txt says:

    14.47 Expect

    The Expect request-header field is used to indicate that particular
    server behaviors are required by the client.  A server that does not
    understand or is unable to comply with any of the expectation values in
    the Expect field of a request MUST respond with appropriate error
    status.
       [...ABNF...]
    The server SHOULD respond with a 417 (Expectation Failed) status if any
    of the expectations cannot be met.

    This header field is defined with extensible syntax to allow for future
    extensions.  If a server receives a request containing an Expect field
    that includes an expectation-extension that it does not support, it MUST
    respond with a 417 (Expectation Failed) status.

  I'm not sure why the SHOULD in the second paragraph above is not a
  MUST other than to allow for the possibility of responding with some
  other 4xx status because of another problem with the request.  If
  so, this should be clarified, perhaps by rewording it as something
  like:

    The server MUST respond with 417 (Expectation Failed) if any of
    the expectations cannot be met or, if there are other problems
    with the request, some other 4xx status.

  If the thinking was/is that the SHOULD is to allow for
  2068-compliant implementations then it should be changed - a
  2068-complaint implementation may not be compliant with the Draft
  Standard, and that is OK.

--
Scott Lawrence           EmWeb Embedded Server       <lawrence@agranat.com>
Agranat Systems, Inc.        Engineering            http://www.agranat.com/
Received on Tuesday, 17 February 1998 14:01:53 EST

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