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Versions: 00 01 02

HTTP Working Group                                     J. Mogul, DECWRL,
Internet-Draft                                       J. Cohen, Netscape,
Expires: 30 January 1997                    S. Lawrence, Agranat Systems
                                                            28 July 1997

               Specification of HTTP/1.1 OPTIONS messages

                     draft-ietf-http-options-00.txt


STATUS OF THIS MEMO

        This document is an Internet-Draft. Internet-Drafts are
        working documents of the Internet Engineering Task Force
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ABSTRACT

        RFC2068 defined a new OPTIONS method for HTTP/1.1.  The
        purpose of OPTIONS is to allow a client to determine the
        options and/or requirements associated with a resource, or
        the capabilities of a server, without implying a resource
        action or initiating a resource retrieval.  However,
        RFC2068 did not defined a specific syntax for using OPTIONS
        to make such a determination.  This proposal clarifies the
        original specification of OPTIONS, adds several new HTTP
        message headers to provide syntactic support for OPTIONS,
        and establishes new IANA registries to avoid namespace
        conflicts.


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                           TABLE OF CONTENTS

1 Introduction                                                         2
2 Outline of proposed solution                                         2
3 Proposed solution                                                    3
     3.1 Changes to section 5.1.2, Request-URI                         3
     3.2 Changes to section 9.2, OPTIONS                               4
     3.3 Changes to section 14.31, Max-Forwards                        5
     3.4 The Compliance header                                         6
     3.5 The Non-Compliance header                                     7
     3.6 Changes to sections 14.7 and 14.35, Allow and Public          8
          3.6.1 Alternative A: proxies MUST NOT modify Allow/Public    9
          3.6.2 Alternative B: proxies MUST modify Allow/Public        9
     3.7 Examples                                                     10
4 Security Considerations                                             10
5 Acknowledgements                                                    11
6 References                                                          11
7 Authors' addresses                                                  11


1 Introduction

   Section 9.2 of RFC2068 [2] defines an OPTIONS method, to allow a
   "client to determine the options and/or requirements associated with
   a resource, or the capabilities of a server, without implying a
   resource action or initiating a resource retrieval."  For example, a
   client may wish to determine if a particular HTTP method is supported
   by a server, or for a specific resource.  Or, a client may wish to
   determine if a server supports the use of a particular HTTP
   request-header.

   The description of OPTIONS in RFC2068 has left some implementors
   confused about what is required, and does not provide a specific
   syntax for determining support for specific options or extensions.
   While some of this might be obviated in the future by the Protocol
   Extension Protocol (PEP) [1], there exists an immediate need to
   define a simple and well-specified OPTIONS mechanism for HTTP/1.1.


2 Outline of proposed solution

      - The intended recipient of an OPTIONS request may be any
        server (including proxies) along the request path.  RFC2068
        supported this by requiring a transformation of the
        request-URI for a set of methods (actually, only for
        OPTIONS); in the current proposal, one can either use the
        Host header to address a specific server or proxy, or the
        Max-Forwards header to address the Nth server on a path.

      - As in RFC2068, the URI '*' refers to the server,
        independent of any specific resource.  Any other URI refers
        to the resource normally identified by that URI.
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      - The descriptions of the Allow and Public headers, and of
        the OPTIONS method, are made consistent in their
        requirements for proxy editing of OPTIONS responses.  (In
        RFC2068, these sections were contradictory).

      - A (new) Compliance header is proposed, which allows a
        client to specify exactly what options it is asking about,
        and which allows a server to specify exactly what subset of
        those options are supported.

      - The Compliance header allows several namespaces for
        options; the set of namespaces is under IANA control.  One
        namespace is that of IETF-issued RFCs; this allows a more
        specific definition of compliance than is available using
        protocol version numbers.  While various interpretations
        can and do exist about the specific meaning of a protocol
        version number (such as "HTTP/1.1"), the meaning of an RFC
        is both well-defined and (more important) immutable.

      - A (new) Non-Compliance header is proposed, allowing a proxy
        processing an OPTIONS response to indicate its
        non-compliance with one or more options, and without
        requiring the proxy to edit the rest of the response (which
        would result in loss of information).


3 Proposed solution

   Here we propose specific changes to RFC2068.

3.1 Changes to section 5.1.2, Request-URI
   Remove this:

    If a proxy receives a request without any path in the Request-URI
    and the method specified is capable of supporting the asterisk form
    of request, then the last proxy on the request chain MUST forward
    the request with "*" as the final Request-URI. For example, the
    request

          OPTIONS http://www.ics.uci.edu:8001 HTTP/1.1

    would be forwarded by the proxy as

          OPTIONS * HTTP/1.1
          Host: www.ics.uci.edu:8001

    after connecting to port 8001 of host "www.ics.uci.edu".





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3.2 Changes to section 9.2, OPTIONS
   Replace:

    Unless the server's response is an error, the response MUST NOT
    include entity information other than what can be considered as
    communication options (e.g., Allow is appropriate, but Content-Type
    is not). Responses to this method are not cachable.

   with:

    An OPTIONS request MAY include Compliance headers (see section
    14.ZZZ) that indicate the set of options the sender wants
    information about.

    Responses to OPTIONS are not cachable, unless caching is explicitly
    allowed by the originating sender (see section 13.4).

   Replace:

    If the Request-URI is an asterisk ("*"), the OPTIONS request is
    intended to apply to the server as a whole. A 200 response SHOULD
    include any header fields which indicate optional features
    implemented by the server (e.g., Public), including any extensions
    not defined by this specification, in addition to any applicable
    general or response-header fields. As described in section 5.1.2, an
    "OPTIONS *" request can be applied through a proxy by specifying the
    destination server in the Request-URI without any path information.

   with:

    If the Request-URI is an asterisk ("*"), the OPTIONS request is
    intended to apply to the server as a whole.  A 200 response SHOULD
    include a Public header field (see section 14.35).  If the request
    includes a Compliance header field, a 200 response SHOULD include a
    Compliance header field, indicating the subset of the requested
    Compliance options supported by the server as a whole.  The response
    SHOULD include any other applicable general or response-header
    fields.

    If an OPTIONS request includes a Host header (see section 14.23),
    this is the intended destination of the OPTIONS method.
    Proxy servers MUST forward such a message until it reaches
    the specified host.  If the specified host has more than
    one `virtual server', the OPTIONS request applies to the
    specified virtual server.

       Note: An OPTIONS request may also include a Max-Forwards header,
       as described in section 14.31.  This allows the sender to select
       the Nth proxy on a path, without knowing its hostname.



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   Replace:

    If the Request-URI is not an asterisk, the OPTIONS request applies
    only to the options that are available when communicating with that
    resource.  A 200 response SHOULD include any header fields which
    indicate optional features implemented by the server and applicable
    to that resource (e.g., Allow), including any extensions not defined
    by this specification, in addition to any applicable general or
    response-header fields. If the OPTIONS request passes through a
    proxy, the proxy MUST edit the response to exclude those options
    which apply to a proxy's capabilities and which are known to be
    unavailable through that proxy.

   with:

    If the Request-URI is not an asterisk, the OPTIONS request applies
    only to the options that are available when communicating with that
    resource.  A 200 response SHOULD include an Allow header field (see
    section 14.7).  If the request includes a Compliance header field, a
    200 response SHOULD include a Compliance header field, indicating
    the subset of the requested Compliance options supported by the
    server as a whole.  If the subset is empty, the response SHOULD
    include a Compliance header with an empty field-value.  The response
    SHOULD include any other applicable general or response-header
    fields.

       Note: if an OPTION request contains a Compliance header, and the
       response does not, the response may have been generated by
       RFC2068-compliant implementation, which would not support
       Compliance.  In this case, it is not possible to infer that the
       sender fails to support all of the options listed in the
       Compliance header of the request.

    If the OPTIONS request passes through a
    proxy, the proxy SHOULD add a Non-Compliance header field (see
    section 14.QQQ) to the response, to list those options that apply to
    a proxy's capabilities and that are known to be unavailable through
    that proxy.

3.3 Changes to section 14.31, Max-Forwards
   Replace:

    Each proxy or gateway recipient of a TRACE request containing a Max-
    Forwards header field SHOULD check and update its value prior to
    forwarding the request.

   with:

    Each proxy or gateway recipient of a TRACE or OPTIONS request
    containing a Max-Forwards header field SHOULD check and update its
    value prior to forwarding the request.

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3.4 The Compliance header
   Insert in section 14, as a new subsection titled ``14.ZZZ
   Compliance''

    The Compliance general header field lists a set of options
    that may or may not be supported by a server.  In a request
    message, this header lists the set of options that a client
    wishes to know about.  In a response message, this header
    lists the set of options that the server complies with.

    A compliance header MAY appear on any message, but is
    normally used with the OPTIONS request (see section 9.2).

        Compliance = "Compliance" ":" ("*" | *(compliance-option))

        compliance-option = compliance-namespace "="
                                option-item [ option-params ]

        compliance-namespace = token

        option-item = token | quoted-string

        option-params = 1#( ";" option-param)

        option-param = "cond" | "uncond" | token | quoted-string

    A Compliance header field with the field-value of "*" MAY
    be used in a request, to ask about all options complied
    with by the recipient.  This field-value MUST NOT be used
    in a response.

    The compliance-namespace is used to select from one of several
    namespaces for compliance options.  The option-item is used
    to specify one or more options within a namespace.


















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    The Internet Assigned Numbers Authority (IANA) acts as a registry
    for compliance-namespace tokens. Initially, the registry contains
    the following tokens:

        "RFC"   Compliance is with an RFC, specified by an RFC number.
                For example, "rfc=1945".

        "HDR"   Compliance is with a named HTTP header.  For example,
                "HDR=Authorization".  There is no IANA registry for
                HTTP header names, but to avoid potential namespace
                confusion, only those HTTP headers listed in an
                IETF standards-track document should be used in
                this namespace.

        "PEP"   Compliance is with a PEP-specified extension, identified
                using a quoted-string containing the PEP extension
                declaration.

    The option-param is used to provide additional parameters.
    Unconditional compliance with a compliance-option is indicated
    using the "uncond" option-param; for example, "rfc=1945;uncond".
    Conditional compliance is indicated using the "cond" option-param;
    for example, "HDR=Authorization;uncond".  Additional option-param
    values might be defined as part of another specification.

    Examples:

        Compliance: rfc=2068;uncond
        Compliance: rfc=1945;uncond, rfc=2068;cond
        Compliance: rfc=2068, hdr=PEP, hdr=SetCookie2
        Compliance: rfc=9999999;uncond;"onlyOn=Tuesdays"

3.5 The Non-Compliance header
   Insert in section 14, as a new subsection titled ``14.QQQ
   Non-Compliance''

    A proxy server SHOULD add this response-header to a response
    containing a Compliance header if the proxy does not implement one
    or more of the options described in the Compliance header.

         Non-Compliance =  "Non-Compliance" ":" 1#non-compliance-option

         proxy-host = host [ ":" port ]

         non-compliance-option = compliance-option "@" proxy-host







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    A non-compliance-option listed in a Non-Compliance response-header
    field indicates that the proxy server named by the proxy-host value
    does not support the listed compliance-option.  The set of
    non-compliance options SHOULD be a subset of the compliance-options
    listed in a Compliance header field of the forwarded message.

       Note: because the proxy-host value is not authenticated,
       this is only for advisory purposes (e.g., for debugging).

    A proxy MUST NOT delete a Non-Compliance header that it has
    received from another server.

3.6 Changes to sections 14.7 and 14.35, Allow and Public
   The problem we address here is that RFC2068's specifications for the
   Allow and Public headers are inconsistent as to whether a proxy
   "MUST" or "MUST NOT" edit them.  We believe that they should be
   consistent.  Given that, there are arguments for either alternative:

      - Requiring proxies to edit these headers provides the
        ultimate client with a simple way to determine if a method
        is allowed along the entire path to the origin server.

      - However, requiring proxies not to edit these headers allows
        a client to find out about the capabilities of the origin
        server, since (as RFC2068 says about the Allow header) "the
        user agent may have other means of communicating with the
        origin server."

   The second alternative seems more robust.  Although we do not
   currently have an efficient mechanism for finding out if a method is
   supported along the entire path, presumbly any request using an
   unsupported method would immediately be rejected.  However, we list
   both alternatives in the hope that further discussion will lead to a
   more satisfying solution.

      Note: one possibility, not yet explored in detail, is that the
      compliance-namespace could be extended to include a "METH"
      token, allowing the Compliance header (and hence the
      Non-Compliance header) to completely replace the Allow and
      Public headers.  E.g., the client could send

       Compliance: METH=*

      to which the origin server might respond

       Compliance: METH=GET,METH=PUT,METH=HEAD

      If this passes through a proxy that bans (e.g.) PUT, the proxy
      could forward the response as

       Compliance: METH=GET,METH=PUT,METH=HEAD
       Non-Compliance: METH=PUT@roproxy.net
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3.6.1 Alternative A: proxies MUST NOT modify Allow/Public
   In section 14.35 (Public), replace

    This header field applies only to the server directly connected to
    the client (i.e., the nearest neighbor in a chain of connections).
    If the response passes through a proxy, the proxy MUST either remove
    the Public header field or replace it with one applicable to its own
    capabilities.

   with:

    A proxy MUST NOT modify the Public header field even if it does not
    understand all the methods specified, since the user agent might
    have other means of communicating with the origin server.

   Also, in section 14.7 (Allow), replace

    A proxy MUST NOT modify the Allow header field even if it does not
    understand all the methods specified, since the user agent MAY have
    other means of communicating with the origin server.

   with:

    A proxy MUST NOT modify the Allow header field even if it does not
    understand all the methods specified, since the user agent might
    have other means of communicating with the origin server.

   (removes an incorrect use of the term "MAY").

3.6.2 Alternative B: proxies MUST modify Allow/Public
   In section 14.7 (Allow), replace

    A proxy MUST NOT modify the Allow header field even if it does not
    understand all the methods specified, since the user agent MAY have
    other means of communicating with the origin server.

   with:

    A proxy MUST remove methods from an Allow header field if it
    does not support the use of those methods for the resource
    identified by the Request-URI.

   and in section 14.35 (Public), replace this paragraph:

    This header field applies only to the server directly connected to
    the client (i.e., the nearest neighbor in a chain of connections).
    If the response passes through a proxy, the proxy MUST either remove
    the Public header field or replace it with one applicable to its own
    capabilities.

   with:

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    A proxy MUST remove methods from a Public header field if it
    does not support the use of those methods.

3.7 Examples

    To list all extensions supported by proxy "proxy4.microscape.com"

      Client sends:
          OPTIONS * HTTP/1.1
          Host: proxy4.microscape.com
          Compliance: *

      proxy4.microscape.com responds:
          HTTP/1.1 200 OK
          Date: Tue, 22 Jul 1997 20:21:51 GMT
          Server: SuperProxy/1.0
          Public: OPTIONS, GET, HEAD, PUT, POST, TRACE
          Compliance: rfc=1543, rfc=2068, hdr=set-proxy
          Compliance: hdr=wonder-bar-http-widget-set
          Compliance: PEP="http://foobar.pep.org/pepmeister/"
          Content-Length: 0

   [Editorial note: check syntax of PEP extensions]

    Probing for a feature which is not supported by
    "proxy4.microscape.com"

      Client sends:
          OPTIONS * HTTP/1.1
          Host: proxy4.microscape.com
          Compliance: PEP="http://foobar.pep.org/evil-not-implemented"

      proxy4.microscape.com responds:
          HTTP/1.1 200 OK
          Date: Tue, 22 Jul 1997 20:21:52 GMT
          Server: SuperProxy/1.0
          Public: OPTIONS, GET, HEAD, PUT, POST, TRACE
          Compliance:
          Content-Length: 0


4 Security Considerations

   Because the proxy-host value in a Non-Compliance header is not
   authenticated, in theory, a malicious proxy along the path could
   insert a Non-Compliance header with the name of some other proxy,
   perhaps one not even involved in the response.  However, because the
   proxy-host value is used only for advisory purposes (e.g., for
   debugging), there does not appear to be a serious security problem
   with this lack of authentication.


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   Besides, any proxy along the request/response path for an HTTP
   interaction is able to perform far more disruptive (and far less
   easily detected) modifications of the messages it forwards; this
   proposal does not change that.


5 Acknowledgements

   We would like to thank Roy Fielding, Jim Gettys, Paul Leach, and
   Larry Masinter for help in constructing this proposal.


6 References

   1.  D. Connolly, R. Khare, H. Frystyk Nielsen.  PEP - an Extension
   Mechanism for HTTP.  Internet Draft draft-ietf-http-pep-04, HTTP
   Working Group, July, 1997. This is a work in progress.

   2.  Roy T. Fielding, Jim Gettys, Jeffrey C. Mogul, Henrik Frystyk
   Nielsen, and Tim Berners-Lee.  Hypertext Transfer Protocol --
   HTTP/1.1.  RFC 2068, HTTP Working Group, January, 1997.


7 Authors' addresses

   Jeffrey C. Mogul
   Western Research Laboratory
   Digital Equipment Corporation
   250 University Avenue
   Palo Alto, California, 94305, USA
   Email: mogul@wrl.dec.com

   Josh Cohen
   Netscape Communications Corporation
   501 E. Middlefield Rd
   Mountain View, CA 94043
   Phone (415) 937-4157
   EMail: josh@netscape.com

   Scott Lawrence
   Agranat Systems, Inc.
   1345 Main St.
   Waltham, MA 02154
   Phone:  +1-617-893-7868
   Fax:    +1-617-893-5740
   Email:  lawrence@agranat.com






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