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RFC 2169 - A Trivial Convention for using HTTP in URN Resolution


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Network Working Group                                       R. Daniel
Request for Comments: 2169             Los Alamos National Laboratory
Category: Experimental                                      June 1997

         A Trivial Convention for using HTTP in URN Resolution

Status of this Memo
===================

   This memo defines an Experimental Protocol for the Internet
   community.  This memo does not specify an Internet standard of any
   kind.  Discussion and suggestions for improvement are requested.
   Distribution of this memo is unlimited.

Abstract:
=========

   The Uniform Resource Names Working Group (URN-WG) was formed to
   specify persistent, location-independent names for network accessible
   resources, as well as resolution mechanisms to retrieve the resources
   given such a name. At this time the URN-WG is considering one
   particular resolution mechanism, the NAPTR proposal [1]. That
   proposal specifies how a client may find a "resolver" for a URN. A
   resolver is a database that can provide information about the
   resource identified by a URN, such as the resource's location, a
   bibliographic description, or even the resource itself. The protocol
   used for the client to communicate with the resolver is not specified
   in the NAPTR proposal.  Instead, the NAPTR resource record provides a
   field that indicates the "resolution protocol" and "resolution
   service requests" offered by the resolver.

   This document specifies the "THTTP" resolution protocol - a trivial
   convention for encoding resolution service requests and responses as
   HTTP 1.0 or 1.1 requests and responses.  The primary goal of THTTP is
   to be simple to implement so that existing HTTP servers may easily
   add support for URN resolution. We expect that the databases used by
   early resolvers will be useful when more sophisticated resolution
   protocols are developed later.

1.0  Introduction:
==================

   The NAPTR specification[1] defined a new DNS resource record which
   may be used to discover resolvers for Uniform Resource Identifiers.
   That resource record provides the "services" field to specify the
   "resolution protocol" spoken by the resolver, as well as the
   "resolution services" it offers. Resolution protocols mentioned in

   that specification are Z3950, THTTP, RCDS, HDL, and RWHOIS. (That
   list is expected to grow over time). The NAPTR specification also
   lists a variety of resolution services, such as N2L (given a URN,
   return a URL); N2R (Given a URN, return the named resource), etc.

   This document specifies the "THTTP" (Trivial HTTP) resolution
   protocol.  THTTP is a simple convention for encoding resolution
   service requests and responses as HTTP 1.0 or 1.1 requests and
   responses. The primary goal of THTTP is to have a URN resolution
   protocol that can easily be added to existing HTTP daemons. Other
   resolution protocols are expected to arise over time, so this
   document serves a secondary purpose of illustrating the information
   that needs to be specified for a URN resolution protocol. One of the
   resolution protocols we expect to be developed is an extension of
   HTTP with new methods for the resolution services. Therefore, we use
   "THTTP" as the identifier for this protocol to leave "HTTP" for later
   developments.

   The reader is assumed to be familiar with the HTTP/1.0 [2] and 1.1
   [3] specifications. Implementors of this specification should be
   familiar with CGI scripts, or server-specific interfaces, for
   database lookups.

2.0 General Approach:
=====================

   The general approach used to encode resolution service requests in
   THTTP is quite simple:

       GET /uri-res/<service>?<uri>  HTTP/1.0

   For example, if we have the URN "urn:foo:12345-54321" and want a URL,
   we would send the request:

       GET /uri-res/N2L?urn:foo:12345-54321 HTTP/1.0

   The request could also be encoded as an HTTP 1.1 request. This would
   look like:

       GET /uri-res/N2L?urn:foo:12345-54321 HTTP/1.1
       Host: <whatever host we are sending the request to>

   Responses from the HTTP server follow standard HTTP practice. Status
   codes, such as 200 (OK) or 404 (Not Found) shall be returned.  The
   normal rules for determining cachability, negotiating formats, etc.
   apply.

   Handling these requests on the server side is easy to implement using
   CGI or other, server-specific, extension mechanisms.  CGI scripts
   will see the incoming URI in the QUERY_STRING environment variable.
   Any %encoded characters in the URN will remain in their %encoded
   state in that string. The script can take the URN, look it up in a
   database, and return the requested information.

   One caveat should be kept in mind. The URN syntax document [4]
   discusses the notion of lexical equivalance and requires that
   resolvers return identical results for URNs that are lexically
   equivalent. Implementors of this specification must be careful to
   obey that rule. For example, the two requests below MUST return
   identical results, since the URNs are lexically equivalent.
       GET /uri-res/N2L?urn:cid:foo@huh.com HTTP/1.0
       GET /uri-res/N2L?URN:CID:foo@huh.com HTTP/1.0

3.0 Service-specific details:
=============================

   This section goes through the various resolution services established
   in the URN services document [5] and states how to encode each of
   them, how the results should be returned, and any special status
   codes that are likely to arise.

   Unless stated otherwise, the THTTP requests are formed according to
   the simple convention above, either for HTTP/1.0 or HTTP/1.1. The
   response is assumed to be an entity with normal headers and body
   unless stated otherwise. (N2L is the only request that need not
   return a body).

3.1  N2L (URN to URL):
----------------------

   The request is encoded as above. The URL MUST be returned in a
   Location:  header for the convienience of the user in the most common
   case of wanting the resource. If the lookup is successful, a 30X
   status line SHOULD be returned. HTTP/1.1 clients should be sent the
   303 status code. HTTP/1.0 clients should be sent the 302 (Moved
   temporarily) status code unless the resolver has particular reasons
   for using 301 (moved permanently) or 304 (not modified) codes.

   Note that access controls may be applied to this, or any other,
   resolution service request. Therefore the 401 (unauthorized) and 403
   (forbidden) status codes are legal responses. The server may wish to
   provide a body in the response to explain the reason for refusing
   access, and/or to provide alternate information about the resource,
   such as the price it will cost to obtain the resource's URL.

3.2  N2Ls (URN to URLs):
------------------------

   The request is encoded as above. The result is a list of 0 or more
   URLs. The Internet Media Type (aka ContentType) of the result may be
   negotiated using standard HTTP mechanisms if desired. At a minimum
   the resolver should support the text/uri-list media type.  (See
   Appendix A for the definition of this media type). That media type is
   suitable for machine-processing of the list of URLs. Resolvers may
   also return the results as text/html, text/plain, or any other media
   type they deem suitable.

   No matter what the particular media type, the result MUST be a list
   of the URLs which may be used to obtain an instance of the resource
   identified by the URN. All URIs shall be encoded according to the URI
   specification [6].

   If the client has requested the result be returned as text/html or
   application/html, the result should be a valid HTML docment
   containing the fragment:
   <UL>
   <LI><A HREF="...url 1...">...url 1...</A>
   <LI><A HREF="...url 2...">...url 2...</A>
    etc.
   </UL>
   where the strings ...url n... are replaced by the n'th URL in the
   list.

3.3  N2R (URN to Resource):
---------------------------

   The request is encoded as above. The resource is returned using
   standard HTTP mechanisms. The request may be modified using the
   Accept: header as in normal HTTP to specify that the result be given
   in a preferred Internet Media Type.

3.4  N2Rs (URN to Resources):
-----------------------------

   This resolution service returns multiple instances of a resource, for
   example, GIF and JPEG versions of an image. The judgment about the
   resources being "the same" resides with the naming authority that
   issued the URN.

   The request is encoded as above. The result shall be a MIME
   multipart/alternative message with the alternative versions of the
   resource in seperate body parts. If there is only one version of the
   resource identified by the URN, it MAY be returned without the

   multipart/alternative wrapper. Resolver software SHOULD look at the
   Accept: header, if any, and only return versions of the resource that
   are acceptable according to that header.

3.5  N2C (URN to URC):
----------------------

   URCs (Uniform Resource Characteristics) are descriptions of other
   resources. This request allows us to obtain a description of the
   resource identified by a URN, as opposed to the resource itself.  The
   description might be a bibliographic citation, a digital signature, a
   revision history, etc. This document does not specify the content of
   any response to a URC request. That content is expected to vary from
   one resolver to another.

   The format of any response to a N2C request MUST be communicated
   using the ContentType header, as is standard HTTP practice. The
   Accept: header SHOULD be honored.

3.6  N2Ns (URN to URNs):
------------------------

   While URNs are supposed to identify one and only one resource, that
   does not mean that a resource may have one and only one URN. For
   example, consider a resource that has something like "current-
   weather-map" for one URN and "weather-map-for-datetime-x" for another
   URN. The N2Ns service request lets us obtain lists of URNs that are
   believed equivalent at the time of the request. As the weathermap
   example shows, some of the equivalances will be transitory, so the
   standard HTTP mechanisms for communicating cachability MUST be
   honored.

   The request is encoded as above. The result is a list of all the
   URNs, known to the resolver, which identify the same resource as the
   input URN. The result shall be encoded as for the N2Ls request above
   (text/uri-list unless specified otherwise by an Accept: header).

3.7  L2Ns (URL to URNs):
----------------------

   The request is encoded as above. The response is a list of any URNs
   known to be assigned to the resource at the given URL. The result
   shall be encoded as for the N2Ls and N2Ns requests.

3.8  L2Ls (URL to URLs):
------------------------

   The request is encoded as described above. The result is a list of
   all the URLs that the resolver knows are associated with the resource
   located by the given URL. This is encoded as for the N2Ls, N2Ns, and
   L2Ns requests.

3.9  L2C (URL to URC):
----------------------

   The request is encoded as above, the response is the same as for the
   N2C request.

Appendix A: The text/uri-list Internet Media Type
=================================================
[This appendix will be augmented or replaced by the registration of the
text/uri-list IMT once that registration has been performed].

   Several of the resolution service requests, such as N2Ls, N2Ns, L2Ns,
   L2Ls, result in a list of URIs being returned to the client. The
   text/uri-list Internet Media Type is defined to provide a simple
   format for the automatic processing of such lists of URIs.

   The format of text/uri-list resources is:

   1) Any lines beginning with the '#' character are comment lines
      and are ignored during processing. (Note that '#' is a character
      that may appear in URIs, so it only denotes a comment when it is the
      first character on a line).
   2) The remaining non-comment lines MUST be URIs (URNs or URLs), encoded
      according to the URI specification RFC[6]. Each URI shall appear on
      one and only one line.
   3) As for all text/* formats, lines are terminated with a CR LF pair,
      although clients should be liberal in accepting lines with only
      one of those characters.

   In applications where one URI has been mapped to a list of URIs, such
   as in response to the N2Ls request, the first line of the text/uri-
   list response SHOULD be a comment giving the original URI.

   An example of such a result for the N2L request is shown below in
   figure 1.

        # urn:cid:foo@huh.org
        http://www.huh.org/cid/foo.html
        http://www.huh.org/cid/foo.pdf
        ftp://ftp.foo.org/cid/foo.txt

               Figure 1: Example of the text/uri-list format

Appendix B:  n2l.pl script
==========================

   This is a simple CGI script for the N2L resolution service. It
   assumes the presence of a DBM database to store the URN to URL
   mappings. This script does not specify standard behavior, it is
   provided merely as a courtesy for implementors. In fact, this script
   does not process incoming Accept: headers, nor does it generate
   status codes. Such behavior should be part of a real script for any
   of the resolution services.

    #!/bin/perl
    # N2L  - performs urn to url  resolution

    $n2l_File = "...filename for DBM database...";

    $urn = $ENV{'QUERY_STRING'} ;

    # Sanity check on the URN. Minimum length of a valid URN is
    # 7 characters - "urn:", a 1-character Namespace ID, ":", and
    # a 1-character namespace-specific string. More elaborate
    # sanity checks should be part of a real resolver script.
    if(length($urn)<7)
    {
        $error=1;
    }

    if(!$error)
    {
        # Convert lexically equivalent versions of a URI into
        # a canonical version for DB lookups.
        $urn =~ s/^urn:([^:]*):(.*)$/sprintf("urn:%s:%s", lc $1, $2)/ie;

        dbmopen(%lu,$n2l_File,0444);
        if($lu{$urn})
        {
            $url=$lu{$urn};
            print STDOUT "Location: $url\n\n";
        }else{
            $error=2;
        }
        dbmclose(%lu);
    }

    if($error)
    {
        print "Content-Type: text/html \n\n";
        print "<html>\n";
        print "<head><title>URN Resolution: N2L</title></head>\n";
        print "<BODY>\n";
        print "<h1>URN to URL resolution failed for the URN:</h1>\n";
        print "<hr><h3>$urn</h3>\n";
        print "</body>\n";
        print "</html>\n";
    }

    exit;

References:
===========

   [1] Daniel, Ron and Michael Mealling, RFC 2168, "Resolution of Uniform
       Resource Identifiers using the Domain Name System", June 1997.

   [2] Berners-Lee, T, R. Fielding, H. Frystyk, RFC 1945, "Hypertext
       Transfer Protocol -- HTTP/1.0", T. Berners-Lee, May 1996.

   [3] Fielding, R., J. Gettys, J.C. Mogul, H. Frystyk, T. Berners-Lee,
       RFC 2068, "Hypertext Transfer Protocol -- HTTP/1.1", Jan. 1997.

   [4] Moats, R., RFC 2141, "URN Syntax", May 1997.

   [5] URN-WG. "URN Resolution Services". Work In Progress.

   [6] Berners-Lee, T., RFC 1630, "Universal Resource Identifiers in WWW:
       A Unifying Syntax for the Expression of Names and Addresses of
       Objects on the Network as used in the World-Wide Web", June 1994.

Security Considerations
=======================

   Communications with a resolver may be of a sensitive nature. Some
   resolvers will hold information that should only be released to
   authorized users. The results from resolvers may be the target of
   spoofing, especially once electronic commerce transactions are common
   and there is money to be made by directing users to pirate
   repositories rather than repositories which pay royalties to
   rightsholders. Resolution requests may be of interest to traffic
   analysts. The requests may also be subject to spoofing.

   The requests and responses in this draft are amenable to encoding,
   signing, and authentication in the manner of any other HTTP traffic.

Author Contact Information:
===========================

   Advanced Computing Lab, MS B287
   Los Alamos National Laboratory
   Los Alamos, NM, USA, 87545
   voice:  +1 505 665 0597
   fax:    +1 505 665 4939
   email:  rdaniel@lanl.gov

 

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