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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.


 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

Daniel Experimental [Page 1] RFC 2169 HTTP in URN Resolution June 1997

 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
 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.

Daniel Experimental [Page 2] RFC 2169 HTTP in URN Resolution June 1997

 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? HTTP/1.0
     GET /uri-res/N2L? 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.

Daniel Experimental [Page 3] RFC 2169 HTTP in URN Resolution June 1997

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:
 <LI><A HREF="...url 1...">...url 1...</A>
 <LI><A HREF="...url 2...">...url 2...</A>
 where the strings ...url n... are replaced by the n'th URL in the

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

Daniel Experimental [Page 4] RFC 2169 HTTP in URN Resolution June 1997

 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
 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.

Daniel Experimental [Page 5] RFC 2169 HTTP in URN Resolution June 1997

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.

Daniel Experimental [Page 6] RFC 2169 HTTP in URN Resolution June 1997

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

Appendix B: 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.

Daniel Experimental [Page 7] RFC 2169 HTTP in URN Resolution June 1997

  # 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.
      # 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;
          print STDOUT "Location: $url\n\n";
      print "Content-Type: text/html \n\n";
      print "<html>\n";
      print "<head><title>URN Resolution: N2L</title></head>\n";
      print "<BODY>\n";
      print "<h1>URN to URL resolution failed for the URN:</h1>\n";
      print "<hr><h3>$urn</h3>\n";
      print "</body>\n";
      print "</html>\n";

Daniel Experimental [Page 8] RFC 2169 HTTP in URN Resolution June 1997


 [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

Daniel Experimental [Page 9]

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