[URN] new HTTP conventions draft

["Ronald E. Daniel" <[e-mail]>]

INTERNET DRAFT                                                  Ron Daniel
draft-ietf-urn-http-conv-01.txt             Los Alamos National Laboratory
                                                               9 Jan, 1997


             Conventions for the Use of HTTP for URN Resolution


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

    This document is an Internet-Draft.  Internet-Drafts are working
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    This draft expires 21 July, 1997.
  
  

Abstract:
=========

The URN-WG was formed to specify persistent, location-independent names
for network accessible resources, as well as resolution mechanisms to retrive
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, when the URN
does not contain a domain name for that resolver. A resolver is a database
that can tell the client where the resource is, can provide information (such
as a bibliographic citation) on the resource, or may even be able to provide
the resource itself to the client. While the NAPTR draft specifies how to
locate a resolver, it does not specify how the client should speak to the
resolver. Instead, the NAPTR draft provides a field that can be used to
specify the "resolution protocol" the client may use to speak to the
resolver.

This draft establishes conventions for using HTTP as one such resolution
protocol. It specifies how to encode URN resolution requests and responses
in HTTP 1.0 (and 1.1) requests and responses. Once a client has used
NAPTR records to locate a resolver that speaks the "http" resolution
protocol, these are the conventions it must follow. The primary goal of
this draft is to define a convention that is simple to implement
and will allow existing HTTP servers to easily add support for URN
resolution. We expect that the resolution databases that arise will be
useful when more sophisticated resolution protocols are developed later.


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

The NAPTR draft[1] describes a way of using DNS to locate resolvers for
URIs.  That draft provides the services field to specify the "resolution
protocol" spoken by the resolver, as well as the "resolution services"
it offers. As of this writing, the "resolution protocols" allowed by the
NAPTR draft are HTTP, RCDS, HDL, and RWHOIS. (That list is expected to grow
over time). The NAPTR draft 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 draft specifies the conventions to follow to
encode resolution service requests in the HTTP protocol, allowing
widely available HTTP daemons to serve as URN resolvers. This is the
specification to follow when the services field ina NAPTR record
begins with "http".

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 for database lookups.


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

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

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

For example, if we have the URN "urn:cid:[e-mail]" and want a URL,
we would send the request:

    GET /uri-res/N2L/urn:cid:[e-mail] HTTP/1.0

Because of the character set limitations on URIs, we might wish to
encode the '@' character as its hex equivalent, thus the request would be

    GET /uri-res/N2L/urn:cid:foo%40huh.com HTTP/1.0

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

    GET /uri-res/N2L/urn:cid:foo%40huh.com HTTP/1.1
    Host: <whatever host we are sending the request to>

Handling these requests on the server side is easy to implement in a
number of ways. The N2L request could be handled by a CGI script that
took the incoming URN, looked it up in a database, and returned the URL
as an HTTP redirect. Service requests like N2R or N2C could be set up
so that the daemon answered the request by returning files out of N2R/
and N2C/ directories, or they could also be handled by a script that
accessed a database of information.

One caveat should be kept in mind. The URN syntax draft[4] discusses
the notion of lexical equivalance. This means that two URIs are equivalent
under certain conditions of case-insensitivity and/or %encoding of characters.
Resolvers MUST return identical results for all lexically equivalent encodings
of a URI. For example, the requests below must return identical results:
    GET /uri-res/N2L/urn:cid:[e-mail] HTTP/1.0
    GET /uri-res/N2L/URN:CID:[e-mail] HTTP/1.0
    GET /uri-res/N2L/urn:cid:foo%40huh%2ecom HTTP/1.0

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.


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

This section goes through the various resolution services established
in the URN services draft[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 HTTP 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 resons 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 encoded as:
<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 draft 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.

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:[e-mail]
     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 = $ENV{'PATH_INFO'} ;
    if(length($urn)<3)
    {
        $error=1;
    }

    if(!$error)
    {
        $urn =~s/^(\/)(urn:)?(.*)/$3/i;
        # Additional operations should be performed here
        # to convert lexically equivalent versions of a URI into
        # a canonical version for DB lookups.

        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] Ron Daniel and Michael Mealling, "Resolution of Uniform Resource
    Identifiers using the Domain Name System", draft-ietf-urn-naptr-01.txt,
    November, 1996.

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

[3] R. Fielding, J. Gettys, J.C. Mogul, H. Frystyk, T. Berners-Lee,
    "Hypertext Transfer Protocol -- HTTP/1.1", draft-ietf-http-v11-spec-06,
    July 1996.

[4] R. Moats, "URN Syntax", draft-ietf-urn-syntax-02, Jan. 1997.

[5] URN Resolution Services Draft -  (That document is in preparation.
    It will actually be strongly based on the content of this document and
    the NAPTR draft[1]). 

[6] 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", T. Berners-Lee, 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:
===========================

Ron Daniel
Advanced Computing Lab, MS B287
Los Alamos National Laboratory
Los Alamos, NM, USA, 87545
voice:  [phone]
fax:    [phone]
email:  [e-mail]


    This draft expires 21 July, 1997.
Ron Daniel Jr.                   email: [e-mail]
Advanced Computing Lab, MS B287  voice: [phone]
Los Alamos National Laboratory     fax: [phone]
Los Alamos, NM, USA 87545         http://www.acl.lanl.gov/~rdaniel/
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