RFC2169 - 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, RFC2168, "Resolution of Uniform
Resource Identifiers using the Domain Name System", June 1997.

[2] Berners-Lee, T, R. Fielding, H. Frystyk, RFC1945, "Hypertext
Transfer Protocol -- HTTP/1.0", T. Berners-Lee, May 1996.

[3] Fielding, R., J. Gettys, J.C. Mogul, H. Frystyk, T. Berners-Lee,
RFC2068, "Hypertext Transfer Protocol -- HTTP/1.1", Jan. 1997.

[4] Moats, R., RFC2141, "URN Syntax", May 1997.

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

[6] Berners-Lee, T., RFC1630, "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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