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Network Working Group                                      M. MeallingRequest for Comments: 2483                     Network Solutions, Inc.Category: Experimental                                  R. Daniel, Jr.                                        Los Alamos National Laboratory                                                          January 1999URI Resolution ServicesNecessary for URN ResolutionStatus of this Memo   This memo defines an Experimental Protocol for the Internet   community.  It does not specify an Internet standard of any kind.   Discussion and suggestions for improvement are requested.   Distribution of this memo is unlimited.Copyright Notice   Copyright (C) The Internet Society (1999).  All Rights Reserved.Abstract   Retrieving the resource identified by a Uniform Resource Identifier   (URI) [1] is only one of the operations that can be performed on a   URI.  One might also ask for and get a list of other identifiers that   are aliases for the original URI or a bibliographic description of   the resource the URI denotes, for example. This applies to both   Uniform Resource Names (URNs) and Uniform Resource Locators (URLs).   Uniform Resource Characteristics (URCs) are discussed in this   document but only as descriptions of resources rather than   identifiers.   A service in the network providing access to a resource may provide   one or some of these options, but it need not provide all of them.   This memo specifies an initial set of these operations that can be   used to describe the interactions provided by a given access service.   It also suggests guidelines that should be adhered to when those   operations are encoded in a protocol.1. Introduction   In the course of formulating current proposals [2] regarding URNs   [3], it became apparent that requiring servers to manage all of the   desired functions or requiring clients to process varied information   returned by a server was unrealistic and a barrier to adoption. There   needed to be some way for a client to be able to identify a server   that specialized in the complex and another that specialized in theMealling & Daniel             Experimental                      [Page 1]

RFC 2483                URI Resolution Services             January 1999   simple (but fast). Also, in subsequent conversations it became   obvious that, in most cases, some of the operations were   inappropriate or difficult for certain identifiers.   The Problem   In the process of learning about a resource in the Internet, there   are a variety of possible functions that may be important and/or   useful, such as discovery of locators, names, descriptions, and   accessing the resource itself. A given service may support only a   subset of these; hence, it is important to describe such an access   service by the types of functions supported and the resources of   which it has some knowledge. For example, in the framework for an RDS   described in [5] the RDS itself may provide URLs [6][7], while the   resolvers may provide descriptions, URLs, or even the resources   themselves. The design of an RDS, as proposed inRFC 2168 [2], may be   more generous and provide all of the above.   This problem requires some well understood set of identifiers that   specify those operations. But an exhaustive set would both be   impossible and not very necessary. Thus, this document will list   several operations, as well as, lay out requirements for specifying   new operations.   The purpose of this document is to define a list of such functions   and short names for them and then use them in defining the interface   to an access service. Previous versions of this document referred to   services where the arguments were specific types of URIs such as URNs   or URLs.  These services were called "N2L" and "L2L",for example.   Their use has been changed in favor of the more general URI form.   Design Criteria   To meet these requirements a fairly simple design criteria was used.   The need to identify the operation with some token such that its   operands, algorithm, and errors were known proved sufficient to meet   these requirements.2. General Specification   To provide a framework both for the specifications in this document   and for future work to be written by others, the guidelines below are   suggested for documents that seek to specify new operations. Any   specification of a member of this set of operations should address   these issues with respect to its operands, algorithm, output, and   errors.Mealling & Daniel             Experimental                      [Page 2]

RFC 2483                URI Resolution Services             January 1999   Due to the small number of listed functions, a registration mechanism   was dismissed as premature. If this list grows, a registration   mechanism will probably be needed.   Also, due to the experimental nature of this document and the systems   that use its specifications, the use of words like MUST and SHALL are   limited. Where used they reflect a case where this specification   could cause harm to existing, non-experimental systems such as HTTP   and URNs.  Thus, the key words "MUST", "MUST NOT", "REQUIRED",   "SHALL", "SHALL NOT", "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY",   and "OPTIONAL" in this document are to be interpreted as described inRFC 2119.2.1 Operands   Operands must contain the following pieces of information:      * name of the operation      * case insensitive mnemonic for the operation      * number of operands      * type of each operand      * format of each operand2.2 Algorithm   The exact algorithm for the operation must either be specified   completely or it must be considered opaque and defined by the server   or application.2.3 Output   Output must specify one of the following:      * there is no output      * the output is undefined      * the output itself and its content      * the fact that the output is an object and the object's        type and format      * any non-protocol specific errors2.4 Error Conditions   All errors that are considered applicable across all implementations   and application environments must be included. Errors that depend on   the system conveying the service are not included. Thus, many of the   expected errors such as service availability or operation syntax are   not included in this document since they are implementation   dependent.Mealling & Daniel             Experimental                      [Page 3]

RFC 2483                URI Resolution Services             January 19992.5 Security Considerations   Any security considerations relating to the service provided must be   specified. This does NOT include considerations dealing with the   protocol used to convey the service or to those that normally   accompany the results of the service. For example, a service that   returned a single URL would need to discuss the situation where   someone maliciously inserts an incorrect URL into the resolver but   NOT the case where someone sends personal information across the   Internet to the resource identified by the correct URL.3. Encoding The Operations   To be useful, these operations have to be used within some system or   protocol. In many cases, these systems and protocols will place   restrictions on which operations make sense and how those that do are   syntactically represented. It is sufficient for those protocols to   define new operations within their own protocol specification   documents but care should be taken to make this fact well known.   Also, a given system or protocol will have its own output   specifications that may restrict the output formats of a given   operation.  Additionally, a given protocol may have better solution   for output than the ones given here. For example, the result of an   operation that converts a URI to more than one URL may be encoded in   a protocol-specific manner that conveys information about the   closeness of each resource on the network.   Thus, the requirements on encoding these operations within a given   system are as follows:      * which subset of the operations are allowed      * how the operator is encoded      * how the operands are encoded      * how the error codes are returned   The text/uri-list MIME Media Type is specified inSection 5. This   Media Type is merely a suggestion for experimental systems that need   a simple implementation. It is included here merely as an example to   show completeness (however simple it may be).Mealling & Daniel             Experimental                      [Page 4]

RFC 2483                URI Resolution Services             January 19994. The Incomplete Set4.1 I2L (URI to URL)      * Name: URI to URL      * Mnemonic: I2L      * Number of Operands: 1      * Type of Each Operand: First operand is a URI.      * Format of Each Operand: First operand is encoded as a URI.      * Algorithm: Opaque      * Output: One and only one URL      * Errors Conditions:           o Malformed URI           o URI is syntactically valid but does not exist in any form.           o URI exists but there is no available output from this             operation.           o URI existed in the past but nothing is currently known             about it.           o Access denied      * Security Considerations:           o Malicious Redirection             One of the fundamental dangers related to any service such             as this is that a malicious entry in a resolver's database             will cause clients to resolve the URI into the wrong URL.             The possible intent may be to cause the client to retrieve             a resource containing fraudulent or damaging material.           o Denial of Service             By removing the URL to which the URI maps, a malicious             intruder may remove the client's ability to retrieve the             resource.   This operation is used to map a single URI to a single URL. It is   used by lightweight clients that do not have the ability to select   from a list of URLs or understand a URC. The algorithm for this   mapping is dependent on the URI scheme.4.2 I2Ls (URI to URLs)      * Name: URI to URLs      * Mnemonic: I2LS      * Number of Operands: 1      * Type of Each Operand: First operand is a URI.      * Format of Each Operand: First operand is encoded as a URI.      * Algorithm: Opaque      * Output: A list of zero or more URLs      * Errors:           o Malformed URIMealling & Daniel             Experimental                      [Page 5]

RFC 2483                URI Resolution Services             January 1999           o URI is syntactically valid but does not exist in any form.           o URI exists but there is no available output from this             operation.           o URI existed in the past but nothing is currently known             about it.           o Access denied      * Security Considerations:           o Malicious Redirection (see I2L)           o Denial of Service (see I2L)   This operation is used to map a single URI to 0 or more URLs. It is   used by a client that can pick from a list of URLs based on some   criteria that are important to the client. The client should not make   any assumptions about the order of the URLs returned. No matter what   the particular media type, the result should be a list of the URLs   that may be used to obtain an instance of the resource identified by   the URI. All URIs shall be encoded according to the URL [7] and URN   [3] specifications.4.3 I2R (URI to Resource)      * Name: URI to Resource      * Mnemonic: I2R      * Number of Operands: 1      * Type of Each Operand: First operand is a URI.      * Format of Each Operand: First operand is encoded as a URI.      * Algorithm: Opaque      * Output: An instance of the resource named by the URI.      * Errors:           o Malformed URI           o URI is syntactically valid but does not exist in any form.           o URI exists but there is no available output from this             operation.           o URI existed in the past but nothing is currently known             about it.           o Access denied      * Security Considerations:           o Malicious Redirection (see I2L)           o Denial of Service (see I2L)   This operation is used to return a single instance of the resource   that is named by the URI. The format of the output is dependent on   the resource itself.Mealling & Daniel             Experimental                      [Page 6]

RFC 2483                URI Resolution Services             January 19994.4 I2Rs (URI to Resources)      * Name: URI to Resources      * Mnemonic: I2Rs      * Number of Operands: 1      * Type of Each Operand: First operand is a URI.      * Format of Each Operand: First operand is encoded as a URI.      * Algorithm: Opaque      * Output: Zero or more instances of the resource named by the URI.      * Errors:           o Malformed URI           o URI is syntactically valid but does not exist in any form.           o URI exists but there is no available output from this             operation.           o URI existed in the past but nothing is currently known             about it.           o Access denied      * Security Considerations:           o Malicious Redirection (see I2L)           o Denial of Service (see I2L)   This operation is used to return 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 URI.   The output shall be a MIME multipart/alternative [4] message with the   alternative versions of the resource in separate body parts. If there   is only one version of the resource identified by the URN, it MAY be   returned without the multipart/alternative wrapper.4.5 I2C (URI to URC)      * Name: URI to URC * Mnemonic: I2C * Number of Operands: 1 * Type      of Each Operand: First operand is a URI.  * Format of Each      Operand: First operand is encoded as a URI.  * Algorithm: Opaque *      Output: A URC * Errors:           o Malformed URI           o URI is syntactically valid but does not exist in any form.           o URI exists but there is no available output from this             operation.           o URI existed in the past but nothing is currently known             about it.           o Access denied * Security Considerations:           o Malicious Redirection (see I2L)           o Denial of Service (see I2L)Mealling & Daniel             Experimental                      [Page 7]

RFC 2483                URI Resolution Services             January 1999   Uniform Resource Characteristics are descriptions of resources. This   request allows the client to obtain a description of the resource   identified by a URI, as opposed to the resource itself or simply the   resource's URLs. The description might be a bibliographic citation, a   digital signature, or a revision history. This memo does not specify   the content of any response to a URC request. That content is   expected to vary from one server to another.4.6 I2CS (URI to URCs)      * Name: URI to URCs      * Mnemonic: I2CS      * Number of Operands: 1      * Type of Each Operand: First operand is a URI.      * Format of Each Operand: First operand is encoded as a URI.      * Algorithm: Opaque      * Output: Zero or more URCs      * Errors:           o Malformed URI           o URI is syntactically valid but does not exist in any form.           o URI exists but there is no available output from this             operation.           o URI existed in the past but nothing is currently known             about it.           o Access denied      * Security Considerations:           o Malicious Redirection (see I2L)           o Denial of Service (see I2L)   URCs can come in different formats and types. This operation returns   zero or more URCs that are appropriate for the given URI.4.7 I2N (URI to URN)      * Name: URI to URN      * Mnemonic: I2N      * Number of Operands: 1      * Type of Each Operand: First operand is a URN.      * Format of Each Operand: First operand is encoded as a URI.      * Algorithm: Opaque      * Output: One and only one URN      * Errors:           o Malformed URI           o URI is syntactically valid but does not exist in any form.           o URI exists but there is no available output from this             operation.           o URI existed in the past but nothing is currently known             about it.Mealling & Daniel             Experimental                      [Page 8]

RFC 2483                URI Resolution Services             January 1999           o Access denied      * Security Considerations:           o Malicious Redirection (see I2L)           o Denial of Service (see I2L)   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 one organization wishes to name   'foo'; another organization, in agreement with the first, wants to   call the resource 'bar'. Both organizations can agree that both names   'name' the same resource and that the URNs 'foo' and 'bar' are   equivalent.   The result is a URN, known to the server, that identifies the same   resource as the input URN.   Extreme care should be taken with this service as it toys with the   idea of equality with respect to URNs. As mentioned in several URN   documents, the idea of equality is very domain specific. For example,   a URN pointing to a weather map for a particular day and a URN   pointing to the map as it changes from day to day would NOT be   returned in this example because they point to do different   resources. Some other concept of temporary equivalence is at work.   This service instead deals with resources that have two different   names where there is a binding between the names that is agreed by   both name assigners. I.e., both namespaces MUST have agreed that the   each name can be used in place of the other and the meaning does not   change.4.8 I2Ns (URI to URNs)      * Name: URI to URNs      * Mnemonic: I2NS      * Number of Operands: 1      * Type of Each Operand: First operand is a URI.      * Format of Each Operand: First operand is encoded as a URI.      * Algorithm: Opaque      * Output: A list of URNs      * Errors:           o Malformed URI           o URI is syntactically valid but does not exist in any form.           o URI exists but there is no available output from this             operation.           o URI existed in the past but nothing is currently known             about it.           o Access denied      * Security Considerations:           o Malicious Redirection (see I2L)Mealling & Daniel             Experimental                      [Page 9]

RFC 2483                URI Resolution Services             January 1999           o Denial of Service (see I2L)   This operation simply returns zero or more URNs following the same   criteria and cautions as the I2N operation.4.9 I=I (Is URI equal to URI):      * Name: URI = URI      * Mnemonic: I=I      * Number of Operands: 2      * Type of Each Operand: Both operands are URIs.      * Format of Each Operand: Both operands are encoded as a URIs.      * Algorithm: Opaque      * Output: TRUE or FALSE      * Errors:           o Malformed URIs           o URIs are syntactically valid but do not exist in any form.           o URIs exist but there is no available output from this             operation.           o URIs existed in the past but nothing is currently known             about them.           o Access denied      * Security Considerations:           o Malicious Redirection (see I2L)           o Denial of Service (see I2L)   This operation is used to determine whether two given URIs are   considered to be equal by the server being asked the question. The   algorithm used to determine equality is opaque. No assertions are   made about whether or not the URIs exhibits characteristics of URNs   or URLs.Mealling & Daniel             Experimental                     [Page 10]

RFC 2483                URI Resolution Services             January 19995. The text/uri-list Internet Media Type   Several of the resolution service requests, such as I2Ls, I2Ns,   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.   This is a copy of the IANA registration of the text/uri-list Media   Type.    Date: Fri, 18 Apr 97 08:36:07 PDT    From: Ron Daniel Jr. <rdaniel@lanl.gov>    To: iana@iana.org, rdaniel@lanl.gov    Subject: Request for MIME media type Text/IETF Tree - uri-list    Name : Ron Daniel Jr.    E-mail : rdaniel@lanl.gov    MIME media type name : Text    MIME subtype name : IETF Tree -uri-list    Required parameters : none    Optional parameters : charset    Currently, URIs can be represented using US-ASCII. However, there    are many non-standard URIs which use special character sets.    Discussion of how to best achieve internationalization of URIs is    underway. This registration will be updated with a discussion of the    URI charsets once that discussion has concluded.    Encoding considerations : Some transfer protocols, such as SMTP,    place limits on the length of lines. Very long URIs might exceed    those limits. Systems must therefore be prepared to use a suitable    content transfer encoding. This is anticipated to be a rare    occurance.    Security considerations : Client software should be aware of the    security considerations of URIs.  For example, accessing some URIs    can result in sending a death threat to a head of state, frequently    prompting a visit from the relevant protective service.  Accessing    other URIs may result in financial obligations, or access to    resources considered inappropriate by one's employer.Mealling & Daniel             Experimental                     [Page 11]

RFC 2483                URI Resolution Services             January 1999    While the legitimate provider of a uri-list could exploit these    properties for good or ill, it is more likely that uri-lists will be    falsified in order to exploit such characteristics of URIs.    Additionally, the lookup and reverse lookup potential of the uri-    list may be attractive to traffic analysts. URI lists may also    reveal confidential information, such as the location of sensitive    information.    Because of these considerations, external confidentiality measures    should be available to protect uri-list responses when appropriate.    Interoperability considerations : none known    Published specification : Uniform Resource Locators (URLs) and    Uniform Resource Names (URNs) are two instances of the more general    class of identifiers known as Uniform Resource Identifiers (URIs).    URN resolution methods frequently wish to return lists of URLs for a    resource so that fault-tolerance and load balancing can be achieved.    The text/uri-list format is intended to be a very simple format for    communicating such lists of URLs (and URNs) in a form suitable for    automatic processing.    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 URIs may contain        the '#' character, so it is only a comment character when it is        the first character on a line.)    2) The remaining non-comment lines shall be URIs (URNs or URLs),        encoded according to the URL or URN specifications (RFC2141,RFC1738 andRFC2396). Each URI shall appear on one and only one        line. Very long URIs are not broken in the text/uri-list format.        Content-transfer-encodings may be used to enforce line length        limitations.    3) As for all text/* formats, lines are terminated with a CRLF pair.    In applications where one URI has been mapped to a list of URIs, the    first line of the text/uri-list response SHOULD be a comment giving    the original URI.Mealling & Daniel             Experimental                     [Page 12]

RFC 2483                URI Resolution Services             January 1999    An example of the format is given below:      # urn:isbn:0-201-08372-8http://www.huh.org/books/foo.htmlhttp://www.huh.org/books/foo.pdfftp://ftp.foo.org/books/foo.txt    Applications which use this media : URN resolvers are the initial    applications. Web clients and proxies are applications that are    likely to support this format in the future.    Additional information :    1. Magic number(s) : none at this time    2. File extension(s) : .uris or .uri recommended    3. Macintosh file type code : URIs recommended    This media type is the product of the URN working group of the IETF.    Person to contact for further information :    1. Name : Ron Daniel Jr.    2. E-mail : rdaniel@lanl.gov    Intended usage : Limited Use    The text/uri-list media type is intended for use in applications    which utilize URIs for replicated resources.    Author/Change controller : Ron Daniel Jr.    Los Alamos National Laboratory    rdaniel@lanl.govMealling & Daniel             Experimental                     [Page 13]

RFC 2483                URI Resolution Services             January 1999   In applications where one URI has been mapped to a list of URIs, such   as in response to the I2Ls 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 I2L request is shown below in Figure 1.6. Security Considerations   Communications with a server may be of a sensitive nature. Some   servers will hold information that should only be released to   authorized users. The results from servers 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 that pay royalties to rights-   holders. Server requests may be of interest to traffic analysts. The   requests may also be subject to spoofing.   The "Access denied" error message assumes a system within which the   operation is being performed that can convey an authenticated concept   of access control. Thus, the "Access denied" message should only be   returned by systems that have an appropriate method of determining   access control.7. References   [1] Berners-Lee, T., "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",RFC 1630,       June 1994.   [2] Daniel, R., and Mealling, M., "Resolution of Uniform Resource       Identifiers using the Domain Name System",RFC 2168, February       1997.   [3] Moats, R., "URN Syntax",RFC 2141, January 1997.   [4] Freed, N. and N. Borenstein, "Multipurpose Internet Mail       Extensions (MIME) Part One: Format of Internet Message Bodies",RFC 2045, November 1996.   [5] Sollins, K., "Architectural Principles of Uniform Resource Name       Resolution",RFC 2276, January 1998.   [6] Kunze, J., "Functional Recommendations for Internet Resource       Locators",RFC 1736, February 1995.   [7] Berners-Lee, T., Masinter, L. and M. McCahill, "Uniform Resource       Locators (URL)",RFC 1738, December 1994.Mealling & Daniel             Experimental                     [Page 14]

RFC 2483                URI Resolution Services             January 19998. Authors' Addresses   Michael Mealling   Network Solutions   505 Huntmar Park Drive   Herndon, VA 22070   Phone: (703) 742-0400   Fax:   (703) 742-9552   EMail: michaelm@rwhois.net   Ron Daniel   Advanced Computing Lab, MS B287   Los Alamos National Laboratory   Los Alamos, NM, USA, 87545   Phone: (505) 665-0597   Fax:   (505) 665-4939   EMail: rdaniel@lanl.govMealling & Daniel             Experimental                     [Page 15]

RFC 2483                URI Resolution Services             January 19999.  Full Copyright Statement   Copyright (C) The Internet Society (1999).  All Rights Reserved.   This document and translations of it may be copied and furnished to   others, and derivative works that comment on or otherwise explain it   or assist in its implementation may be prepared, copied, published   and distributed, in whole or in part, without restriction of any   kind, provided that the above copyright notice and this paragraph are   included on all such copies and derivative works.  However, this   document itself may not be modified in any way, such as by removing   the copyright notice or references to the Internet Society or other   Internet organizations, except as needed for the purpose of   developing Internet standards in which case the procedures for   copyrights defined in the Internet Standards process must be   followed, or as required to translate it into languages other than   English.   The limited permissions granted above are perpetual and will not be   revoked by the Internet Society or its successors or assigns.   This document and the information contained herein is provided on an   "AS IS" basis and THE INTERNET SOCIETY AND THE INTERNET ENGINEERING   TASK FORCE DISCLAIMS ALL WARRANTIES, EXPRESS OR IMPLIED, INCLUDING   BUT NOT LIMITED TO ANY WARRANTY THAT THE USE OF THE INFORMATION   HEREIN WILL NOT INFRINGE ANY RIGHTS OR ANY IMPLIED WARRANTIES OF   MERCHANTABILITY OR FITNESS FOR A PARTICULAR PURPOSE.Mealling & Daniel             Experimental                     [Page 16]