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Versions: 00 01 02 03 04 05 RFC 2718

INTERNET-DRAFT                                           Larry Masinter
<draft-ietf-urlreg-guide-05.txt>                   Harald T. Alvestrand
March 25, 1999                                              Dan Zigmond
                                                             Rich Petke


                  Guidelines for new URL Schemes


Status of this Memo

   This document is an Internet-Draft and is in full conformance with
   all provisions of Section 10 of RFC 2026.  Internet-Drafts are
   working documents of the Internet Engineering Task Force (IETF), its
   areas, and its working groups.  Note that other groups may also
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   are draft documents valid for a maximum of six months and may be
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   is inappropriate to use Internet-Drafts as reference material or to
   cite them other than as "work in progress."  The list of current
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   http://www.ietf.org/ietf/1id-abstracts.txt  The list of
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   http://www.ietf.org/shadow.html.

   Distribution of this memo is unlimited.

   This Internet Draft expires September 25, 1999.

Copyright Notice

   Copyright (C) The Internet Society (1999).  All Rights Reserved.

Abstract

   A Uniform Resource Locator (URL) is a compact string representation
   of the location for a resource that is available via the Internet.
   This document provides guidelines for the definition of new URL
   schemes.


1. Introduction

   A Uniform Resource Locator (URL) is a compact string representation
   of the location for a resource that is available via the Internet.
   RFC 2396 [1] defines the general syntax and semantics of URIs, and,
   by inclusion, URLs.  URLs are designated by including a "<scheme>:"
   and then a "<scheme-specific-part>".  Many URL schemes are already
   defined.

   This document provides guidelines for the definition of new URL
   schemes, for consideration by those who are defining and
   registering or evaluating those definitions.

   The process by which new URL schemes are registered is defined in
   RFC [URL-PROCESS] [2].


2. Guidelines for new URL schemes

   Because new URL schemes potentially complicate client software, new
   schemes must have demonstrable utility and operability, as well as
   compatibility with existing URL schemes.  This section elaborates
   these criteria.


2.1 Syntactic compatibility

   New URL schemes should follow the same syntactic conventions of
   existing schemes when appropriate.  If a URI scheme that has
   embedded links in content accessed by that scheme does not share
   syntax with a different scheme, the same content cannot be served up
   under different schemes without rewriting the content.  This can
   already be a problem, and with future digital signature schemes,
   rewriting may not even be possible.  Deployment of other schemes in
   the future could therefore become extremely difficult.


2.1.1 Motivations for syntactic compatibility

   Why should new URL schemes share as much of the generic URI syntax
   (that makes sense to share) as possible?  Consider the following:

   o If fragment syntax isn't shared between two schemes, (e.g. "<a
     href="#foo">"), you can't move individual completely self
     referential documents between schemes without rewriting the
     embedded references within the document.  In the Web, the fragment
     syntax is a property of the media type, and evaluated by the
     client.

   o If fragment syntax is not shared between different media types of
     the same capability (e.g. HTML, XML, Word, or image types such as
     GIF, JPEG, PNG) then you can't have a URI reference that can
     evolve to superior media types as they become available, or even
     likely work properly today with content negotiation.

   o If relative syntax (to the extent of understanding the URI is
     relative, and what part of the URI string is relative) isn't
     shared between two schemes, (e.g. "<a href="foo">"), you can't
     move sets of documents that are internally self referential
     between schemes without rewriting the embedded URIs.

   o If the ".." syntax as a path component in relative URI's isn't
     shared between schemes, you can't easily have sets of document
     sets and refer to them between schemes without rewriting the
     embedded references.

   o If the "/" syntax (to the extent of understanding that the URI
     refers to a path relative to the current naming authority, see
     section 2.1.1) isn't shared, you can't have multiple sets of
     documents easily be moved up or down in a relative hierarchy of
     names and share a common set of documents between them, without
     rewriting the content, shared either in that scheme or between
     schemes.  The best example is a site that has a common set of
     GIF's, JPEG and PNG images, and you want to reorganize the site
     changing the depth of a subtree from one depth to another, or
     from one directory to another where the depth isn't the same.

   o If naming authority syntax (e.g. what comes after "//" in most URL
     schemes, see section 2.1.1) and relative path syntax is shared, to
     the extent of understanding that the URI has a naming authority,
     and what part of the URI string is the naming authority vs. path),
     isn't shared between two schemes, you can't share identical name
     spaces and serve them up via different schemes.  (The naming
     authority syntax is a property of the scheme).  The fact that
     HTTP, and FTP have the same syntax, for example, has often been
     exploited by sites transitioning from ftp archive service to HTTP
     archive service so that the URL's can be identical between schemes
     except for the scheme; the same content can be served via two
     schemes simultaneously.


2.1.2 Improper use of "//" following "<scheme>:"

   Contrary to some examples set in past years, the use of double
   slashes as the first component of the <scheme-specific-part> of a
   URL is not simply an artistic indicator that what follows is a URL:
   Double slashes are used ONLY when the syntax of the URL's
   <scheme-specific-part> contains a hierarchical structure as
   described in RFC 2396.  In URLs from such schemes, the use of double
   slashes indicates that what follows is the top hierarchical element
   for a naming authority.  (See section 3 of RFC 2396 for more
   details.)  URL schemes which do not contain a conformant
   hierarchical structure in their <scheme-specific-part> should not
   use double slashes following the "<scheme>:" string.


2.1.3 Compatibility with relative URLs

   URL schemes should use the generic URL syntax if they are intended
   to be used with relative URLs.  A description of the allowed
   relative forms should be included in the scheme's definition.
   Many applications use relative URLs extensively.  Specifically,

   o Can the scheme be parsed according to RFC 2396 - that is, if the
     tokens "//", "/", ";", "?" and "#" are used, do they have the
     meaning given in RFC 2396?

   o Does the scheme make sense to use it in relative URLs like those
     RFC 2396 specifies?

   o If the scheme syntax is designed to be broken into pieces, does
     the documentation for the scheme's syntax specify what those
     pieces are, why it should be broken in this way, and why the
     breaks aren't where RFC 2396 says that they usually should be?

   o If the scheme has a hierarchy, does it go left-to-right and with
     slash separators like RFC 2396?  If not, why not?


2.1.4 Compatibility with fragment syntax

   Fragment syntax should be shared across URL schemes whenever
   possible.  Fragments indicate a location within a particular
   document, of a particular media type.  As media types evolve,
   and content negotiation becomes deployed, a shared fragment syntax
   allows a fragment to point to the correct location within documents
   of different media types.  For example, a named fragment (#foo),
   should to be able to point to the foo label in either a HTML
   document or an XML document.  Similarly for fragments identifying a
   location in an image, where the image may want to evolve from GIF,
   to JPEG, to PNG, the fragment ID should point to the same location.


2.2 Is the scheme well defined?

   It is important that the semantics of the "resource" that a URL
   "locates" be well defined.  This might mean different things
   depending on the nature of the URL scheme.


2.2.1 Clear mapping from other name spaces

   In many cases, new URL schemes are defined as ways to translate
   other protocols and name spaces into the general framework of
   URLs.  The "ftp" URL scheme translates from the FTP protocol, while
   the "mid" URL scheme translates from the Message-ID field of
   messages.

   In either case, the description of the mapping must be complete,
   must describe how characters get encoded or not in URLs, must
   describe exactly how all legal values of the base standard can be
   represented using the URL scheme, and exactly which modifiers,
   alternate forms and other artifacts from the base standards are
   included or not included.  These requirements are elaborated
   below.


2.2.2 URL schemes associated with network protocols

   Most new URL schemes are associated with network resources that
   have one or several network protocols that can access them.  The
   'ftp', 'news', and 'http' schemes are of this nature.  For such
   schemes, the specification should completely describe how URLs are
   translated into protocol actions in sufficient detail to make the
   access of the network resource unambiguous.  If an implementation
   of the URL scheme requires some configuration, the configuration
   elements must be clearly identified.  (For example, the 'news'
   scheme, if implemented using NTTP, requires configuration of the
   NTTP server.)


2.2.3 Definition of non-protocol URL schemes

   In some cases, URL schemes do not have particular network protocols
   associated with them, because their use is limited to contexts
   where the access method is understood.  This is the case, for
   example, with the "cid" and "mid" URL schemes.  For these URL
   schemes, the specification should describe the notation of the
   scheme and a complete mapping of the locator from its source.


2.2.4 Definition of URL schemes not associated with data resources

   Most URL schemes locate Internet resources that correspond
   to data objects that can be retrieved or modified.  This is the
   case with "ftp" and "http", for example.  However, some URL schemes
   do not; for example, the "mailto" URL scheme corresponds to an
   Internet mail address.

   If a new URL scheme does not locate resources that are data
   objects, the properties of names in the new space must be clearly
   defined.


2.2.5 Character encoding

   When describing URL schemes in which (some of) the elements of
   the URL are actually representations of sequences of characters,
   care should be taken not to introduce unnecessary variety in the
   ways in which characters are encoded into octets and then into
   URL characters.  Unless there is some compelling reason for a
   particular scheme to do otherwise, translating character sequences
   into UTF-8 (RFC 2279) [3] and then subsequently using the %HH
   encoding for unsafe octets is recommended.


2.2.6 Definition of operations

   In some contexts (for example, HTML forms) it is possible to
   specify any one of a list of operations to be performed on a
   specific URL.  (Outside forms, it is generally assumed to be
   something you GET.)

   The URL scheme definition should describe all well-defined
   operations on the URL identifier, and what they are supposed to
   do.

   Some URL schemes (for example, "telnet") provide location
   information for hooking onto bi-directional data streams, and don't
   fit the "infoaccess" paradigm of most URLs very well; this should
   be documented.

   NOTE: It is perfectly valid to say that "no operation apart from
   GET is defined for this URL".  It is also valid to say that "there's
   only one operation defined for this URL, and it's not very
   GET-like".  The important point is that what is defined on this type
   is described.


2.3 Demonstrated utility

   URL schemes should have demonstrated utility.  New URL schemes are
   expensive things to support.  Often they require special code in
   browsers, proxies, and/or servers.  Having a lot of ways to say the
   same thing needless complicates these programs without adding value
   to the Internet.

   The kinds of things that are useful include:

   o Things that cannot be referred to in any other way.

   o Things where it is much easier to get at them using this scheme
     than (for instance) a proxy gateway.


2.3.1 Proxy into HTTP/HTML

   One way to provide a demonstration of utility is via a gateway
   which provides objects in the new scheme for clients using an
   existing protocol.  It is much easier to deploy gateways to a new
   service than it is to deploy browsers that understand the new URL
   object.

   Things to look for when thinking about a proxy are:

   o Is there a single global resolution mechanism whereby any proxy
     can find the referenced object?
   o If not, is there a way in which the user can find any object of
     this type, and "run his own proxy"?
   o Are the operations mappable one-to-one (or possibly using
     modifiers) to HTTP operations?
   o Is the type of returned objects well defined?
      - as MIME content-types?
      - as something that can be translated to HTML?
   o Is there running code for a proxy?


2.4 Are there security considerations?

   Above and beyond the security considerations of the base mechanism
   a scheme builds upon, one must think of things that can happen in
   the normal course of URL usage.

   In particular:

   o Does the user need to be warned that such a thing is happening
     without an explicit request (GET for the source of an IMG tag,
     for instance)?  This has implications for the design of a proxy
     gateway, of course.

   o Is it possible to fake URLs of this type that point to different
     things in a dangerous way?

   o Are there mechanisms for identifying the requester that can be
     used or need to be used with this mechanism (the From: field in a
     mailto: URL, or the Kerberos login required for AFS access in the
     AFS: URL, for instance)?

   o Does the mechanism contain passwords or other security
     information that are passed inside the referring document in the
     clear (as in the "ftp" URL, for instance)?


2.5 Does it start with UR?

   Any scheme starting with the letters "U" and "R", in particular if
   it attaches any of the meanings "uniform", "universal" or
   "unifying" to the first letter, is going to cause intense debate,
   and generate much heat (but maybe little light).

   Any such proposal should either make sure that there is a large
   consensus behind it that it will be the only scheme of its type, or
   pick another name.


2.6 Non-considerations

   Some issues that are often raised but are not relevant to new URL
   schemes include the following.


2.6.1 Are all objects accessible?

   Can all objects in the world that are validly identified by a
   scheme be accessed by any UA implementing it?

   Sometimes the answer will be yes and sometimes no; often it will
   depend on factors (like firewalls or client configuration) not
   directly related to the scheme itself.


3. Security considerations

   New URL schemes are required to address all security considerations
   in their definitions.


4. References

   [1] Berners-Lee, T., Fielding, R., Masinter, L., "Uniform Resource
       Identifiers (URI): Generic Syntax", RFC 2396, August 1998

   [2] Petke, R., "Registration Procedures for URL Scheme Names",
       RFC [URL-PROCESS], November 1998

   [3] Yergeau, F., "UTF-8, A Transformation Format of Unicode and ISO
       10646", RFC 2279, January 1998.


5. Authors' Addresses

   Larry Masinter
   Xerox Corporation
   Palo Alto Research Center
   3333 Coyote Hill Road
   Palo Alto, CA 94304
   Fax: +1-415-812-4333
   EMail: masinter@parc.xerox.com

   Harald Tveit Alvestrand
   Maxware, Pirsenteret
   N-7005 Trondheim
   NORWAY
   Voice: +47 73 54 57 00
   EMail: harald.alvestrand@maxware.no

   Dan Zigmond
   WebTV Networks, Inc.
   305 Lytton Avenue
   Palo Alto, CA 94301
   USA
   Voice: +1-650-614-6071
   EMail: djz@corp.webtv.net

   Rich Petke
   UUNET Technologies
   5000 Britton Road
   P. O. Box 5000
   Hilliard, OH 43026-5000
   Voice: +1-614-723-4157
   Fax: +1-614-723-1333
   EMail: rpetke@wcom.net


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