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Network Working Group                                       L. MasinterRequest for Comments: 2718                            Xerox CorporationCategory: Informational                                   H. Alvestrand                                                   Maxware, Pirsenteret                                                             D. Zigmond                                                   WebTV Networks, Inc.                                                               R. Petke                                                     UUNET Technologies                                                          November 1999Guidelines for new URL SchemesStatus of this Memo   This memo provides information for the Internet community.  It does   not specify an Internet standard of any kind.  Distribution of this   memo is unlimited.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 inRFC2717 [2].Masinter, et al.             Informational                      [Page 1]

RFC 2718             Guidelines for new URL Schemes        November 19992. 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.Masinter, et al.             Informational                      [Page 2]

RFC 2718             Guidelines for new URL Schemes        November 1999   o  If the "/" syntax (to the extent of understanding that the URI      refers to a path relative to the current naming authority, seesection 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, seesection 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 inRFC2396.  In URLs from such schemes, the use of double slashes indicates   that what follows is the top hierarchical element for a naming   authority.  (Seesection 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 toRFC 2396 - for example, if      the tokens "//", "/", ";", or "?" are used, do they have the      meaning given inRFC 2396?Masinter, et al.             Informational                      [Page 3]

RFC 2718             Guidelines for new URL Schemes        November 1999   o  Does the scheme make sense to use it in relative URLs like thoseRFC 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 whereRFC 2396 says that they usually should be?   o  If the scheme has a hierarchy, does it go left-to-right and with      slash separators likeRFC 2396?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.)Masinter, et al.             Informational                      [Page 4]

RFC 2718             Guidelines for new URL Schemes        November 19992.2.3 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.Masinter, et al.             Informational                      [Page 5]

RFC 2718             Guidelines for new URL Schemes        November 1999      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?Masinter, et al.             Informational                      [Page 6]

RFC 2718             Guidelines for new URL Schemes        November 19992.4 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.Masinter, et al.             Informational                      [Page 7]

RFC 2718             Guidelines for new URL Schemes        November 19992.6.1 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. and L. Masinter, "Uniform Resource       Identifiers (URI): Generic Syntax",RFC 2396, August 1998.   [2] Petke, R. and I. King, "Registration Procedures for URL Scheme       Names",BCP 35,RFC 2717, November 1999.   [3] Yergeau, F., "UTF-8, A Transformation Format of Unicode and ISO       10646",RFC 2279, January 1998.Masinter, et al.             Informational                      [Page 8]

RFC 2718             Guidelines for new URL Schemes        November 19995. Authors' Addresses   Larry Masinter   Xerox Corporation   Palo Alto Research Center   3333 Coyote Hill Road   Palo Alto, CA 94304   URL:http://purl.org/NET/masinter   EMail: masinter@parc.xerox.com   Harald Tveit Alvestrand   Maxware, Pirsenteret   N-7005 Trondheim   NORWAY   Phone: +47 73 54 57 00   EMail: harald.alvestrand@maxware.no   Dan Zigmond   WebTV Networks, Inc.   305 Lytton Avenue   Palo Alto, CA 94301   USA   Phone: +1-650-614-6071   EMail: djz@corp.webtv.net   Rich Petke   UUNET Technologies   5000 Britton Road   P. O. Box 5000   Hilliard, OH 43026-5000   Phone: +1-614-723-4157   Fax: +1-614-723-8407   EMail: rpetke@wcom.netMasinter, et al.             Informational                      [Page 9]

RFC 2718             Guidelines for new URL Schemes        November 19996. 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.Acknowledgement   Funding for the RFC Editor function is currently provided by the   Internet Society.Masinter, et al.             Informational                     [Page 10]