RFC 2718

Network Working Group L. Masinter
Request for Comments: 2718 Xerox Corporation
Category: Informational H. Alvestrand
Maxware, Pirsenteret
D. Zigmond
WebTV Networks, Inc.
R. Petke
UUNET Technologies
November 1999

Guidelines for new URL Schemes

Status 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.

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 ":"
and then a "". 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
2717 [2].

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RFC 2718 Guidelines for new URL Schemes November 1999

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. ""), 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. ""), 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.

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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 ":"

Contrary to some examples set in past years, the use of double
slashes as the first component of the 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 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 should not use double slashes following
the ":" 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 - for example, if
the tokens "//", "/", ";", or "?" are used, do they have the
meaning given in RFC 2396?

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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?

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.)

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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.

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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?

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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.

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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. 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.

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5. 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.net

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6. Full Copyright Statement

This document and translations of it may be copied and furnished to
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The limited permissions granted above are perpetual and will not be
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HEREIN WILL NOT INFRINGE ANY RIGHTS OR ANY IMPLIED WARRANTIES OF
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Acknowledgement

Funding for the RFC Editor function is currently provided by the
Internet Society.

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