Network Working Group J. Holsten
Internet-Draft
Intended status: Informational L. Hunt
Expires: February 13, 2010 Opera Software, ASA.
August 12, 2009
The 'about' URI scheme
draft-holsten-about-uri-scheme-02
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Abstract
This document specifies the URI (Uniform Resource Identifier) scheme
"about". About URIs are designed to be an internal, application-
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level identifier. Unlike many other URI schemes, the resolution of,
and resources represented by, about URIs are left largely to each
individual application. Only the "about:blank" URI must be the same.
1. Editorial Note (To be removed by RFC Editor)
Discussion of this draft should take place on the URI Review mailing
list (uri-review@ietf.org).
2. Introduction
An about URI is designed to be used internally by applications for
almost any desired purpose. Such URIs have commonly been used by web
browsers for providing access to built-in functionality, such as
application information, preferences, settings, or "easter eggs".
While any number of existing schemes could be used to identify such
resources, about URIs have become the de facto standard. Browsers
already use the about scheme. The about:blank (Section 5.1) URI is
ubiquitous, and some browsers also provide other resources with the
about scheme, including those in Section 5.2.
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 in [RFC2119].
3. URI Syntax
The general syntax of an about URI is defined below using ABNF
[RFC5234]:
abouturi = "about:" segment [ "?" query ]
where "segment" and "query" are defined in [RFC3986].
4. Encoding Considerations
Because many characters are not permitted with this syntax, the
"segment" and "query" elements may contain characters from the
Unicode Character Set [UCS] as suggested by URI [RFC3986], by first
encoding those characters as octets to the UTF-8 character encoding
[RFC3629]; then only those octets that do not correspond to
characters in the unreserved set should be percent-encoded.
By using UTF-8 encoding, there are no known compatibility issues with
mapping Internationlized Resource Identifiers to about URIs according
to [RFC3987]. Since about URIs do not use domain names, "ireg-name"
conversion is unnecessary.
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5. Resolving About URIs
In general, applications are free to resolve any about URI to any
resource, either internal or external, or redirect to an alternative
URI, with about:blank being the only exception.
As about URIs are designed to be internal to each application, there
is no expectation of any URI, except about:blank, returning the same
resource among different applications. However, it is worth noting
that some conventions have arisen for providing particular
functionality via common about URIs.
Because about URIs identify application specific information,
applications SHOULD NOT retrieve remote information for such a
resource.
The about:blank URI is the only about URI reserved by this
specification.
5.1. about:blank
Applications resolving this URI MUST return an empty resource, with
the media type "text/html" and the character encoding "UTF-8".
5.2. Examples
The following examples illustrate some known URIs supported by
existing applications. They are not guaranteed to be resolvable by
every application.
about:config Commonly provides access to application preferences and
settings
about:cache Commonly provides access to information about resources
stored in the browsers cache.
about:plugins Commonly provides access to information about
installed plugins
about:mozilla An easter egg supported by Mozilla showing a passage
from the fictional Book of Mozilla
Applications are also permitted to redirect such URIs. For example,
Opera redirects all about URIs, with the exception of "about:blank",
to the equivalent URI using their internal opera: scheme. e.g.
"about:config" redirects to "opera:config".
This is not an exhaustive list. Many more are supported by numerous
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applications. For more examples, consult Wikipedia's entry on the
"about: URI Scheme" [wikiabout].
6. Normalization
About URIs use the standard URI normalization rules [RFC3986],
specifically Simple String Comparison, Case Normalization, and
Percent-Encoding Normalization. For example, "about:blank",
"about:blan%6B" and "about:blan%6b" are equivalent, though the
percent-encoded forms are discouraged. Due to the structure of about
URIs, some normalizations do not apply, specifically Syntax-Based
Normalization, Scheme-Based Normalization, and Protocol-Based
Normalization. For example, "about:blank" is not equivalent to
"about:BLANK", "about:blank?" or "about:blank:", each may represent a
different resource. Similarly, "about:blank%3F" is not equivalent to
"about:blank?".
7. Relative About URIs
As about URIs do not use a hierarchical path, relative about URIs are
not permitted
8. Security Considerations
Implementations of the scheme SHOULD NOT modify data when processing
about resource identifiers. Applications SHOULD NOT use about URIs
to access or erase files or other sensitive information.
About URIs MAY identify resources which show sensitive information.
Applications SHOULD ensure appropriate restrictions are in place to
protect such information from access or modification by untrusted
sources.
The HTML representation of the URI "about:blank" MUST use the origin
and the effective script origin as defined by HTML5 Section 5.4
Origin [W3C.WD-html5]. The origin of the "about:blank" Document is
set when the Document is created. If the new browsing context has a
creator browsing context, then the origin of the "about:blank"
Document is the origin of the creator Document. Otherwise, the
origin of the "about:blank" Document is a globally unique identifier
assigned when the new browsing context is created.
9. IANA Considerations
This specification requests the IANA provisionally register the about
URI scheme as specified in this document and summarized in the
following template, per [RFC4395]:
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URI scheme name: about
Status: Provisional
URI scheme syntax: See Section 3
URI scheme semantics: See Section 2
Encoding considerations: Percent-encoding is allowed in "segment"
and "query" components. Internationalization is handled by IRI
processing. See Section 4.
Intended usage: See Section 2
Applications and/or protocols that use this URI scheme name: Any
applications that use URIs as identifiers for private resources,
such as web browsers.
Interoperability considerations: Applications are only required to
support about:blank, and may choose to interpret other about URIs
differently.
Security considerations: See Section 8
Relevant publications: None
Contact: Joseph Holsten (joseph@josephholsten.com)
Author/Change controller: Joseph Holsten
10. Acknowledgements
This document was made possible thanks to the input of Henri Sivonen,
Ian Hickson and Larry Masinter.
11. References
11.1. Normative References
[RFC3986] Berners-Lee, T., Fielding, R., and L. Masinter,
"Uniform Resource Identifier (URI): Generic Syntax",
STD 66, RFC 3986, January 2005.
[RFC5234] Crocker, D. and P. Overell, "Augmented BNF for Syntax
Specifications: ABNF", STD 68, RFC 5234,
January 2008.
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
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Requirement Levels", BCP 14, RFC 2119, March 1997.
[RFC3987] Duerst, M. and M. Suignard, "Internationalized
Resource Identifiers (IRIs)", RFC 3987, January 2005.
[RFC3629] Yergeau, F., "UTF-8, a transformation format of ISO
10646", STD 63, RFC 3629, November 2003.
[UCS] International Organization for Standardization,
"Information Technology - Universal Multiple-Octet
Coded Character Set (UCS)", ISO/IEC Standard 10646,
December 2003.
[W3C.WD-html5] Hickson, I. and D. Hyatt, "HTML 5", World Wide Web
Consortium TR html5, February 2009,
<http://www.w3.org/TR/html5>.
11.2. Informative References
[RFC4395] Hansen, T., Hardie, T., and L. Masinter, "Guidelines
and Registration Procedures for New URI Schemes",
BCP 35, RFC 4395, February 2006.
[wikiabout] Wikipedia, The Free Encyclopedia, "About: URI
scheme", March 2009.
Authors' Addresses
Joseph Anthony Pasquale Holsten
EMail: joseph@josephholsten.com
URI: http://josephholsten.com
Lachlan Hunt
Opera Software, ASA.
EMail: lachlan.hunt@lachy.id.au
URI: http://lachy.id.au/
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