The 'icon' URI scheme
This document specifies the icon URI scheme. Icons have a fundamental role in user interaction with computer operating systems. In a graphical user interface, they may represent a file, folder, application, or device. They live on the desktop, in toolbars, and in menus. Most operating systems provide a standard set of icons that match the look and feel of the system. The icon URI scheme lets web page designers leverage these existing platform icons.
Editorial Note (To be removed by RFC Editor)
Discussion of this draft should take place on the URI Review mailing list (email@example.com).
URI Scheme versus URN Namespace
It has been suggested that a URN namespace may be more appropriate for icon resolution than the new URI scheme. Discussion on these matters has been ongoing in the firstname.lastname@example.org mailing list. The arguments will be taken into consideration for the final decision as whether this draft should be considered for permanent registration.
Status of This Memo
This Internet-Draft is submitted in full conformance with the provisions of BCP 78 and BCP 79.
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Similar icon URI schemes are currently being used in some modern browsers. The Mozilla project has a moz-icon:// URI scheme that is web accessible and provides native and browser themed icons. The Chromium project has a chrome://fileicon scheme that is not web accessible, but does provide native icons for internal pages. This new scheme is an attempt to standardize icon URIs so that their core functionality may become available to all web pages.
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] (Bradner, S., “Key words for use in RFCs to Indicate Requirement Levels,” March 1997.).
3. URI Syntax
The general syntax of an icon URI is defined below using ABNF (Crocker, D. and P. Overell, “Augmented BNF for Syntax Specifications: ABNF,” January 2008.) [RFC5234]:
iconuri = "icon:" ( fextension / mediatype / "unknown" / "directory" / "parentdir" ) [ ";" size ] fextension = "." 1*token ; File extension mediatype = [ type ":" subtype ] ; Internet media type size = pixels / "small" / "medium" / "large" pixels = 1*digit ; Size of icon in square pixels
where token, digit, type and subtype rules are defined in [RFC2045] (Freed, N. and N. Borenstein, “Multipurpose Internet Mail Extensions (MIME) Part One: Format of Internet Message Bodies,” November 1996.). The sections of an icon URI are ALWAYS case-insensitive.
4. Encoding Considerations
The encoding of the type and subtype rules is defined in [RFC2045] (Freed, N. and N. Borenstein, “Multipurpose Internet Mail Extensions (MIME) Part One: Format of Internet Message Bodies,” November 1996.). The fextension rule may include characters from the Unicode Character Set (International Organization for Standardization, “Information Technology - Universal Multiple-Octet Coded Character Set (UCS),” December 2003.) [UCS], as suggested by URI (Berners-Lee, T., Fielding, R., and L. Masinter, “Uniform Resource Identifier (URI): Generic Syntax,” January 2005.) [RFC3986], by first encoding those characters as octets to the UTF-8 character encoding (Yergeau, F., “UTF-8, a transformation format of ISO 10646,” November 2003.) [RFC3629]. 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 Internationalized Resource Identifiers to icon URIs according to [RFC3987] (Duerst, M. and M. Suignard, “Internationalized Resource Identifiers (IRIs),” January 2005.). Since icon URIs do not use domain names, ireg-name conversion is unnecessary.
5. Resolving "icon" URIs
An icon URI MUST resolve to an image resource representing an icon. The directory and parentdir keywords SHOULD resolve to the system icons for a directory and a parent directory (i.e. the "up" icon). The resolved icon SHOULD be the platform icon for a the specified fextension or mediatype. If an icon is not available for the specified filetype, applications MUST return a default icon which SHOULD be the system icon for files of unknown type. The icon resource's dimensions MUST match the size indicated in the URI or use the default size.
5.1. File extension
The filetype for an icon is specifiable by the fextension rule. An acceptable value for this section is a file extension for which a platform icon can be retrieved. Applications are REQUIRED to support icon URI resolution by file extension.
In the event that a platform icon is not available for the provided file extension, the application SHOULD return a default "unknown file" icon in the appropriate size.
5.2. Internet media type
The filetype for an icon is also specifiable by the mediatype rule. An acceptable value for this section is an Internet media type, as defined in [RFC2045] (Freed, N. and N. Borenstein, “Multipurpose Internet Mail Extensions (MIME) Part One: Format of Internet Message Bodies,” November 1996.) and [RFC2046] (Freed, N. and N. Borenstein, “Multipurpose Internet Mail Extensions (MIME) Part Two: Media Types,” November 1996.), for which a platform icon can be retrieved. Applications are REQUIRED to support icon URI resolution by Internet media type.
In the event that a platform icon is not available for the provided media type, the application SHOULD return a default "unknown file" icon in the appropriate size.
5.3. Icon Size
An icon URI MAY specify the size of a requested icon through the size rule. The size MUST be either an integer value representing the width/height of the square icon in pixels or one of: "small", "medium", or "large". Applications are REQUIRED to support both user-provided integer values and the 3 keyword values.
If the size is specified as "small" the application MUST return a 16x16 pixels icon.
If the size is specified as "medium" the application MUST return a 64x64 pixels icon.
If the size is specified as "large" the application MUST return a 256x256 pixels icon.
If the size is not specified in the URI, the application MUST return a 16x16 pixels icon.
In the event that the platform does not support one of the keyword icon sizes or a user-provided size, the application SHOULD scale the icon appropriately.
icon: — Displays a 16x16 pixels platform icon for an unknown filetype.
icon:;medium — Displays a 64x64 pixels platform icon for an unknown filetype.
icon:.zip —Displays a 16x16 pixels platform icon for the .zip file extension.
icon:.MP3;32 —Displays a 32x32 pixels platform icon for the .mp3 file extension.
ICON:unknown —Displays a 16x16 pixels default icon for an unknown filetype.
icon:directory —Displays a 16x16 pixels platform icon for a directory.
icon:parentdir;32 —Displays a 32x32 pixels platform icon for a parent directory.
icon:image:jpeg;128 —Displays a 128x128 pixels platform icon for the image/jpeg media type.
icon:AUDIO:MPEG;large —Displays a 256x256 platform icon for the audio/mpeg media type.
icon:.doc;SMALL —Displays a 16x16 platform icon for the .doc file extension.
icon:.pdf;medium —Displays a 64x64 platform icon for the .pdf file extension.
Icon URIs adhere to the standard URI normalization rules (Berners-Lee, T., Fielding, R., and L. Masinter, “Uniform Resource Identifier (URI): Generic Syntax,” January 2005.) [RFC3986]; specifically Simple String Comparison, Case Normalization, and Percent-Encoding Normalization. Due to the structure of icon URIs, the Syntax-Based, Scheme-Based, and Protocol-Based Normalization rules do not apply. The sections of an icon URI are ALWAYS case-insensitive.
7. Security Considerations
There are potential privacy risks that need to be taken into consideration with this new scheme.
Icon URIs specify private icon resources located on a user's machine. Web pages MUST NOT be able to retrieve the platform icons that this scheme provides to the user. Further, web pages MUST NOT be able to retrieve information about the platform icons that might expose the applications installed on a user's machine. The following sections explore specific issues that applications MUST address in their respective implementations.
7.1. Canvas Images
HTML5 introduces the canvas element which allows for dynamic scriptable rendering of 2D bitmap images.
1. Retrieve <img> elements already on the page using the DOM.
2. Retrieve other canvas elements using the document.getElementsById method or the document.getElementsByTagName method.
3. Create an image from scratch, i.e. var img = new Image().
4. Embed an image using its data URI.
Drawing the Image to the canvas provides web pages with two potential means of accessing the images: the getImageData and toDataURL methods of the canvas's 2D context. The canvas's 2D drawing API is defined in [Canvas2D] (Hickson, I., Hyatt, D., Schepers, D., and E. Graff, “Canvas 2D API Specification 1.0,” .).
An Image object with an icon URI as its src attribute MUST NOT be considered as same-origin as any other origin. If an Image with an icon URI src is drawn to a canvas, the canvas MUST be considered tainted and have its origin-clean flag set to false. As such, if getImageData or toDataURL is called on a canvas that has been tainted by icon URI Image data, the method MUST raise a SECURITY_ERR exception. See Security with canvas elements in the HTML5 Draft Standard (Hickson, I., “HTML5 Draft Standard,” .) [HTML5] for further details.
7.2. Image Size
Because the width and height of an Image object are always accessible, applications MUST use the default size (16x16) for platform icons if the size has not been provided by the icon URI. This is a requirement because evil web pages may be able to use the width and height of icons to infer details about the applications a user has installed.
8. IANA Considerations
This specification requests the IANA provisionally register the icon URI scheme as specified in this document and summarized in the following template, per [RFC4395] (Hansen, T., Hardie, T., and L. Masinter, “Guidelines and Registration Procedures for New URI Schemes,” February 2006.):
- URI scheme name:
- URI scheme syntax:
- See Section 3 (URI Syntax)
- URI scheme semantics:
- See Section 1 (Introduction)
- Encoding considerations:
- See Section 4 (Encoding Considerations)
- Intended usage:
- See Section 1 (Introduction) and Section 5.4 (Examples)
- 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:
- See Section 6 (Normalization)
- Security considerations:
- See Section 7 (Security Considerations)
- Relevant publications:
- Pierre-Antoine LaFayette (email@example.com)
- Author/Change controller:
- Pierre-Antoine LaFayette
This document was made possible thanks to the input of Adam Barth, Lachlan Hunt, Maciej Stachowiak, Jonas Sicking, Robert O'Callahan, Josh Soref, and Ian Fette.
10.1. Normative References
10.2. Informative References
|[Canvas2D]||Hickson, I., Hyatt, D., Schepers, D., and E. Graff, “Canvas 2D API Specification 1.0.”|
|[HTML5]||Hickson, I., “HTML5 Draft Standard.”|
|[RFC4395]||Hansen, T., Hardie, T., and L. Masinter, “Guidelines and Registration Procedures for New URI Schemes,” BCP 35, RFC 4395, February 2006 (TXT).|
|Pierre-Antoine Benoit LaFayette|