SMIL media types: The application/smil and application/smil+xml Media Types
Official MIME types for synchronized multimedia presentations.
Hoschka, P., "The application/smil and application/smil+xml Media Types", RFC 4536, DOI 10.17487/RFC4536, July 2006, <https://www.rfc-editor.org/info/rfc4536>.
@misc{rfc4536,
series = {Request for Comments},
number = 4536,
howpublished = {RFC 4536},
publisher = {RFC Editor},
doi = {10.17487/RFC4536},
url = {https://www.rfc-editor.org/info/rfc4536},
author = {P. Hoschka},
title = {{The application/smil and application/smil+xml Media Types}},
pagetotal = 8,
year = 2006,
month = jul,
} RFC text
Contents
Network Working Group P. Hoschka
Request for Comments: 4536 W3C
Category: Informational May 2006
The application/smil and application/smil+xml Media Types
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.
Copyright Notice
Copyright (C) The Internet Society (2006).
Abstract
This document specifies the media type for versions 1.0, 2.0, and 2.1
of the Synchronized Multimedia Integration Language (SMIL 1.0, SMIL
2.0, SMIL 2.1). SMIL allows integration of a set of independent
multimedia objects into a synchronized multimedia presentation.
1. Introduction
The World Wide Web Consortium (W3C) has issued specifications that
define versions 1.0 [1], 2.0 [2] and 2.1 [3] of the Synchronized
Multimedia Integration Language (SMIL). This memo provides
information about the application/smil and application/smil+xml media
types.
The definition is based on RFC 3023, which defines the use of the
"application/xml" media type [4]. Before using the
"application/smil" or "application/smil+xml" media type, implementors
must thus be familiar with [4].
2. Synchronized Multimedia Integration Language
SMIL allows integrating a set of independent multimedia objects into
a synchronized multimedia presentation. Using SMIL, an author can
1. describe the temporal behavior of the presentation,
2. describe the layout of the presentation on a screen,
3. associate hyperlinks with media objects, and
4. define conditional content inclusion/exclusion based on
system/network properties.
3. Registration Information
3.1. Registration of MIME media type application/smil
MIME media type name: application
MIME subtype name: smil
Required parameters: none
Optional parameters:
charset
Same as charset parameter considerations of application/xml in
RFC 3023.
profile
See Section 5 of this document.
Encoding considerations:
Same as encoding considerations of application/xml in RFC 3023
Security considerations: See Section 6, "Security Considerations", of
this document.
Interoperability considerations:
SMIL documents contain links to other media objects. The SMIL
player must be able to decode the media types of these media in
order to display the whole document. To increase
interoperability, SMIL has provisions for including alternate
versions of a media object in a document.
Published specification: See [1], [2], and [3]
Applications which use this media type:
SMIL players and editors
Additional information:
Semantics of fragment identifiers in URIs: The SMIL media type
allows a fragment identifier to be appended to a URI pointing to a
SMIL resource (e.g., http://www.example.com/test.smil#foo). The
semantics of fragment identifiers for SMIL resources are defined
in the SMIL specification.
Magic number(s):
There is no single initial byte sequence that is always present
for SMIL files. However, Section 4 of this document gives some
guidelines for recognizing SMIL files.
File extension(s): .smil, .smi, .sml
NOTE: On the Windows operating system and the Macintosh platform, the
".smi" extension is used by other formats. To avoid conflicts, it is
thus recommended to use the extension ".smil" for storing SMIL files
on these platforms.
Macintosh File Type Code(s): "TEXT", ".SMI", "SMIL"
Object Identifier(s) or OID(s): none
Person & email address to contact for further information:
The author of this memo.
Intended usage: OBSOLETE
Author/Change controller:
The SMIL specification is a work product of the World Wide Web
Consortium's SYMM Working Group.
The W3C has change control over the specification.
3.2. Registration of MIME media type application/smil+xml
MIME media type name: application
MIME subtype name: smil+xml
Required parameters: See registration of application/smil.
Optional parameters: See registration of application/smil.
Encoding considerations: See registration of application/smil.
Security considerations: See Section 6, "Security Considerations", of
this document
Interoperability considerations: See registration of
application/smil.
Published specification: See registration of application/smil.
Applications which use this media type: See registration of
application/smil.
Additional information: See registration of application/smil.
Magic number(s): See registration of application/smil.
File extension(s): See registration of application/smil.
Macintosh File Type Code(s): See registration of application/smil.
Object Identifier(s) or OID(s): See registration of application/smil.
Person & email address to contact for further information: See
registration of application/smil.
Intended usage: COMMON
Author/Change controller: See registration of application/smil.
4. Recognizing SMIL Files
All SMIL files will have the string "<smil" near the beginning of the
file. Some will also begin with an XML declaration that begins with
"<?xml", though that alone does not indicate a SMIL document.
All SMIL 2.0 files must include a declaration of the SMIL 2.0
namespace. This should appear shortly after the string "<smil", and
should read 'xmlns="http://www.w3.org/2001/SMIL20/Language"'.
All SMIL 2.1 files must include a declaration of a SMIL 2.1
namespace, appearing shortly after the string "<smil". The namespace
string depends on the language profile. Please refer to the SMIL 2.1
specification for the definition of the relevant namespace names.
5. The "profile" Optional Parameter
This parameter is meant to be used in MIME media-type-based content
negotiation (such as that done with the HTTP "Accept" header) to
negotiate for a variety of SMIL-based languages. It is modelled
after the "profile" parameter in the application/xhtml+xml MIME type
registration [5] and is motivated by very similar considerations.
The parameter is intended to be used only during content negotiation.
It is not expected that it be used to deliver content, or that origin
web servers have any knowledge of it (though they are welcome to).
It is primarily targeted for use on the network by proxies in the
HTTP chain that manipulate data formats (such as transcoders).
The value of the profile attribute is a URI that can be used as a
name to identify a language. Though the URI need not be resolved in
order to be useful as a name, it could be a namespace, schema, or
language specification.
For example, user agents supporting only SMIL Basic (see
http://www.w3.org/TR/smil20/smil-basic.html) currently have no
standard means to convey their inability to fully support SMIL 2.0.
While SMIL 2.0 Basic user agents are required to parse the full SMIL
2.0 language, there is potentially a substantial burden in receiving
and parsing document content that will not be presented to the user,
since its functionality is not included in SMIL Basic.
In the future, the functionality afforded by this parameter will also
be achievable by the emerging work on a protocol to transfer
Composite Capability/Preferences Profiles (CC/PP) descriptions [6].
It is suggested that the "profile" parameter be used until the CC/PP
protocol work has been finalized.
An example use of this parameter as part of a HTTP GET transaction
would be:
Accept: application/smil+xml;
profile="http://www.w3.org/2001/SMIL20/HostLanguage"
6. Security Considerations
SMIL documents contain a construct that allows "infinite loops".
This is indispensable for a multimedia format. However, SMIL clients
should foresee provisions such as a "stop" button that lets users
interrupt such an "infinite loop".
As with HTML, SMIL documents contain links to other media (images,
sounds, videos, text, etc.), and those links are typically followed
automatically by software, resulting in the transfer of files without
the explicit request of the user for each one. The security
considerations of each linked file are those of the individual
registered types.
The SMIL language contains "switch" elements. SMIL provides no
mechanism that ensures that the media objects contained in a "switch"
element provide equivalent information. An author knowing that one
SMIL player will display one alternative of a "switch" and another
will display a different part can put different information in the
two parts. While there are legitimate use cases for this, it also
gives rise to a security consideration: The author can fool viewers
into thinking that the same information was displayed when in fact it
was not.
In addition, all of the security considerations of RFC 3023 also
apply to SMIL.
7. Normative References
[1] "Synchronized Multimedia Integration Language (SMIL) 1.0
Specification", W3C Recommendation REC-smil-19980615,
http://www.w3.org/TR/1998/REC-smil/, July 1998.
[2] "Synchronized Multimedia Integration Language (SMIL 2.0) -
[Second Edition]", W3C Recommendation,
http://www.w3.org/TR/2005/REC-SMIL2-20050107/, January 2005.
[3] "Synchronized Multimedia Integration Language (SMIL 2.1)", W3C
Recommendation, http://www.w3.org/TR/2005/REC-SMIL2-20051213/,
December 2005.
[4] Murata, M., St. Laurent, S., and D. Kohn, "XML Media Types", RFC
3023, January 2001.
8. Informative References
[5] Baker, M. and P. Stark, "The 'application/xhtml+xml' Media
Type", RFC 3236, January 2002.
[6] H. Ohto, J. Hjelm, G. Klyne, M. Butler, L. Tran, F. Reynolds, C.
Woodrow "Composite Capability/Preferences Profiles (CC/PP):
Structure and Vocabularies 1.0", W3C Recommendation
http://www.w3.org/TR/CCPP-struct-vocab/, January 2004.
Author's Address
Philipp Hoschka
W3C/ERCIM
2004, route des Lucioles - B.P. 93
06902 Sophia Antipolis Cedex
FRANCE
EMail: ph@w3.org
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On this page
§1What SMIL Is
The Synchronized Multimedia Integration Language (SMIL) is a language, defined by the World Wide Web Consortium (W3C), that combines separate multimedia objects—such as video, audio, images, and text—into a single synchronized presentation. A SMIL document describes when each piece plays, how it is positioned on the screen, what hyperlinks attach to it, and which pieces to include or exclude based on system capabilities.
RFC 4536 registers official MIME media types for SMIL versions 1.0, 2.0, and 2.1, so that servers, browsers, and players can correctly identify and handle SMIL files.
SMIL files are scripts that say: play this video here, then show this picture there, and at the same time play this audio. Instead of a person directing the play, a computer reads the SMIL script and makes it happen.
§2Registered MIME Types
The RFC registers two MIME types:
application/smilis marked as OBSOLETE and covers SMIL 1.0 and 2.0.application/smil+xmlis marked as COMMON and covers SMIL 2.0, 2.1, and newer versions that use XML syntax.
Both types use the same optional parameters: charset (for character encoding) and profile (a URI identifying a specific SMIL dialect or profile). The profile parameter is intended for content negotiation, such as in HTTP Accept headers, to negotiate support for SMIL Basic (a subset of SMIL 2.0).
SMIL files get two official file-type names. One is old (for SMIL 1.0 and 2.0), and one is current (for newer versions). Both can optionally say what encoding they use and what features they need.
§3File Format and Recognition
SMIL files are XML documents, so they start with optional XML declarations and contain SMIL elements. All SMIL files begin with the string <smil near the start. SMIL 2.0 and 2.1 files declare a namespace URL (for example, xmlns='http://www.w3.org/2001/SMIL20/Language'), which helps identify which version of SMIL the file uses.
Common file extensions are .smil, .smi (though this collides with other formats on Windows and Mac), and .sml. On Macintosh systems, file type codes include 'TEXT', 'SMIL', and '.SMI'.
SMIL files are text that starts with <smil and looks like HTML but with special rules. They can have different file endings (.smil, .smi, .sml) and the computer recognizes them by looking inside.
§4The Profile Parameter
The optional 'profile' parameter lets clients and proxies negotiate which SMIL features are needed. A SMIL player that only supports SMIL Basic (a simplified subset of SMIL 2.0) can advertise this in an HTTP Accept header, for example: 'Accept: application/smil+xml; profile="http://www.w3.org/2001/SMIL20/HostLanguage"'. Proxies and transcoders can use this to decide whether to deliver or transcode a SMIL document.
This parameter is inspired by the same mechanism in the application/xhtml+xml media type and is intended for use during content negotiation, not for delivery. Origin servers are not required to understand or use it.
The profile parameter is like saying 'I can only play basic SMIL, not the fancy version.' Servers and proxies can use this to send you a simpler version if you ask for it.
§5Security Risks in SMIL
SMIL documents can define infinite loops, which is necessary for multimedia playback but allows a malicious author to create content that plays forever. SMIL players should provide a stop button or other controls so users can interrupt playback.
SMIL documents link to other media (images, sounds, videos, text). These links are typically followed automatically by SMIL players, so a malicious document can cause unwanted downloads without explicit user consent. Each linked media file carries its own security risks.
SMIL 'switch' elements can contain different alternatives for different players or systems. A switch does not guarantee that all alternatives present the same information, so a malicious author can show different content to different viewers while claiming to show the same thing.
Beyond these SMIL-specific concerns, all security considerations of RFC 3023 (XML Media Types) apply.
SMIL can cause problems if someone makes a file that loops forever, automatically downloads lots of stuff, or tricks different players into showing different pictures. SMIL players need safeguards, and users should be careful where they get SMIL files from.