(6) Browsers and Resources

• Web browsers retrieve resources and render them
  • HTTP can transfer any kind of resource (binary resources must be transfer encoded)
  • resource types cannot (and should not) be inferred from the URI
• HTTP combines data transfer, transfer management, and metadata
  • basic information about a resource (modification date)
  • information describing the resource's type (media type) and content (language)
• The resource type received may or may not be supported by the browser
  • built-in support is provided for the core Web resource types (HTML, GIF, JPEG)
  • plug-in support is an add-on to the browser for popular types (PDF, Flash)
  • external applications are standalone applications invoked by the browser

(7) Firefox Media Type Handling

(8) Media Type Misconfiguration

• Problems accessing web content are often the results of media type misconfigurations
• Example:
  • Correctly configured MP3 (MIME type audio/mpeg) [http://courses.ischool.berkeley.edu/i190-waim/s10/media/correct/unpossible]
  • Inorrectly configured MP3 (MIME type text/plain) [http://courses.ischool.berkeley.edu/i190-waim/s10/media/incorrect/unpossible]
• Browsers try to work around the most common errors
  • if I had left the .mp3 suffix on the files in the examples above, they would have been treated as MP3s regardless of the MIME type

(10) Content Types

• MIME splits the world of resource types into Content Types and Subtypes
  • Content types are the main classification of a resource type
  • Subtypes [Subtypes (1)] qualify the format and encoding used for the content
• Content types classify the world of resource type into 8 areas
  • audio for media types representing exclusively audio signals
  • image for any media type representing two-dimensional images
  • message for resources representing e-mail messages
  • model for complex representations of physical objects (very unpopular)
  • multipart for MIME entities containing multiple individual MIME-tagged resources
  • text for mainly textual material (e.g., HTML is considered to be text)
  • video for media types combining moving pictures with audio
  • application for any resource which cannot by classified anywhere else
  • (example is only used for media type examples, not for real-world resources)

(11) Subtypes

• Within each content type, many different data formats are in use
  • content types only allow a broad classification
  • subtypes allow the identification of a specific data format of a resource
• Subtypes are expected to be registered [Media Type Registration (1)] with the IANA [http://www.iana.org/]
  • unregistered subtypes can be used but must have a x- prefix
• Additional qualifiers can be used to be more specific
  • text/plain is the media type for plain text files
  • plain text files have additional properties such as character encoding and language
  • text/plain can be further qualified to text/plain; charset=iso-8859-1

(12) Media Type Registration

• Media types need to be registered together with documentation
  • this makes sense if it is assumed that registered types should be openly accessible
  • this becomes complicated if the types are proprietary and not publicly documented
• It makes sense to register types even if they are not publicly documented
  • if a Word document is sent by e-mail it should be opened by the Word application
  • IANA registers vnd. prefixed subtypes with less requirements than regular types
  • vendor specific types are often undocumented and may change significantly over time
• Using well-defined types makes handling resources more stable
  • the IANA registry contains hundreds of types (most of them application types)
  • when designing applications dealing with various content types, use media types as the foundation

(13) application/msword Media Type

SECURITY CONSIDERATIONS:
None known.


PUBLISHED SPECIFICATION:

Specification by example:

   From any microsoft word application select "Save As..." from the
   "File" menu.  Enter a filename, make sure that "Normal" is specified
   for the file type, and click "Save".

Company Contact:

   Microsoft Inc.

   16011 NE 36th Way
   Box 97017
   Redmond WA, 98073-9717

(24) Identification of Resource Fragments

• URIs identify a resource (based on a scheme and a scheme-specific part)
  • URIs do not necessarily identify a specific representation of a resource
  • any representation-specific operation needs to look at the resource type
• Fragment identifiers can be used to identify a part of a resource
  • http://courses.ischool.berkeley.edu/i190-waim/s10/media-types#frag-id [media-types#frag-id]
  • fragments are a client side concept (the HTTP GET requests the complete resource)
  • if the client supports fragment handling, the identifier is interpreted

(25) HTML Fragment Identifiers

• HTML allows to address named/identified elements in the HTML document
  • the first HTML versions required named <a name="html-frag-id">incoming anchors</a>
  • newer HTML versions allow <div id="html-frag-id">every element to have an id</div> [media-types#html-frag-id]
  • browsers support both ways, but the id variant should be preferred
• Only named/identified fragments can be identified
  • 99.99% of all page authors do not routinely add identifications
  • tools may be smarter and take over that task (e.g., Movable Type identifies all relevant elements)
  • for most pages on the Web this means users cannot link to most elements in them
• Keeping fragment identifiers stable should be a goal of Web authors
  • identify the key fragments (and maybe provide a better UI than view source)
  • never change identifiers once they have been assigned

(26) Media Fragments

• There are a number of non-standard approaches to define fragments for media types other than HTML
  • link to a specific page of a PDF [http://kb2.adobe.com/cps/317/317300.html] by appending #page (only works with a PDF browser plugin):
    http://cue.berkeley.edu/ecm.pdf#page=16 [http://cue.berkeley.edu/ecm.pdf#page=16]
  • link to a specific time in a YouTube video [http://www.wikihow.com/Link-to-a-Certain-Time-in-a-YouTube-Video] by appending #t:
    http://www.youtube.com/watch?v=8HE9OQ4FnkQ#t=59s [http://www.youtube.com/watch?v=8HE9OQ4FnkQ#t=59s]
• There is ongoing work [http://www.w3.org/TR/media-frags/] to standardize linking to multimedia documents (e.g. audio and video)
  • demo [http://annodex.net/~silvia/itext/mediafrag.html] of linking to points in an HTML5 video (Firefox 3.6 only)
  • standard support for linking to parts of media opens many possibilities [http://www.w3.org/TR/media-frags-reqs/], including simple web-based remixing [http://media.sffs.org/remix/RemixerHome.php]