One document matched: draft-masinter-media-features-00.txt

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Copyright (C) The Internet Society (1997).  All Rights Reserved.

Abstract

This specification grew out of work in the HTTP working group to allow
for HTTP clients and servers to negotiate elements of the presentation
of documents that were not naturally captured by the MIME Media
Type. However, the applicability of media features is important in a
number of broader contexts, including document distribution, facsimile
transmission, push technology.  The document [FEATURES] defines the
framework for document features, and the document [REG] defines the
registration procedures for them. This specification establishes
a small number of generally useful feature descriptions.

Introduction
    
This work was originally motivated by the requirements from web
browsers.  The Hypertext Transfer Protocol (HTTP) is protocol for
distributed, collaborative, hypermedia information systems.  At
present the server relies on the client's ability to present visual
information in a usable fashion without information about the client's
display characteristics.  The presence of large images, video, and
other visual information in HTML documents has strained this model.
HTML documents suitable for a certain video monitor size are often
less usable on displays of much smaller or larger resolution, such as
PDA's and high-resolution printers.
     
This specification defines feature tags [5].  These tags are the means
by which a recipient may inform a sender as to the characteristics of
its message handling.  The sender may then provide the variant of the
message that is most suitable for the recipient.

Different variants would typically be higher or lower resolution
images (for example) as appropriate.  In the case of a sending to a
printer, the result would be higher quality output.  In the case of a
small screen device (cellphone, portable digital assistant), the
result would be faster transmission.

Feature tags may be used in many different protocol situations.  Those
defined in this specification can indicate the display or printer
dimensions (in pixels), display resolution (in pixels/inch), color
capability and bit-depth, and display media type. The physical
dimensions of the display can be inferred from the display size and
display resolution. In the case of paper output, the paper size may be
expressed as a token from a list of certain standard paper sizes.
These are presented formally in the Notation section.
    
pix-x<=n  
pix-y<=m 
    
  These features indicate the maximum display size that the recipient
  can conveniently display or print, measured in pixels; they indicate
  horizontal (n) and vertical (m) dimensions.

res<=n  
    
   This feature indicates the maximum resolution that the recipient
   can display or print without loss, measured in pixels per inch.
   For example: res<=72.  Certain resources such as images may have
   similar total pixel size but differing data size and quality
   depending on degree of compression.  UA-res can be used to resolve
   a preferred image in this case.
     
   While English units are not universal, it is preferable to avoid
   multiple unit definitions.
    
res-x<=n
res-y<=m

   In cases where non-square aspect ratio is supported, these
   features can be used instead.

UA-media=token  
    
   This feature indicates the recipients device media, indicated with
   an simple token.  Basic token values are: screen, stationary,
   transparency, envelope, or continuous-long.  Other values may be
   defined.  Except for `screen', these tokens are a subset of the
   Printer MIB MediaType set defined in RFC-1759 [6].  Other tokens
   may be registered and used as needed.
    
   They are defined as:  
     screen:           a refreshable display 
     screen-paged:     a refreshable display which cannot scroll 
     stationary:       separately cut sheets of an opaque material  
     transparency:     separately cut sheets of a transparent material  
     envelope:         envelopes that can be used for conventional  
                       mailing purposes 
     continuous-short: continuously connected sheets of an opaque   
                       material connected along the short edge  
    
papersize=token

   For ua-media types such as stationary, it is often useful to have
   information about the size of display used.  While it is more
   precise and predictable to use absolute resolution and pixel sizes,
   some applications find it useful to provide paper size in lieu of
   or in addition to this information.  Paper sizes names and
   definitions are taken from the the Printer MIB RFC [6].

   Examples of paper size tokens, with names from [6], are:
     na-letter:        8.5x11.0 inches 
     iso-A4:           210x297 mm    
     iso-B4:           250x353 mm
     iso-A3:           297x420 mm            
     na-legal:         8.5x14 inches

color<=n 
grey<=n  
    
   The color capabilities of the recipient are indicated with feature
   tag and a parameter describing the number of color channel bits
   available.  Values of n are typically (but not limited to) 2, 8, or
   24.  For example: grey=8 indicates a display capable of
   representing an image in 256 levels of a single color, while
   color=8 indicates a display capable of representing an image with a
   palette of 256 colors.
  
Examples    

   pix-x<=1024 
   pix-y<=768 
      indicates a 1024x768 display 
    
   res<=72  
      indicates a 72 dpi display  
    
   UA-media=stationery  
      indicates the display is a cut sheet of opaque material, such as   
      paper.  
  
   papersize=iso-a4
      indicates the display size is 210x297mm. 
   
   color<=24  
      indicates the display supports 24-bit (8-bit/channel) color.   
       
Future work and other issues 
    
Acknowledgments   
    
   This document is based on a previous draft co-authored with Lou
   Montoulli, Koen Holtman and Andy Mutz. It had benefited from the
   comments of Ho John Lee, Brian Behlendorf, and Jeff Mogul.
    
References   
    
   [1]     T.  Berners-Lee.  "Universal Resource Identifiers in WWW." A   
   Unifying Syntax for the Expression of Names and Addresses of Objects   
   on the Network as used in the World-Wide Web." RFC 1630, CERN, June   
   1994.    
    
   [2]     T.  Berners-Lee, L.  Masinter, M.  McCahill.    
   "Uniform Resource Locators (URL)." RFC 1738, CERN, Xerox PARC,   
   University of Minnesota, December 1994.   
     
   [3]     T.  Berners-Lee, R.  Fielding, H.  Frystyk.    
   "Hypertext Transfer Protocol -- HTTP/1.0." RFC 1945." MIT/LCS, UC   
   Irvine, May 1996.    
    
   [4]     T.  Berners-Lee, R.  Fielding,I J.  Gettys, J.  Mogul, H.   
   Frystyk.   "Hypertext Transfer Protocol - HTTP/1.1"  MIT/LCS,   
   UC Irvine, May 1996.   
    
   [5]     K. Holtman, A. Mutz, "Transparent Content Negotiation in HTTP"
   IETF Internet Draft draft-holtman-http-negotiation-04.txt, Nov. 1996. 
    
   [6]     R.  Smith, F.  Wright, T.  Hastings, S.  Zilles, J.  Gyllenskog.   
   "Printer MIB." RFC 1759." IETF, March 1995 

   [7]     K. Holtman, A. Mutz, "Feature Tag Registration Procedures" IETF
   Internet Draft draft-ietf-http-feature-reg-00.txt, October 1996. 
     
Author's Addresses     
   
   Larry Masinter  
   Xerox Palo Alto Research Center  
   3333 Coyote Hill Road  
   Palo Alto CA 94304  
   Fax +1 415 812 4333  
   Email: masinter@parc.xerox.com