INTERNET-DRAFT Larry Masinter draft-masinter-media-features-00.txt Xerox Corporation expires in 6 months November 20, 1997 Features for Media Display Status of this memo This document is an Internet-Draft. Internet-Drafts are working documents of the Internet Engineering Task Force (IETF), its areas, and its working groups. Note that other groups may also distribute working documents as Internet-Drafts. Internet-Drafts are draft documents valid for a maximum of six months and may be updated, replaced, or obsoleted by other documents at any time. It is inappropriate to use Internet-Drafts as reference material or to cite them other than as "work in progress." To learn the current status of any Internet-Draft, please check the "1id-abstracts.txt" listing contained in the Internet-Drafts Shadow Directories on ftp.is.co.za (Africa), ftp.nordu.net (Europe), munnari.oz.au (Pacific Rim), ds.internic.net (US East Coast), or ftp.isi.edu (US West Coast). 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