rfc1614.txt

<VC++网络游戏建摸与实现>源代码

Network Working Group                                            C. Adie
Request for Comments: 1614        Edinburgh University Computing Service
RARE Technical Report: 8                                        May 1994
Category: Informational


                Network Access to Multimedia Information

Status of this Memo

   This memo provides information for the Internet community.  This memo
   does not specify an Internet standard of any kind.  Distribution of
   this memo is unlimited.

Abstract

   This report summarises the requirements of research and academic
   network users for network access to multimedia information.  It does
   this by investigating some of the projects planned or currently
   underway in the community.  Existing information systems such as
   Gopher, WAIS and World-Wide Web are examined from the point of view
   of multimedia support, and some interesting hypermedia systems
   emerging from the research community are also studied.  Relevant
   existing and developing standards in this area are discussed.  The
   report identifies the gaps between the capabilities of
   currentlydeployed systems and the user requirements, and proposes
   further work centred on the World-Wide Web system to rectify this.

   The report is in some places very detailed, so it is preceded by an
   extended summary, which outlines the findings of the report.

Publication History

   The first edition was released on 29 June 1993.  This second edition
   contains minor changes, corrections and updates.

Table of Contents

    Acknowledgements                                                2
    Disclaimer                                                      2
    Availability                                                    3
    0. Extended Summary                                             3
    1. Introduction                                                10
      1.1. Background                                              10
      1.2. Terminology                                             11
    2. User Requirements                                           13
      2.1. Applications                                            13
      2.2. Data Characteristics                                    18



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      2.3. Requirements Definition                                 19
    3. Existing Systems                                            24
      3.1. Gopher                                                  24
      3.2. Wide Area Information Server                            30
      3.3. World-Wide Web                                          34
      3.4. Evaluating Existing Tools                               42
    4. Research                                                    47
      4.1. Hyper-G                                                 47
      4.2. Microcosm                                               48
      4.3. AthenaMuse 2                                            50
      4.4. CEC Research Programmes                                 51
      4.5. Other                                                   53
    5. Standards                                                   55
      5.1. Structuring Standards                                   55
      5.2. Access Mechanisms                                       62
      5.3. Other Standards                                         63
      5.4. Trade Associations                                      66
    6. Future Directions                                           68
      6.1. General Comments on the State-of-the-Art                68
      6.2. Quality of Service                                      70
      6.3. Recommended Further Work                                71
    7. References                                                  76
    8. Security Considerations                                     79
    9. Author's Address                                            79

Acknowledgements

   The following people have (knowingly or unknowingly) helped in the
   preparation of this report: Tim Berners-Lee, John Dyer, Aydin Edguer,
   Anton Eliens, Tony Gibbons, Stewart Granger, Wendy Hall, Gary Hill,
   Brian Marquardt, Gunnar Moan, Michael Neuman, Ari Ollikainen, David
   Pullinger, John Smith, Edward Vielmetti, and Jane Williams.  The
   useful role which NCSA's XMosaic information browser tool played in
   assembling the information on which this report was based should also
   be acknowledged - many thanks to its developers.

   All trademarks are hereby acknowledged as being the property of their
   respective owners.

Disclaimer

   This report is based on information supplied to or obtained by
   Edinburgh University Computing Service (EUCS) in good faith.  Neither
   EUCS nor RARE nor any of their staff may be held liable for any
   inaccuracies or omissions, or any loss or damage arising from or out
   of the use of this report.





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   The opinions expressed in this report are personal opinions of the
   author.  They do not necessarily represent the policy either of RARE
   or of ECUS.

   Mention of a product in this report does not constitute endorsement
   either by EUCS or by RARE.

Availability

   This document is available in various forms (PostScript, text,
   Microsoft Word for Windows 2) by anonymous FTP through the following
   URL:

    ftp://ftp.edinburgh.ac.uk/pub/mmaccess/

    ftp://ftp.rare.nl/rare/pub/rtr/rtr8-rfc.../

    Paper copies are available from the RARE Secretariat.

0. Extended Summary

   Introduction

   This report is concerned with issues in the intersection of
   networked information retrieval, database and multimedia
   technologies.  It aims to establish research and academic user
   requirements for network access to multimedia data, to look at
   existing systems which offer partial solutions, and to identify
   what needs to be done to satisfy the most pressing requirements.

   User Requirements

   There are a number of reasons why multimedia data may need to be
   accessed remotely (as opposed to physically distributing the data,
   e.g., on CD-ROM).  These reasons centre on the cost of physical
   distribution, versus the timeliness of network distribution.  Of
   course, there is a cost associated with network distribution, but
   this tends to be hidden from the end user.

   User requirements have been determined by studying existing and
   proposed projects involving networked multimedia data.  It has
   proved convenient to divide the applications into four classes
   according to their requirements: multimedia database applications,
   academic (particularly scientific) publishing applications, cal
   (computeraided learning), and general multimedia information
   services.





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   Database applications typically involve large collections of
   monomedia (non-text) data with associated textual and numeric
   fields. They require a range of search and retrieval techniques.

   Publishing applications require a range of media types,
   hyperlinking, and the capability to access the same data using
   different access paradigms (search, browse, hierarchical, links).
   Authentication and charging facilities are required.

   Cal applications require sophisticated presentation and
   synchronisation capabilities, of the type found in existing
   multimedia authoring tools.  Authentication and monitoring
   facilities are required.

   General multimedia information services include on-line
   documentation, campus-wide information systems, and other systems
   which don't conveniently fall into the preceding categories.
   Hyperlinking is perhaps the most common requirement in this area.

   The analysis of these application areas allows a number of
   important user requirements to be identified:

      o    Support for the Apple Macintosh, UNIX and PC/MS Windows
           environments.

      o    Support for a wide range of media types - text, image,
           graphics and application-specific media being most
           important, followed by video and sound.

      o    Support for hyperlinking, and for multiple access structures
           to be built on the same underlying data.

      o    Support for sophisticated synchronisation and presentation
           facilities.

      o    Support for a range of database searching techniques.

      o    Support for user annotation of information, and for user-
           controlled display of sequenced media.

      o    Adequate responsiveness - the maximum time taken to retrieve
           a node should not exceed 20s.

      o    Support for user authentication, a charging mechanism, and
           monitoring facilities.

      o    The ability to execute scripts.




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      o    Support for mail-based access to multimedia documents, and
           (where appropriate) for printing multimedia documents.

      o    Powerful, easy-to-use authoring tools.

   Existing Systems

   The main information retrieval systems in use on the Internet are
   Gopher, Wais, and the World-Wide Web.  All work on a client-server
   paradigm, and all provide some degree of support for multimedia data.

   Gopher presents the user with a hierarchical arrangement of nodes
   which are either directories (menus), leaf nodes (documents
   containing text or other media types), or search nodes (allowing some
   set of documents to be searched using keywords, possibly using WAIS).
   A range of media types is supported.  Extensions currently being
   developed for Gopher (Gopher+) provide better support for multimedia
   data.  Gopher has a very high penetration (there are over 1000 Gopher
   servers on the Internet), but it does not provide hyperlinks and is
   inflexibly hierarchical.

   Wais (Wide Area Information Server) allows users to search for
   documents in remote databases.  Full-text indexing of the databases
   allows all documents containing particular (combinations of) words to
   be identified and retrieved.  Non-text data (principally image data)
   can be handled, but indexing such documents is only performed on the
   document file name, severely limiting its usefulness.  However, WAIS
   is ideally suited to text search applications.

   World-Wide Web (WWW) is a large-scale distributed hypermedia system.
   The Web consists of nodes (also called documents) and links.  Links
   are connections between documents: to follow a link, the user clicks
   on a highlighted word in the source document, which causes the
   linkedto document to be retrieved and displayed.  A document can be
   one of a variety of media types, or it can be a search node in a
   similar sense to Gopher.  The WWW addressing method means that WAIS
   and Gopher servers may also be accessed from (indeed, form part of)
   the Web.  WWW has a smaller penetration than Gopher, but is growing
   faster.  The Web technology is currently being revised to take better
   account of the needs of multimedia information.

   These systems all go some way to meet the user requirements.

      o    Support for multiple platforms and for a wide range of media
           types (through "viewer" software external to the client
           program) is good.

      o    Only WWW has hyperlinks.



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      o    There is little or no support for sophisticated presentation
           and synchronisation requirements.

      o    Support for database querying tends to be limited to
           "keyword" searches, but current developments in Gopher and
           WWW should make more sophisticated queries possible.

      o    Some clients support user annotation of documents.

      o    Response times for all three systems vary substantially
           depending on the network distance between client and server,
           and there is no support for isochronous data transfer.

      o    There is little in the way of authentication, charging and
           monitoring facilities, although these are planned for WWW.

      o    Scripting is not supported because of security issues

      o    WWW supports a mail responder.

      o    The only system sufficiently complex to warrant an authoring
           tool is WWW, which has editors to support its hypertext