rfc2542.txt

<VC++网络游戏建摸与实现>源代码

Network Working Group                                        L. Masinter
Request for Comments: 2542                             Xerox Corporation
Category: Informational                                       March 1999


                 Terminology and Goals for Internet Fax

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 (1999).  All Rights Reserved.

Abstract

   This document defines a number of terms useful for the discussion of
   Internet Fax. In addition, it describes the goals of the Internet Fax
   working group and establishes a baseline of desired functionality
   against which protocols for Internet Fax can be judged. It
   encompasses the goals for all modes of facsimile delivery, including
   'real-time', 'session', and 'store and forward'.  Different levels of
   desirability are indicated throughout the document.

Table of Contents

   1. Introduction ..................................................  2
   2. Definitions and Operational Modes .............................  3
    2.1 User model of fax ...........................................  3
    2.2 Definition of Internet Fax ..................................  4
    2.3 Internet Fax Roles ..........................................  5
    2.4 Internet Fax Devices ........................................  5
    2.5 Operational modes ...........................................  8
   3. Goals for Internet Fax ........................................  8
   4. Operational Goals for Internet Fax ............................  9
    4.1 Functionality ...............................................  9
    4.2 Interoperability ............................................  9
    4.3 Confirmation ................................................ 10
    4.4 Quick Delivery .............................................. 11
    4.5 Capabilities ................................................ 12
    4.6 Simplicity .................................................. 12
    4.7 Security .................................................... 13
    4.8 Reliability ................................................. 14
    4.9 Fax-like use ................................................ 14
    4.10 Legal ...................................................... 15



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RFC 2542         Terminology and Goals for Internet Fax       March 1999


   5. Functional Goals for Internet Fax ............................. 15
    5.1 Goals for image data representation ......................... 15
    5.2 Goals for transmission ...................................... 16
    5.3 Goals for addressing ........................................ 16
    5.4 Goals for security .......................................... 17
    5.5 Goals for capability exchange ............................... 17
   6. Security Considerations ....................................... 18
   7. Acknowledgements .............................................. 18
   8. Author's Address .............................................. 18
   9. References .................................................... 19
   10. Full Copyright Statement ..................................... 20

1. Introduction

   Facsimile (Fax) has a long tradition as a telephony application for
   sending a document from one terminal device to another.

   Many mechanisms for sending fax documents over the Internet have been
   demonstrated and deployed and are currently in use. The general
   application of using the Internet for facsimile is called "Internet
   Fax".

   This document defines a number of terms useful for the discussion of
   Internet Fax. In addition, it describes the goals for Internet Fax and
   establishes a baseline of desired functionality against which
   protocols for Internet Fax can be judged. It encompasses the goals for
   all modes of facsimile delivery, including "real-time", "session", and
   "store and forward" (terms defined in Section 2 of this document).

   1.1 Terminology used within this document

   Within this document, different levels of desirability for a protocol
   for Internet Fax are indicated by different priorities, indicated in
   {braces}:

      {1} there is general agreement that this is a critical
          characteristic of any definition of Internet Fax.
      {2} most believe that this is an important characteristic
          of Internet Fax.
      {3} there is general belief that this is a useful feature
          of Internet Fax, but that other factors might override;
          a definition that does not provide this element is
          acceptable.








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RFC 2542         Terminology and Goals for Internet Fax       March 1999


   In addition, the following terms are used:

   "service"      An operational service offered by a service provider.
   "application"  A use of systems to perform a particular function.
   "terminal"     The endpoint of a communication application.
   "goal"         An objective of the standarization process.

2. Definitions and Operation Modes

   This section defines some of the basic terms for Internet Fax.

2.1 User model of fax and basic operations

   The phrase "traditional facsimile" or "G3Fax" is used to denote
   implementations of [T.30]. Facsimile (fax) is a telephony application
   for sending a document from one terminal device to another.

   The telephone network is often referred to as the Public Switched
   Telephone Network (PSTN) or Global Switched Telephone Network (GSTN).

   Communication over the telephone network is accomplished using
   modems.  The transmission of data end-to-end is accompanied by
   negotiation (to ensure that the scanned data can be rendered at the
   recipient) and confirmation of delivery (to give the sender assurance
   that the final data has been received and processed.)  Over time,
   facsimile has been extended to allow for PCs using fax modems to send
   and receive fax, to send data other than scanned facsimile images. In
   addition, there have been many extensions to the basic image model,
   to allow for additional compression methods and for representation of
   images with grey-scale and color. Other delivery extensions have
   included sub-addressing (additional signals after the call is
   established to facilitate automated routing of faxes to desktops or
   mailboxes), and enhanced features such as fax-back and polling.

   Typically, the terminal device consists of a paper input device
   (scanner), a paper output device (printer), with (a limited amount
   of) processing power. Traditional facsimile has a simple user
   operational model; the user

      1) inserts paper into a device
      2) dials a number corresponding to the destination
      3) presses the 'start' button on the device
      4) the sending device connects to the receiving device using the
         telephone network
      5) the sending device scans the paper and transmits the image of
         the paper
      6) simultaneously, the remote device receives the transmission and
         prints the image on paper



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RFC 2542         Terminology and Goals for Internet Fax       March 1999


      7) upon completion of transmission and successful processing by
         the recipient, the sending user is notified of success

   Although not usually visible to the user, the operation (5) of
   transmission consists of

      5a) negotiation: the capabilities of the recipient are obtained,
          and suitable mutually available parameters for the
          communication are selected
      5b) scanning: creating digitized images of pages of a document
      5c) compression: the image data is encoded using a data
          compression method
      5d) transmission: the data is sent from one terminal to the other

   In addition, the terminiation of operations (5d) and (6) may be
   characterized as consisting of:

      6a) completed delivery: the message has completed transmission
      6b) completed receipt:  the message has been accepted by the
          recipient
      6c) processing and disposition: the message has been processed

   From a protocol perspective, the information conveyed in the
   transmission consists of both "protocol" (control information,
   capabilities, identification) and also "document content".

   The document content consists primarily of the "document image" plus
   additional metadata accompanying the image. The means by which an
   image of a document is encoded within the fax content is the "image
   data representation".

   When the fax has been successfully transmitted, the sender receives a
   "confirmation": an indication that the fax content was delivered.
   This "confirmation" is an internal signal and is not normally visible
   to the sending user, although some error messages are visible, to
   allow a page to be retransmitted.

2.2 Definition of Internet Fax

   The phrase "Internet Fax" is used to denote an application which
   supports an approximation to the user model of fax (Section 2.1), but
   where Internet protocols are used instead of the telephone network
   for (some portion of) the transmission. The exact modes and
   operations of traditional facsimile need not be duplicated exactly.







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RFC 2542         Terminology and Goals for Internet Fax       March 1999


2.3 Internet Fax Roles

   Internet Fax is a document transmission mechanism between various
   different devices and roles. Those devices and roles might come in a
   wide variety of configurations. To allow for a wide variety of
   configurations, it is useful to separate out the roles, as they may
   be made available separately or in combination. These roles are:

      * Network scanner
        A device that can scan a paper document and transmit the scanned
        image via the Internet

      * Network printer
        A device that can accept an image transmission via the Internet
        and print the received document automatically

      * Fax onramp gateway
        A device that can accept a facsimile telephone call and
        automatically forward it via the Internet

      * Fax offramp gateway
        A device that can accept a transmission from the Internet and
        forward it to a traditional fax terminal

   In addition, other traditional Internet applications might also
   participate in Internet Fax, including Internet mail users, Web
   browsers, Internet printing hosts.

2.4 Internet Fax Devices

   The Internet Fax roles may be embedded in a variety of combinations
   and configurations within devices and larger applications.  They may
   be combined with other elements, e.g., a traditional T.30 fax device.
   Many different configurations of applications and systems should {2}
   be able to participate in Internet Fax; the specification should not
   unnecessarily restrict the range of devices, applications and
   services that can participate.

   A device that supports Internet Fax might support any combination of
   the roles defined in 2.3.

2.4.1 Gateway devices

   A traditional fax terminal has a telephone line connection (GSTN)
   with a fax modem used to connect over the telephone network. To
   connect a fax terminal to the Internet requires a service which
   offers connections on one side to the GSTN using standard fax
   signals, and on the other side to the Internet. This role might be