rfc841.txt

著名的RFC文档,其中有一些文档是已经翻译成中文的的.

           The  message  format  specification  defines  the  form  and
      meaning  of  message  contents  and their components as they pass
      from one CBMS to another through a message transfer system.   The
      message   format  specification  does  not  address  any  of  the
      following major issues.


        o  Functions or services provided to a user by a CBMS.
                For  example,  the  message  format   specification
                assumes  that  every CBMS allows a user to send and
                receive messages.  It does not specify any  of  the
                details of how a send function or a message-reading
                function  might  work or how it might appear to the
                user.  That is, the  message  format  specification
                neither limits nor mandates functions.

        o  Storage or format of message contents in a CBMS.
                The  message  format specification defines the form
                and contents of messages when they are  transferred
                between  systems.   A CBMS may or may not choose to
                use the same format for internal storage.

        o  Message transfer system protocols.
                The message format specification does  not  specify
                how  a  message  travels  between  CBMSs.   It does
                specify the form of its contents as it  leaves  and
                arrives,  assuming  only  that the message is moved
                transparently by the transfer system.

        o  Message envelopes.
                While a message is traveling between CBMSs,  it  is
                enclosed  in a message envelope.  Message envelopes
                contain all the information about a message that  a
                message transfer system needs to know.  The message
                format  specification does not define the format or
                content of a message envelope.

        o  How message originators and recipients are identified.
                The message format specification does not provide a
                representation scheme for the names or addresses of
                message originators  and  recipients  as  they  are
                known to a CBMS.



                                      6

                                                              Section 1

      1.  INTRODUCTION


           A  computer-based message system (CBMS) allows communication
      between "entities" (usually people) using computers.    Computers
      serve  both  to mediate the actual communications between systems
      and to provide users with facilities for creating and reading the
      messages.

           CBMSs have  been  developing  for  over  ten  years.    More
      recently,  CBMSs  have  been one of the bases in industry for the
      introduction of office automation.  A growing number  of  organi-
      zations  use  either  their own or a commercially available CBMS.
      The design and complexity of these systems  vary  widely.    This
      message  format  specification  provides  a basis for interaction
      between different CBMSs by defining the format of messages passed
      between them.



      1.1  Guide to Reading This Document


           The method of presenting the material in this  specification
      is  to  combine  the technical specification with tutorial infor-
      mation.  This approach has been taken to place the  specification
      in context and improve its readability.

           The  core of the technical information in the document is in
      Section 2, "A Simple Model of a CBMS Environment";  Section  3.1,
      "Semantics  of  Message Fields"; Section 4.2, "Overview of Syntax
      Encoding"; and Section 4.3, "Data Element Syntax".  Appendixes  A
      and  B  consolidate  the technical information.  These appendices
      are designed  for  ease  of  reference  and  should  be  read  in
      conjunction   with   the  body  of  the  report  for  a  complete
      understanding of the message format  presented  in  the  specifi-
      cation.

           Section  2  presents  a simple model of operation of a CBMS.
      Section 3 discusses the components of messages and their  meaning
      (semantics),    including    discussions   of   the   recommended
      relationship between message components and CBMS user  functions.
      (See  Section  3.2.)    Section  4  presents  details of the form
      (syntax) required for components of a message.

           Appendix D summarizes the components of  messages  according
      to  whether  they are required or optional for CBMSs implementing
      the message format  specification.    Appendix  E  organizes  the
      message  components  according  to  the  functional  class of the
      components.  Appendix F provides an  overview  of  the  syntactic
      elements defined by this message format specification; Appendix G




                                      7

                                                            Section 1.1

      summarizes  those elements according to whether they are required
      or optional for a CBMS implementing the message  format  specifi-
      cation.  Examples of each syntactic element appear in Appendix H,
      displaying syntax and describing the associated semantics.



      1.2  Vendor-Defined Extensions to the Specification


           This  specification provides the capability of extending the
      range of functionality by the use  of  vendor-defined  qualifiers
      and  vendor-defined  data  elements.    Any  vendor who uses this
      capability to provide services which are  essentially  equivalent
      to  those already designated as required, basic, or optional does
      not comply with the specification.



      1.3  The Scope of the Message Format Specification


           The purpose of  this  message  format  specification  is  to
      present  the  semantics  and syntax to be used for messages being
      exchanged between CBMSs.  Specifically, it defines the following:


        o  The meaning and form of standard fields to  be  used  in
           messages.

        o  Which fields must be present in all messages.

        o  Which fields complying CBMSs must be able to process.

        o  How  messages,  fields, and the data contained in fields
           are represented.



      1.4  Issues Not Within the Scope of the Message Format Specifi-
           cation


           The  message  format  specification  does  not  address  the
      following  issues,  some  of which are being covered by other NBS
      standards development programs  at  the  Institute  for  Computer
      Sciences and Technology (ICST).  (See [BlaR-80] for a description
      of the ICST network protocols program.)


        o  The nature of a message transfer system, except to state
           the assumption that it transfers messages transparently.



                                      8

                                                            Section 1.4

        o  The  form  or  nature  of the protocols used to transfer
           messages (posting, relay, and delivery protocols).

        o  The content and representation of message envelopes.

        o  Representations for unique identifiers  (in  particular,
           message identifiers).

        o  Network and internetwork addressing.

        o  Representations  for  identities  of message originators
           and recipients.

        o  Certain message processing functions that CBMSs  provide
           for  users,  e.g., those concerned with the creation and
           editing of text.

        o  Presentation of messages to users.

        o  Representations for multi-media objects.

        o  Data representation for messages within CBMSs.

        o  Data sharing or any storage management within CBMSs.

        o  Representations for fixed  or  floating  point  numbers.
           



      1.5  Relationship to Other Efforts


           The  message  format specification is based on several docu-
      ments and the current state  of  many  CBMSs  available  both  in
      industry and the research community.  These documents include the
      standardization efforts in the ARPANet [CroD-77, PosJ-79] and the
      CCITT,  proposed  ISO  and  ANSI  header  format standards [TasG-
      80, ISOD-79], the work of IFIPS Working Group  6.5,  and  various
      papers  about the general nature of mail systems, addressing, and
      mail delivery.  (See [FeiE-79] for references.














                                      9

                                                              Section 2

      2.  A SIMPLE MODEL OF A CBMS ENVIRONMENT


           In  order  to provide a framework for presenting the message
      format specification, this section describes a simple  functional
      model for a CBMS.  The model provides a high-level description of
      both  user  facilities  and  system architecture.  Discussions of
      messages, message originators, and message  recipients  serve  to
      further clarify the nature of a CBMS.

           A  CBMS permits the transfer of a message from an originator
      to a recipient.  "Originator" and "recipient" are used  in  their
      normal  English  senses.    (See Section 2.4.)  A message (in its
      most abstract definition) is simply a unit of communication  from
      an  originator  to a recipient.  A CBMS offers several classes of
      functions to its users:


        o  Message Creation:  The  facilities  used  by  a  message
           originator  to  create messages and specify to whom they
           are to be sent.

        o  Message Transfer: The facilities used to convey  a  mes-
           sage to its recipient(s).

        o  Recipient  Processing:  The facilities used by a message
           recipient to process messages that have arrived.


           These classes of functions are presented in more  detail  in
      Section 3.2.

           CBMSs  differ  from  other  office automation/communications
      systems in a number of ways.


        o  Unlike other types of  electronic  communications,  CBMS
           messages  are  sent  to  particular  individuals, not to
           stations or telephone sets.  If a recipient moves  to  a
           different  location, messages sent to that recipient are
           delivered to the recipient at the new location.

        o  Transmission of CBMS  messages  is  asynchronous.    The
           recipient's  system  need not be available when the mes-
           sage leaves the originator's  system.    That  is,  CBMS
           message transfer facilities are store-and-forward.

        o  CBMS  messages can contain a wide variety of data.  They
           are not constrained to any single kind of communication.
           CBMS messages are often simple  memoranda  but  are  not
           restricted to text.  A CBMS message may contain any kind




                                     10

                                                              Section 2

           of  data  that  an originator wishes to send to a recip-
           ient.  By contrast, Teletex  systems  and  communicating
           word  processors  handle  the  transfer  of  final  form
           documents; compatible communicating word processors  can
           exchange  documents in editable form; Telex and TWX deal
           in unformatted text.

        o  CBMSs offer message creation facilities as an  important
           part  of  the  system.  CBMSs assist users in the prepa-
           ration of messages by  having  text  editing  facilities
           available  and allowing users to include data stored on-
           line in messages.  Some CBMSs also  interface  to  other
           office  automation  facilities,  such  as formatters and
           spelling correctors.  This is not true of Telex, TWX, or
           similar services.

        o  CBMSs offer recipient processing facilities as an impor-
           tant part of the system.  This is not true of most other
           forms of electronic communications.  For example,  Telex
           and TWX systems simply print messages on paper when they
           are  received,  without  retaining a copy in the system.
           (Teletex systems are similar to Telex systems, but  some
           can  retain  a  copy  of the document in local storage.)
           Communicating  word  processors   might   notify   their
           operators  that  a  document  has  been  received and is
           stored on-line, but they offer  little  in  the  way  of
           other recipient processing facilities.  Most CBMSs offer
           at least the following recipient processing facilities:


             .  The  ability  to retain a copy of a message on-line
                after it has been read.

             .  The ability to examine or  delete  stored  messages
                individually.

             .  The ability to organize messages using some form of
                electronic "file folder."

             .  The  ability  to  determine  if a message is recent
                (has arrived since the last time the recipient used
                the CBMS) or unseen (has never been examined by the
                recipient).

             .  The  ability  to  summarize  stored  messages.    A
                summary   usually   includes  information  such  as
                whether the message is recent or  unseen,  when  it
                was  received,  its length, who it is from, and its
                subject.

             .  The ability to retrieve a stored message based upon




                                     11

                                                              Section 2

                one  or  more of its attributves (for example, when
                the message was received, whether  or  not  it  has
                been  seen  or deleted, and the values contained in
                its fields).

             .  A forward facility that allows users to include all
                or part of a message in a new outgoing message.

             .  A reply facility that allows users to  answer  mes-
                sages  without having to enter a new list of recip-
                ients.



      2.1  Logical Model of a CBMS