rfc806.txt

著名的RFC文档,其中有一些文档是已经翻译成中文的的.

      TABLE 1.    FIELDS USED IN MESSAGE PROCESSING FUNCTIONS        19
      TABLE 2.    TYPE BITS IN THE IDENTIFIER OCTET                  33














































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                                                      Executive Summary


                              EXECUTIVE SUMMARY




           The  message  format  specification addresses the problem of
      exchanging  messages  between  different  computer-based  message
      systems  (CBMSs).    This interchange problem can be addressed on
      several   levels.      One   level   specifies    the    physical
      interconnections,   another  specifies  how  information  travels
      between CBMSs, another specifies form  and  meaning  of  messages
      being  interchanged.  The highest level specifies operations on a
      message.  Each of these levels would be covered  by  a  different
      standard.

           This  message format specification addresses only the issues
      of form and meaning of messages at the points in time  when  they
      are  sent  from  one  CBMS and received by another.  Messages are
      composed of fields, containing different classes of  information.
      These  fields  contain  information about the message originator,
      message recipient, subject matter, precedence and  security,  and
      references  to  previous  messages,  as  well  as the text of the
      message.  Standard formats (syntax) for messages ensure that  the
      contents  of  messages  generated by one CBMS can be processed by
      another CBMS.  Standard meanings (sematics) for the components of
      a message ensure standard interpretation of a  message,  so  that
      everyone  receiving  a  message  gets the meaning intended by its
      sender.

           Each CBMS that implements this message format  specification
      will  be  compatible  with  any  other  CBMS  that implements the
      specification.  Compatibility ensures  that  the  contents  of  a
      message  posted  by one CBMS can be received and interpreted by a
      different CBMS.

           This message format specification has been  developed  as  a
      result  of  examining  CBMSs  currently  in use in commercial and
      research environments.  Three major  design  perspectives  helped
      shape the message format specification.


        o  Viability.     The  message  format  specification  uses
           concepts that already work.  It has been  designed  with
           implementation concerns in mind.

        o  Compatibility.      The   message  format  specification
           contains concepts from existing CBMSs.  For this reason,
           many CBMS would already contain functions and components
           similar to  those  required  to  implement  the  message
           format specification.



                                      1                          

                                                      Executive Summary



        o  Extensibility.      This  message  format  specification
           defines a broad range of message content components  and
           requires  only an elementary subset of them.  This means
           that even a very simple CBMS can implement  the  message
           format  specification.  The message format specification
           contains a rich  set  of  optional  components  and,  in
           addition,  mechanisms  for  user  extensions  and future
           extensions to the message format specification.


           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.

                                      2                          

                                                              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
      organizations  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
      information.     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 informations.   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
      specification.

           Section 2 presents a simple model of operation  of  a  CBMS.
      Section  3 discusses the components of messages and their meaning
      (semantics).    This  includes  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


                                      3                          

                                                            Section 1.1



      summarizes  those elements according to whether they are required
      or  optional  for  a  CBMS  implementing   the   message   format
      specification.    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
           Specification


           The  message  format  specification  does  not  address  the
      following  issues,  some  of which are being covered by other NBS
      standards developments.  (See [BlaR-80] for a description of  the
      NBS protocols program.)


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

                                      4                          

                                                            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  Functions that CBMSs provide for users.

        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
      documents 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.














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                                                              Section 2



      2.  A SIMPLE MODEL OF A CBMS ENVIRONMENT