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.

           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
           message 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
           message 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


                                      6                          

                                                              Section 2



           of   data  that  an  originator  wishes  to  send  to  a
           recipient.      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
           preparation  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
           important  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 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


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                                                              Section 2



                one  or  more  of its attributes (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
                messages  without  having  to  enter  a new list of
                recipients.



      2.1  Logical Model of a CBMS


           CBMS  facilities  for  message   creation,   transfer,   and
      recipient  processing  are reflected in a logical model of a CBMS
      developed by IFIP Working Group 6.5 [SchP-79].   (An  essentially
      identical  model is being used by CCITT Study Group VII, Question
      5, regarding Message Handling Facilities.)  The model consists of
      a Message Transfer System and a number  of  User  Agents.    (See
      Figure 1.)



                    |                       |
                    |     *************     |
      *********  ------>  *  Message  *  ------->  *********
      * User  *  Posting  * Transfer  *  Delivery  * User  *
      * Agent *  Protocol *  System   *  Protocol  * Agent *
      *********  <------- *************  <-------  *********
                    |                       |
                    |                       |
                 Posting                Delivery
                  Slot                    Slot

                          Message Flow
      Originator --------------------------------> Recipient



      FIG. 1.  LOGICAL MODEL OF A COMPUTER BASED MESSAGE SYSTEM


           A User Agent is a functional entity that acts on behalf of a
      user,   assisting  with  creating  and  processing  messages  and
      communicating with the Message Transfer System.

           The Message Transfer System] is an  entity  that  accepts  a


                                      8                          

                                                            Section 2.1



      message from its originator's User Agent and ultimately passes it
      to  each  of  its  recipients' User Agents.  The Message Transfer
      System may perform routing and storage functions  (among  others)
      in order to accomplish its task.

           Transferring  a  message  from an originator's User Agent to
      the Message Transfer System is called Posting;  the  originator's
      User  Agent  and  Message  Transfer  System  engage  in a Posting
      Protocol in order to accomplish Posting.  Transferring a  message
      from  the  Message Transfer System to a recipient's User Agent is
      called Delivery; the recipient's User Agent and Message  Transfer
      System  engage  in  a  Delivery  Protocol  in order to accomplish
      Delivery.

           The  point  at  which  responsibility  for  a   message   is
      transferred  is  called a Slot.  The Posting Slot is the point at
      which responsibility for a message passes  from  an  originator's
      User  Agent  to the Message Transfer System; the Delivery Slot is
      the point at which responsibility for a message passes  from  the
      Message Transfer System to a recipient's User Agent.

           The  model  divides  messages  into  two  parts, the message
      content and the message envelope.  The  message  content  is  the
      information  that the originator wishes to send to the recipient;
      this message format specification deals solely with  the  message
      content.    The  message envelope consists of all the information
      necessary for the Message Transfer System to  do  its  job;  this
      message   format  specification  does  not  specify  the  message
      envelope.  Some of the data appearing  on  the  message  envelope
      could  be  redundant with some data found in the message content.
      The Message Transfer  System  is  not  expected  to  examine  the
      message content unless it is told to do so by the originator's or
      recipient's User Agent.

           This  message format specification places no restrictions on
      the Message Transfer System itself, except that it be transparent
      to the contents of messages.  In addition,  this  message  format
      specification does not dictate the form or nature of any protocol
      used  by  the  Message  Transfer  System.   Finally, this message
      format specification does not specify the content or form of  the
      message  envelope.    That  is,  the message format specification
      defines the format for the contents of messages, not  the  manner
      in which they are transmitted.

           Many of today's commercially available CBMSs incorporate all
      of  the  facilities  represented  in  the  logical  model.  Their
      architectures may reflect the economies that can  be  taken  when
      implementing  systems  that  are  self-contained.    For example,
      stand-alone systems that  store  messages  in  a  single  central
      database  require  no  Message Transfer System; an implementation
      may integrate software for User Agent and Message Transfer System
      functions, doing away with Posting or Delivery Protocols.

                                      9                          

                                                            Section 2.1



      2.2  Relationship to the ISO Reference Model for Open Systems
           Interconnection


           Subcommittee   TC97/SC16   of  the  International  Standards
      Organization (ISO) has developed a reference model for describing
      communications between "open" systems [ISOD-81].  This  model  is
      known as the ISO Reference Model for Open Systems Interconnection
      (OSI).    It  divides communications protocols into seven layers,
      ranging from physical interconnection at the lowest layer to data
      exchange by application programs at the top.

           This message format specification deals with data used by an
      application within a system.   Thus,  the  message  format  being
      specified here is not a protocol.  Since it is not a protocol, it
      lies outside of the model for open systems interconnection.  User
      Agents are application layer entities (layer 7), however, and the
      protocols used by a message transfer system are above the session
      layer (layer 5).



      2.3  Messages and Fields


           A message is a unit of communication from an originator to a
      recipient.    A message consists of a series of components called
      fields.  Fields can be described according to their meaning in  a
      message (semantics) and according to the format required for them
      in a message (syntax).

           Semantically,  a field is just a component of a message; the
      meanings of particular fields are defined by this message  format
      specification.    Syntactically,  a field is a unit of data whose
      form is defined by this message format specification.  Additional
      fields can be defined by users or vendors as long as they conform
      to the syntactic and semantic  rules  that  this  message  format
      specification defines for additional fields.

           (A  note  on  terminology:  A message consists of components
      called fields.  The words "message" and "field" are used both  in
      the  informal  sense  of  the  previous  sentence  and  in a more
      restricted sense as names of particular syntactic elements.    As
      syntactic   element   names,   Message   and   Field  are  always
      capitalized.)

           Some CBMS functions are based on the contents of  particular
      fields;  other  functions (such as the ability to read a message)
      may have little to do with the fields themselves.    Section  3.2
      discusses  some  of  the  specific  functions  that  a CBMS might
      provide to users and the fields that  must  be  used  to  support
      those functions.

                                     10                          

                                                            Section 2.3



      2.4  Message Originators and Recipients


           This   message   format   specification  refers  to  message
      originators  and  recipients.      These   terms   were   defined
      functionally  in Figure 1.  When the message format specification
      refers to the identity of a message originator or  recipient,  it
      means  "that  information  which  uniquely identifies the message
      originator or recipient within the domain of  the  given  message
      system."   The syntax and semantics of message addressing are not
      within the scope of the message format specification.

           Originators  and  Recipients  can  be  people,   roles,   or
      processes.

           People.    People as originators and recipients are specific
      individuals.

           Roles.  Roles identify  functions  within  organizations  as
      opposed  to  the  specific  individuals  who  perform  them.  For
      example, consider a newspaper  that  produces  both  morning  and
      evening editions and therefore operates with more than one shift.
      Someone  wishing to contact the city desk would send a message to
      the city desk role rather than trying to  determine  exactly  who
      was  assigned  to  the city desk at a specific time.  (Of course,
      messages can usually be sent to the individuals directly  whether
      or not they are actually performing a role at the time.)

           Processes.  A process in a computer could serve as either an
      originator  or a recipient for messages.  A computer system might
      originate a message to notify a recipient  about  the  status  of
      some  task.    For example, an archive utility could notify users
      about files that have been archived; a  distributed  file  system
      could  notify  a  user that a remote file has been deposited on a
      local file system.  Messages could be used by computer systems to
      warn about some  impending  condition  or  even  to  monitor  the
      performance  of the computer itself.  Some computer processes may
      also be message recipients,  taking  action  based  upon  message
      contents.

           In addition, some CBMSs allow messages to be sent to groups.
      A  group  is  a  predefined  list of message recipients.  Using a
      group  name  as  a  recipient  permits  message  originators   to
      designate a potentially large number of recipients using a single
      recipient  identifier.  This makes using the CBMS more convenient
      and accurate.







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                                                              Section 3