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.

           CBMS facilities for message creation, transfer,  and  recip-
      ient  processing  are  reflected  in  a  logical  model of a CBMS
      developed by IFIP Working Group 6.5.  (An  essentially  identical
      model  is  being  used  by  CCITT  Study  Group  VII, Question 5,
      regarding  Message  Handling  Systems [CCIT-82].)     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 (UA) 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(MTS) is an entity that accepts a




                                     12

                                                            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  trans-
      ferred  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  able  to
      transfer  messages  between originating and receiving UAs without
      reading or altering the contents  of  messages  unless  otherwise
      instructed by the UAs.  In addition, this message format specifi-
      cation  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




                                     13

                                                            Section 2.1

      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.



      2.2  Relationship to the ISO Reference Model for Open Systems
           Interconnection


           Subcommittee TC97/SC16 of the International Organization for
      Standardization  (ISO)  has  developed  a  reference  model   for
      describing communications between "open" systems [ISOD-82].  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.)




                                     14

                                                            Section 2.3

           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.



      2.4  Message Originators and Recipients


           This message format specification refers to  message  origi-
      nators  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, processes
      or groups.

           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




                                     15

                                                            Section 2.4

      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.



















































                                     16

                                                              Section 3

      3.  SEMANTICS


           This  section discusses two major topics, message processing
      functions and message field meanings.  Section 3.1 describes  the
      six  functional  groups of message fields.  The functional groups
      are Origination, Dates, Recipients,  Cross-referencing,  Message-
      handling, and Message-contents.  They are explained more fully in
      Section 3.1.1, along with detailed discussion of the semantics of
      all  the  fields in each functional group.  Section 3.2 describes
      message processing functions whose  operation  is  based  on  the
      meanings of particular message fields.



      3.1  Semantics of Message Fields


           The  definition  of  a  message  is  discussed  generally in
      Sections 1 and 2.  Semantically valid messages must  contain  one
      From  field,  one  To field, and one Posted-Date field.  They may
      contain, in addition, any number of other  fields,  depending  on
      the  processing  and  functions  supplied  by  the originating or
      receiving CBMS.  (Section  3.2  describes  classes  of  functions
      supplied by CBMSs.)


      3.1.1  Types of fields


           Message  receiving programs are required to interpret fields
      according to the semantics described in  the  remainder  of  this
      section.  The message fields defined in this document are grouped
      into the following functional categories.


        o  Originator  fields  indicate who or what participated in
           the creation of the message and where replies should  be
           directed.  (See Section 3.1.3.)

        o  Date fields record when events take place, for a variety
           of events, such as message creation or expiration.  (See
           Section 3.1.5.)

        o  Recipient  fields  indicate  who  or what is intended to
           receive a message.  (See Section 3.1.4.)

        o  Cross-reference fields label a message or refer to other
           messages.  (See Section 3.1.6.)

        o  Message-handling fields record the  type  of  service  a




                                     17

                                                          Section 3.1.1

           message's  sender requested of a message transfer system
           or indicate how the message should  be  treated  by  its
           recipients.  (See Section 3.1.7.)

        o  Message-content   fields   either  contain  the  primary
           content of a message, or index the message, or summarize
           the message.  (See Section 3.1.8.)

        o  Extension fields provide mechanisms  for  extending  the
           message format specification.  (See Section 3.1.9.)


      3.1.2  Semantic Compliance Categories


           For purposes of determining whether a CBMS complies with the
      semantic  requirements of this message format specification, mes-
      sage fields have been divided into three categories:


      REQUIRED  These fields must be present in all messages  and  must
                be  processed  by message receiving programs as defined
                by the message format specification.