rfc759.txt

RFC 相关的技术文档

  that are produced and consumed respectively by user interface (or
  other) programs.

  



































[Page 4]                                                          Postel


August 1980                                                             
                                               Internet Message Protocol



                       2.  FUNCTIONAL DESCRIPTION

This section gives an overview of the Internet Message System and its
environment.

2.1.  Terminology

  The messages are routed by a process called the Message Processing
  Module or MPM.  Messages are created and consumed by User Interface
  Programs (UIPs) in conjunction with users.

  The basic unit transferred between MPMs is called a message.  A
  message is made up of a transaction identifier (which uniquely
  identifies the message), a command (which contains the necessary
  information for delivery), and document.  The document may have a
  header and a body.

  For a personal letter the document body corresponds to the contents of
  the letter; the document header corresponds to the date line,
  greeting, and signature.

  For an inter-office memo the document body corresponds to the text;
  the document header corresponds to the header of the memo.

  The commands correspond to the information used by the Post Office or
  the mail room to route the letter or memo.  Some of the information in
  the command is supplied by the UIP.

2.2.  Assumptions

  The following assumptions are made about the internetwork environment:

  In general, it is not known what format intranet addresses will
  assume.  Since no standard addressing scheme would suit all networks,
  it is safe to assume there will be several and that they will change
  with time.  Thus, frequent software modification throughout all
  internet MPMs would be required if such MPMs were to know about the
  formats on many networks.  Therefore, each MPM which handles internet
  messages is required to know only the minimum necessary to deliver
  them.

  Each MPM is required to know completely only the addressing format of
  its own network(s).  In addition, the MPM must be able to select an
  output link for each message addressed to another network or host.
  This does not preclude more intelligent behavior on the part of a
  given MPM, but at least this minimum is necessary.  Each network has a
  unique name and numeric address.  Such names and addresses are



Postel                                                          [Page 5]


                                                             August 1980
Internet Message Protocol
Functional Description



  registered with a naming authority and may be listed in documents such
  as Assigned Numbers [4].

  Each MPM will have a unique internet address.  This feature will
  enable every MPM to place a unique "handling-stamp" on a message which
  passes through the MPM enroute to delivery.

2.3.  General Specification

  There are several aspects to a distributed service to be specified.
  First, there is the service to be provided; that is, the
  characteristics of the service as seen by its users.  Second, there is
  the service it uses; that is, the characteristics it assumes to be
  provided by some lower level service.  And third, there is the
  protocol used between the modules of the distributed service.

       User                                          User       
          \                                          /          
          UIP                                      UIP          
            \                                      /            
         --+----------------------------------------+-- Service 
           |   \                                /   | Interface 
           |  +--------+                +--------+  |           
           |  | Module | <--Protocol--> | Module |  |           
           |  +--------+                +--------+  |           
           |        \                       /       |           
           |        +-----------------------+       |           
           |        | Communication Service |       |           
           |        +-----------------------+       |           
           |                                        |           
           +----------------------------------------+           

                            Message Service

                               Figure 1.

  The User/Message Service Interface

    The service the message delivery system provides is to accept
    messages conforming to a specified format, to attempt to deliver
    those messages, and to report on the success or failure of the
    delivery attempt.  This service is provided in the context of an
    interconnected system of networks and may involve relaying a message
    through several intermediate MPMs via different communication
    services.




[Page 6]                                                          Postel


August 1980                                                             
                                               Internet Message Protocol
                                                  Functional Description



  The Message/Communication Service Interface

    The message delivery system calls on a communication service to
    transfer information from one MPM to another.  There may be
    different communication services used between different pairs of
    MPMs, though all communication services must meet the service
    characteristics described below.

    It is assumed that the communication service provides a reliable
    two-way data stream.  Such a data stream can usually be obtained in
    computer networks from the transport level protocol, for example,
    the Transmission Control Protocol (TCP) [3].  In any case, the
    properties the communication service must provide are:

      o  Logical connections for two way simultaneous data flow of
         arbitrary data (i.e., no forbidden codes).  All data sent is
         delivered in order.

      o  Simple commands to open and close the connections, and to send
         and receive data on the connections.

      o  Controlled flow of data so that data is not transmitted faster
         that the receiver chooses to consume it (on the average).

      o  Transmission errors are corrected without user notification or
         involvement of the sender or receiver.  Complete breakdown on
         communication is reported to the sender or receiver.

  The Message-Message Protocol

    The protocol used between the distributed modules of the message
    delivery system, that is, the MPMs, is a small set of commands which
    convey requests and replies.  These commands are encoded in a highly
    structured and rigidly specified format.

2.4.  Mechanisms

  MPMs are processes which use some communication service.  A pair of
  MPMs which can communicate reside in a common interprocess
  communication environment.  An MPM might exist in two (or more)
  interprocess communication environments, and such an MPM might act to
  relay messages between MPMs.  Messages may be held for a time in an
  MPM; the total path required for delivery need not be available
  simultaneously.

  From the time a message is accepted from a UIP by an MPM until it is
  delivered to a UIP by an MPM and an acknowledgment is returned to the


Postel                                                          [Page 7]


                                                             August 1980
Internet Message Protocol
Functional Description



  originating UIP, the message is considered to be active in the message
  system.

     User                                                    User 
       \                                                      /   
       UIP                                                  UIP   
         \                                                  /     
      +---------------------------------------------------------+ 
      |    \                                              /     | 
      |  +-----+                +-----+                +-----+  | 
      |  | MPM | <--Protocol--> | MPM | <--Protocol--> | MPM |  | 
      |  +-----+                +-----+                +-----+  | 
      |     |                    /   \                    |     | 
      |  +-----------------------+   +-----------------------+  | 
      |  |Communication Service A|   |Communication Service B|  | 
      |  +-----------------------+   +-----------------------+  | 
      |                                                         | 
      +---------------------------------------------------------+ 

                 Message Service with Internal Relaying

                               Figure 2.

  It should be clear that there are two roles an MPM can play, an
  end-point MPM or a relay MPM.  Most MPMs will play both roles.  A
  relay MPM acts to relay messages from one communication environment to
  another.  An end-point MPM acts as a source or destination of
  messages.

  The transfer of data between UIPs and MPMs is viewed as the exchange
  of data structures which encode messages.  The transfer of data
  between MPMs is also in terms of the transmission of structured data.

















[Page 8]                                                          Postel


August 1980                                                             
                                               Internet Message Protocol
                                                  Functional Description



                    +-----+     DATA       +-----+         
             USER-->| UIP |-->STRUCTURES-->| MPM |-->other 
                    +-----+    +-----+     +-----+    MPMs 
                               |     |                     
                               |  +-----+                  
                               +--|     |                  
                                  |  +-----+               
                                  +--|     |               
                                     |     |               
                                     +-----+               

                     +-----+     DATA       +-----+        
             other-->| MPM |-->STRUCTURES-->| UIP |-->USER 
             MPMs    +-----+    +-----+     +-----+        
                                |     |                    
                                |  +-----+                 
                                +--|     |                 
                                   |  +-----+              
                                   +--|     |              
                                      |     |              
                                      +-----+              

                              Message Flow

                               Figure 3.

  In the following, a message will be described as a structured data
  object represented in a particular kind of typed data elements.  This
  is how a message is presented when transmitted between MPMs or
  exchanged between an MPM and a UIP.  Internal to an MPM (or a UIP), a
  message may be represented in any convenient form.


















Postel                                                          [Page 9]


                                                             August 1980
Internet Message Protocol
Functional Description



2.5.  Relation to Other Protocols

  This protocol the benefited from the earlier work on message protocols
  in the ARPA Network [5,6,7,8,9], and the ideas of others about the
  design of computer message systems
  [10,11,12,13,14,15,16,17,18,19,20,21].

  Figure 4 illustrates the place of the message protocol in the ARPA
  internet protocol hierarchy:

                                    
   +------+ +-----+ +-------+ +-----+     +-----+                   
   |Telnet| | FTP | |Message| |Voice| ... |     | Application Level 
   +------+ +-----+ +-------+ +-----+     +-----+                   
           \   |   /             |           |                      
            +-----+           +-----+     +-----+                   
            | TCP |           | RTP | ... |     | Host Level        
            +-----+           +-----+     +-----+                   
               |                 |           |                      
              +-------------------------------+                     
              |       Internet Protocol       |   Gateway Level     
              +-------------------------------+                     
                              |                                     
                +---------------------------+                       
                |   Local Network Protocol  |     Network Level     
                +---------------------------+                       
                              |                                     



                         Protocol Relationships

                               Figure 4.

  Note that "local network" means an individual or specific network.
  For example, the ARPANET is a local network.

  The message protocol interfaces on one side to user interface programs
  and on the other side to a reliable transport protocol such as TCP.

  In this internet message system the MPMs communicate directly using
  the lower level transport protocol.  In the old ARPANET system,
  message transmission was part of the file transfer protocol.






[Page 10]                                                         Postel


August 1980                                                             
                                               Internet Message Protocol
                                                  Functional Description



                                    
        +------+   +-----+   +-------+                              
        |Telnet|   | FTP |---|Message|            Application Level 
        +------+   +-----+   +-------+                              
              \     /                                               
    +-----+   +-----+                                               
    |Voice|---| NCP |                             Host Level        
    +-----+   +-----+                                               
                 |                                                  
                 |                                                  
                 |                                Gateway Level     
                 |                                                  
                 |                                                  
         +----------------+                                         
         |    ARPA NET    |                       Network Level     
         +----------------+