rfc1085.txt

著名的RFC文档,其中有一些文档是已经翻译成中文的的.

Network Working Group                                            M. Rose
Request for Comments: 1085                                           TWG
                                                           December 1988


                       ISO Presentation Services
                    on top of TCP/IP-based internets

Status of this Memo

   This memo proposes a standard for the Internet community.
   Distribution of this memo is unlimited.

1. Introduction

   [RFC1006] describes a mechanism for providing the ISO transport
   service on top of the Transmission Control Protocol (TCP) [RFC793]
   and Internet Protocol (IP) [RFC791].  Once this method is applied,
   one may implement "real" ISO applications on top of TCP/IP-based
   internets, by simply implementing OSI session, presentation, and
   application services on top of the transport service access point
   which is provided on top of the TCP.  Although straight-forward,
   there are some environments in which the richness provided by the OSI
   application layer is desired, but it is nonetheless impractical to
   implement the underlying OSI infrastructure (i.e., the presentation,
   session, and transport services on top of the TCP).  This memo
   describes an approach for providing "stream-lined" support of OSI
   application services on top of TCP/IP-based internets for such
   constrained environments.

2. Terminology

   In as much as this memo is concerned primarily with concepts defined
   in the framework of Open Systems Interconnection (OSI) as promulgated
   by the International Organization for Standardization (ISO), the
   terminology used herein is intended to be entirely consistent within
   that domain of discourse.  This perspective is being taken despite
   the expressed intent of implementing the mechanism proposed by this
   memo in the Internet and other TCP/IP-based internets.  For those
   more familiar with the terminology used in this latter domain, the
   author is apologetic but unyielding.

   Although no substitute for the "correct" definitions given in the
   appropriate ISO documents, here is a short summary of the terms used
   herein.






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      Application Context:
         The collection of application service elements which
         cooperatively interact within an application-entity.

      Application Service Element:
         A standardized mechanism, defined by both a service and a
         protocol, which provides a well-defined capability, e.g.,

         ROSE -  the Remote Operations Service Element,
                 which orchestrates the invocation of "total"
                 operations between application-entities [ISO9066/2].

         ACSE -  the Association Control Service Element,
                 which manages associations between application
                 entities [ISO8650].

      Object Identifier:
         An ordered set of integers, used for authoritative
         identification.

      Presentation Service:
         A set of facilities used to manage a connection between two
         application-entities.  The fundamental responsibility of the
         presentation service is to maintain transfer syntaxes which
         are used to serialize application protocol data units for
         transmission on the network and subsequent de-serialization
         for reception.

      Protocol Data Unit (PDU):
         A data object exchanged between service providers.

      Serialization:
         The process of applying an abstract transfer notation to an
         object described using abstract syntax notation one (ASN.1)
         [ISO8824] in order to produce a stream of octets.
         De-serialization is the inverse process.

         It is assumed that the reader is familiar with terminology
         pertaining to the reference model [ISO7498], to the service
         conventions in the model [ISO8509], and to the
         connection-oriented presentation service [ISO8822].

3. Scope

   The mechanism proposed by this memo is targeted for a particular
   class of OSI applications, namely those entities whose application
   context contains only an Association Control Service Element (ACSE)
   and a Remote Operations Service Element (ROSE).  In addition, a



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   Directory Services Element (DSE) is assumed for use by the
   application-entity, but only in a very limited sense.  The
   organization of such an entity is as follows:


      +------------------------------------------------------------+
      |                                                            |
      |                     Application-Entity                     |
      |                                                            |
      |    +------+              +------+              +------+    |
      |    | ACSE |              | ROSE |              | DSE  |    |
      |    +------+              +------+              +------+    |
      |                                                            |
      +------------------------------------------------------------+
      |                                                            |
      |                Presentation Services                       |
      |                                                            |
      |    P-CONNECT         P-RELEASE         P-DATA              |
      |                      P-U-ABORT                             |
      |                      P-P-ABORT                             |
      |                                                            |
      +------------------------------------------------------------+


   The mechanism proposed by this memo is not applicable to entities
   whose application context is more extensive (e.g., contains a
   Reliable Transfer Service Element).  The mechanism proposed by this
   memo could be modified to support additional elements.  However, such
   extensions would, at this time, merely serve to defeat the purpose of
   providing the minimal software infrastructure required to run the
   majority of OSI applications.

   The motivation for this memo was initially derived from a requirement
   to run the ISO Common Management Information Protocol (CMIP) in
   TCP/IP-based internets.  In its current definition, CMIP uses
   precisely the application service elements provided for herein.  It
   may be desirable to offer CMIP users a quality of service different
   than the one offered by a connection with a high-quality level of
   reliability.  This would permit a reduced utilization of connection-
   related resources.  This memo proposes a mechanism to implement this
   less robust -- and less costly -- quality of service.

4. Approach

   The approach proposed by this memo relies on the following
   architectural nuances:





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     - the TCP is a stream-oriented transport protocol

     - ASN.1 objects, when represented as a stream of octets are
       self-delimiting

     - The ISO presentation service permits the exchange of ASN.1
       objects

     - The ACSE and ROSE require the following presentation
       facilities:

           The Connection Establishment Facility

           The Connection Termination Facility

           The Information Transfer Facility (P-DATA
           service only)

     - The majority of the parameters used by the services which
       provide these facilities can be "hard-wired" to avoid
       negotiation

   In principle, these nuances suggest that a "cheap" emulation of the
   ISO presentation services could be implemented by simply serializing
   ASN.1 objects over a TCP connection.  This approach is precisely what
   is proposed by this memo.

   Given this perspective, this memo details how the essential features
   of the ISO presentation service may be maintained while using a
   protocol entirely different from the one given in [ISO8823]. The
   overall composition proposed by this memo is as follows:


   +-----------+                                       +-----------+
   |  PS-user  |                                       |  PS-user  |
   +-----------+                                       +-----------+
        |                                                     |
        | PS interface                           PS interface |
        |  [ISO8822]                                          |
        |                                                     |
   +----------+   ISO Presentation Services on the TCP  +----------+
   |  client  |-----------------------------------------|  server  |
   +----------+              (this memo)                +----------+
        |                                                     |
        | TCP interface                         TCP interface |
        |  [RFC793]                                           |
        |                                                     |




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   In greater detail, the "client" and "server" boxes implement the
   protocol described in this memo.  Each box contains three modules:

      - a dispatch module, which provides the presentation services
        interface,

      - a serialization module, containing a serializer, which takes
        an ASN.1 object and applies the encoding rules of [ISO8825]
        to produce a stream of octets, and a de-serializer, which
        performs the inverse operation, and

      - a network module, which manages a TCP connection.

   The software architecture used to model a network entity using this
   approach is as follows:


   +---------+    +----------+                                   +-----+
   |         |    |          |  output +---------------+  input  |  n  |
   |         |    |          |<--------| de-serializer |<--------|  e  |
   |         |    |          |   queue +---------------+  queue  |  t  |
   | PS-user |----| dispatch |                                   |  w  |
   |         |    |          |  input  +---------------+ output  |  o  |
   |         |    |          |-------->|   serializer  |-------->|  r  |
   |         |    |          |  queue  +---------------+ queue   |  k  |
   +---------+    +----------+                                   +-----+

                                 |---- serialization module ----|


   The ISO presentation layer is concerned primarily with the
   negotiation of transfer syntaxes in addition to the transformation to
   and from transfer syntax.  However, using the mechanism proposed by
   this memo, no negotiation component will be employed.  This memo
   specifies the fixed contexts which exist over each presentation
   connection offered.  This memo further specifies other constants
   which are used in order to eliminate the need for presentation layer
   negotiation.

5. Fundamental Parameters

   There are certain parameters which are used by the presentation
   service and are defined here.

      1. Presentation address:

      The structure of a presentation address is presented in Addendum 3
      to [ISO7498].  This memo interprets a presentation address as an



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      ordered-tuple containing:

         - one or more network addresses
         - a transport selector
         - a session selector
         - a presentation selector

      Each selector is an uninterpreted octet string of possibly zero
      length.  The mechanism proposed in this memo completely ignores
      the values of these selectors.  Note however that the value of the
      presentation selector is preserved by the provider.

      A network address is interpreted as containing three components:

         - a 32-bit IP address

         - a set indicating which transport services are available
           at the IP address  (currently only two members are defined:
           TCP and UDP; as experience is gained, other transport
           services may be added); as a local matter, if a member is
           present it may have an "intensity" associated with it:
           either "possibly present" or "definitely present"

         - a 16-bit port number

      As a consequence of these interpretations, any application-entity
      residing in the network can be identified by its network address.

      2. Presentation context list

      A list of one or more presentation contexts.  Each presentation
      context has three components:

         - a presentation context identifier (PCI), an integer

         - an abstract syntax name, an object identifier

         - an abstract transfer name, an object identifier

      The range of values these components may take is severely
      restricted by this memo.  In particular, exactly two contexts are
      defined: one for association control and the other for the
      specific application service element which is being carried as ROS
      APDUs (see the section on connection establishment for the precise
      values).

      In addition, if the presentation context list appears in a
      "result" list (e.g., the Presentation context result list



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      parameter for the P-CONNECT service), a fourth component is
      present:

         - an acceptance indicator

      which indicates if the context was accepted by both the service
      provider and the remote peer.  If the context was not accept, a
      brief reason, such as "abstract syntax not supported" is given.

      For the novice reader, one might think of the abstract syntax
      notation as defining the vocabulary of some language, that is, it
      lists the words which can be spoken.  In contrast, the abstract
      transfer notation defines the pronunciation of the language.

      3. User data

      User data passes through the presentation service interface as