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


                       <span class="h1">ISO Presentation Services</span>
                    <span class="h1">on top of TCP/IP-based internets</span>

Status of this Memo

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

<span class="h2"><a class="selflink" id="section-1" href="#section-1">1</a>. Introduction</span>

   [<a id="ref-RFC1006">RFC1006</a>] describes a mechanism for providing the ISO transport
   service on top of the Transmission Control Protocol (TCP) [<a href="./rfc793" title="&quot;Transmission Control Protocol - DARPA Internet Program Protocol Specification&quot;">RFC793</a>]
   and Internet Protocol (IP) [<a href="./rfc791" title="&quot;Internet Protocol - DARPA Internet Program Protocol Specification&quot;">RFC791</a>].  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.

<span class="h2"><a class="selflink" id="section-2" href="#section-2">2</a>. Terminology</span>

   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 [<a href="#ref-ISO8650" title="&quot; Protocol Specification for the Association Control Service Element (Final Text of DIS 8650)&quot;">ISO8650</a>].

      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)
         [<a href="#ref-ISO8824" title="&quot; Specification of Abstract Syntax Notation One (ASN.1)&quot;">ISO8824</a>] 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 [<a href="#ref-ISO7498" title="&quot;Basic Reference Model&quot;">ISO7498</a>], to the service
         conventions in the model [<a href="#ref-ISO8509" title="&quot; Service Conventions&quot;">ISO8509</a>], and to the
         connection-oriented presentation service [<a href="#ref-ISO8822" title="&quot; Connection Oriented Presentation Service Definition (Final Text of DIS 8822)&quot;">ISO8822</a>].

<span class="h2"><a class="selflink" id="section-3" href="#section-3">3</a>. Scope</span>

   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.

<span class="h2"><a class="selflink" id="section-4" href="#section-4">4</a>. Approach</span>

   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 [<a href="#ref-ISO8823" title="&quot; Connection Oriented Presentation Protocol Specification (Final Text of DIS 8822)&quot;">ISO8823</a>]. The
   overall composition proposed by this memo is as follows:


   +-----------+                                       +-----------+
   |  PS-user  |                                       |  PS-user  |
   +-----------+                                       +-----------+
        |                                                     |
        | PS interface                           PS interface |
        |  [<a href="#ref-ISO8822" title="&quot; Connection Oriented Presentation Service Definition (Final Text of DIS 8822)&quot;">ISO8822</a>]                                          |
        |                                                     |
   +----------+   ISO Presentation Services on the TCP  +----------+
   |  client  |-----------------------------------------|  server  |
   +----------+              (this memo)                +----------+
        |                                                     |
        | TCP interface                         TCP interface |
        |  [<a href="./rfc793" title="&quot;Transmission Control Protocol - DARPA Internet Program Protocol Specification&quot;">RFC793</a>]                                           |
        |                                                     |




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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 [<a href="#ref-ISO8825" title="&quot;Specification of basic encoding rules for Abstract Syntax Notation One (ASN.1)&quot;">ISO8825</a>]
        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  |
   |         |    |          |&lt;--------| de-serializer |&lt;--------|  e  |
   |         |    |          |   queue +---------------+  queue  |  t  |
   | PS-user |----| dispatch |                                   |  w  |
   |         |    |          |  input  +---------------+ output  |  o  |
   |         |    |          |--------&gt;|   serializer  |--------&gt;|  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.

<span class="h2"><a class="selflink" id="section-5" href="#section-5">5</a>. Fundamental Parameters</span>

   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 [<a href="#ref-ISO7498" title="&quot;Basic Reference Model&quot;">ISO7498</a>].  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
      ASN.1 objects (in a locally defined form).  Associated with each
      object is a presentation context identifier.  The PCI
      distinguishes the context for which the data is intended.  The
      range of values the PCI may take is severely restricted by this
      memo.  Exactly one of two contexts must always be used: either the
      value for the ACSE presentation context or the value for the ROSE.

      4. Quality of Service

      Quality of service is a collection of "elements".  Each element
      denotes some characteristics of the communication, e.g., desired
      throughput, and some value in an arbitrary unit of measure.  For
      our purposes, only one quality of service element is interpreted,
      "transport-mapping".  Currently, the "transport-mapping" element
      takes on one of two values: "tcp-based" or "udp-based".  At
      present, the two values may also be referred to as "high-quality"
      or "low-quality", respectively.

      As experience is gained, other values may be added.  These values
      would correspond directly to the new transport services which are
      listed in the network address.

      5. Version of Session Service

      Some application service elements (e.g., the ACSE) invoke
      different procedures based on the (negotiated) version of the
      session service available.  Implementations of this memo always
      indicate that session service version 2 has been negotiated.






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<span class="h2"><a class="selflink" id="section-6" href="#section-6">6</a>. Choice of Transport Service</span>

   Discussion thus far has centered along the use of the TCP as the
   underlying transport protocol.  However, it has also been noted that
   it may be desirable to permit a quality of service with less
   reliability in order to take advantage of some other characteristic
   of the transport service.

   The introduction of this service has several profound impacts on the
   model, and it is beyond the scope of this memo to enumerate these
   impacts.  However, this memo does propose a mechanism by which such a
   facility is implemented.

   To begin, we use the quality of service parameter for the P-CONNECT
   service to select an underlying transport service.  Only one element
   is currently interpreted, "transport-mapping" which takes the value
   "tcp-based" or "udp-based".  If the value is "tcp-based", then the
   presentation provider will use TCP as the underlying transport
   service. If, however, the value of "transport-mapping" is "udp-
   based", then the presentation provider will use the UDP instead.

   The User Datagram Protocol (UDP) [<a href="./rfc768" title="&quot;User Datagram Protocol&quot;">RFC768</a>] is used to implement the
   udp-based service.  Very few transport-level facilities are placed on
   top of the UDP service, i.e., it is not the intent of this memo to
   "re-invent" the facilities in the TCP.  Hence, It is critical to
   understand that

           low-quality means LOW-QUALITY!

   Because the UDP is a packet-oriented protocol, it is necessary to
   slightly redefine the role of the serialization module.  For the
   serializer, we say that each top-level ASN.1 object placed on the
   input queue will form a single UDP datagram on the output queue which
   is given to the network.  Similarly, for the de-serializer, we say
   that each UDP datagram placed on the input queue from the network
   will form a single top-level ASN.1 object placed on the output queue.
   The term "top-level ASN.1 object" refers, of course, to the protocol
   data units being exchanged by the presentation providers.

   It should be noted that in its current incarnation, this memo permits
   the choice of two different transport protocols, e.g., the TCP or the
   UDP.  However, as experience is gained and as other transport
   protocols are deployed (e.g., the VMTP), then future incarnations of
   this memo will permit these transport protocols to be used.  This is
   a three step process: first, the set of transport services defined
   for the network address is updated; second, a corresponding value is
   added to the range of the quality of service element "transport-
   mapping"; and, third, the following sections of this memo are



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   modified accordingly.

<span class="h2"><a class="selflink" id="section-7" href="#section-7">7</a>. Connection Establishment</span>

   The Connection Establishment facility consists of one service, the
   P-CONNECT service.

<span class="h3"><a class="selflink" id="section-7.1" href="#section-7.1">7.1</a>. The P-CONNECT Service</span>

   This service is used to bring two identified application-entities
   into communication.  Its successful use results in a presentation
   connection, with an initial defined context set, being established
   between then.  This connection is available for their subsequent
   communication.  This is a confirmed service whose effects are
   sequenced and non-destructive.

   If the udp-based service is selected, then a presentation connection
   is formed which should be used infrequently and will have minimal
   reliability characteristics.

   For our purposes, the P-CONNECT service:

      - requests TCP or UDP resources,

      - builds a fixed defined context set, and

      - exchanges initial user data.

   Following are the interpretation of and the defaults assigned to the
   parameters of the P-CONNECT service:

      1. Calling Presentation Address

        This is a presentation address.  Although the ISO presentation
        service states that this parameter is mandatory, in practice, a
        local implementation rule may be used to determine an
        "ephemeral" address to use.

      2. Called Presentation Address

        This is a presentation address.  Note that when issuing the P-
        CONNECT.REQUEST primitive, this parameter may contain more than
        one network address.  In the P-CONNECT.INDICATION primitive
        however, only one network address, the one actually used to
        establish the presentation connection, is present.  (Appendix C
        describes a strategy which might be used to determine the actual
        network address).




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      3. Responding Presentation Address

        This parameter is identical to the value of the Called
        Presentation Address parameter of the P-CONNECT.INDICATION
        primitive.

      4. Multiple defined Contexts

        Always TRUE.  Note that this parameter is present only in the
        DIS version of the presentation service.

      5. Presentation context definition list

      Two contexts are defined:

      PCI     Abstract Syntax Name            Abstract Transfer Name
      ---     --------------------            ----------------------
       1      specific to the application     "iso asn.1 abstract
                                              transfer"
                                              1.0.8825

       3      "acse pci version 1"            "iso asn.1 abstract
                                              transfer"
              2.2.1.0.0                       1.0.8825

      The abstract syntax and transfer names for the ACSE PCI are for
      use with the DIS version of association control.  If the IS
      version is being used, then this PCI is used instead:

       3      "acse pci version 1"            "asn.1 basic encoding"
              2.2.1.0.1                       2.1.1

      6. Presentation context result list

        Identical to the Presentation context definition list with the
        addition that the acceptance indicator for both contexts is
        "accepted".

      7. Default Context Name

        None.

      8. Default Context Result

        Not applicable.






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      9. Quality of Service

        The element "transport-mapping" takes the value "tcp-based" or
        "udp-based".  In the future the range of values may be extended.

      10. Presentation Requirements

        None (the kernel functional unit is always used).

      11. Session Requirements

        Full duplex.

      12. Initial synchronization point serial number

        None.

      13. Initial Assignment of tokens

        None.

      14. Session connection identifier

        Unlike the "real" presentation service, depending on the quality
        of service selected, this parameter may have great significance
        to presentation provider.  Hence, the following format of the
        session connection identifier is mandated by this memo.

        user data:        a local string encoded as a T.61 string
                          using ASN.1, e.g., given string "gonzo":

                          14     05     67   6f   6e   7a   6f
                          tag  length   "g"  "o"  "n"  "z"  "o"

        common data:      a universal time encoding using ASN.1, e.g.,
                          given time "880109170845":

                          17     0c     38   38   30   31   30   ...
                          tag  length   "8"  "8"  "0"  "1"  "0"  ...

        additional data:  any string encoded as a T.61 string using ASN.1
                          (optional)

        As a local convention, the presentation provider may disregard
        the first two octets of each data component for transmission on
        the network as when the session connection identifier is
        represented with ASN.1, the tag and length octets will be added
        anyway.



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      15. User Data

        A single ASN.1 object is present, the appropriate A-ASSOCIATE
        PDU, carried in presentation context 3.

      16. Result

        One of the following values: acceptance, user-rejection,
        provider-rejection (transient), or provider-rejection
        (permanent).

<span class="h2"><a class="selflink" id="section-8" href="#section-8">8</a>. Connection Termination</span>

   The Connection Termination facility consists of three services, the
   P-RELEASE, P-U-ABORT, and P-P-ABORT services.

<span class="h3"><a class="selflink" id="section-8.1" href="#section-8.1">8.1</a>. The P-RELEASE Service</span>

   This service provides the service user with access to a negotiated
   release facility.  This service has effects which are sequenced and
   non-destructive.  Either presentation user is permitted to request
   this service.  However, in the event of collision, a provider-
   initiated abort procedure will be invoked.

   If the udp-based service is selected, then any data in transit may be
   discarded.

      For our purposes, the P-RELEASE service:

      - waits for the serialization module to drain,

      - sends release user data, and

      - releases TCP or UDP resources

   Following are the interpretation of and the defaults assigned to the
   parameters of the P-RELEASE service:

      1. Result

        Release accepted.

      2. User data

        A single ASN.1 object is present, the appropriate A-RELEASE PDU,






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<span class="h3"><a class="selflink" id="section-8.2" href="#section-8.2">8.2</a>. The P-U-ABORT Service</span>

   This service can be used by either presentation user to force the
   release of a presentation connection at any time and have the
   correspondent presentation user informed of this termination.  This
   service has effects which are not sequenced with respect to preceding
   service invocations and may be destructive.  It does not require the
   agreement of both service users.

      For our purposes, the P-U-ABORT service:

      - flushes the serialization module,

      - sends abort user data, and

      - releases TCP or UDP resources

   Following are the interpretation of and the defaults assigned to the
   parameters of the P-U-ABORT service:

      1. Presentation context identifier list

        Contained in the ASN.1 objects, if any, that are delivered as
        user data.

      2. User data

        A single ASN.1 object is present, an A-ABORT PDU, carried in
        presentation context 3.

<span class="h3"><a class="selflink" id="section-8.3" href="#section-8.3">8.3</a>. The P-P-ABORT Service</span>

   This service is the means by which the service provider may indicate
   the termination of the presentation connection for reasons internal
   to the service provider.  This service has effects which are not
   sequenced with respect to preceding service invocations.  The
   execution of this service disrupts any other concurrently active
   service and may thus be destructive.

      For our purposes, the P-P-ABORT service:

      - flushes the serialization module, and

      - releases TCP or UDP resources

   Following are the interpretation of and the defaults assigned to the
   parameters of the P-P-ABORT service.




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      1. Provider reason

        An integer code detailing why the connection was aborted. Codes
        include, but are not limited to: invalid PPDU parameter,
        unexpected PPDU, unrecognized PPDU, and specified reason.

      2. Abort data

        None.

<span class="h2"><a class="selflink" id="section-9" href="#section-9">9</a>. Information Transfer</span>

   Although the Information Transfer facility consists of many services,
   only one, the P-DATA service, is provided by this memo.

<span class="h3"><a class="selflink" id="section-9.1" href="#section-9.1">9.1</a>. The P-DATA Service</span>

   This services provides the service user with a data transfer
   capability.  This service has effects which are sequenced and non-
   destructive.

   If the udp-based service is selected, then there is an upper-bound on
   the size of the serialized ASN.1 objects which may be transmitted.
   This limit, imposed by the UDP, is 65536 octets.  As a practical
   matter, it is probably a good idea to keep datagrams less than or
   equal to 536 octets in size.

   For our purposes, the P-DATA service:

              - sends user data

   Following are the interpretation of and the defaults assigned to the
   parameters of the P-DATA service:

      1. User data

        A single ASN.1 object is present, a remote operations APDU,
        carried in presentation context 1.

<span class="h2"><a class="selflink" id="section-10" href="#section-10">10</a>. Elements of Procedure</span>

   The service provider is in one of the following states:

           IDLE, WAIT1, WAIT2, DATA, WAIT3, or WAIT4

        The possible events are:

           PS-user         P-CONNECT.REQUEST



<span class="grey">Rose                                                           [Page 14]</span></pre>
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                           P-CONNECT.RESPONSE
                           P-RELEASE.REQUEST
                           P-RELEASE.RESPONSE
                           P-DATA.REQUEST
                           P-U-ABORT.REQUEST

           network         TCP closed or errored(*)
                           receive ConnectRequest PDU
                           receive ConnectResponse PDU
                           receive ReleaseRequest PDU
                           receive ReleaseResponse PDU
                           receive UserData(*) or CL-UserData(**) PDU
                           receive user-initiated Abort PDU
                           receive provider-initiated Abort PDU
                           timer expires(**)


        The possible actions are:

           PS-user         P-CONNECT.INDICATION
                           P-CONNECT.CONFIRMATION
                           P-RELEASE.INDICATION
                           P-RELEASE.CONFIRMATION
                           P-DATA.INDICATION
                           P-U-ABORT.INDICATION
                           P-P-ABORT.INDICATION

           network         open TCP(*)
                           close TCP(*)
                           send ConnectRequest PDU
                           send ConnectResponse PDU
                           send ReleaseRequest PDU
                           send ReleaseResponse PDU
                           send UserData(*) or CL-UserData(**) PDU
                           send user-initiated Abort PDU
                           send provider-initiated Abort PDU
                           set timer(**)

           (*)   tcp-based service only
           (**)  udp-based service only

<span class="h3"><a class="selflink" id="section-10.1" href="#section-10.1">10.1</a>. Elements of Procedure specific to the tcp-based service</span>

   The provider maintains the following information for each
   presentation connection:

      - a local designator for the PS-user




<span class="grey">Rose                                                           [Page 15]</span></pre>
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      - a local designator for a TCP connection

      - the state of the connection (e.g., IDLE, WAIT1, and so on)

   Upon receiving an event from the network, the provider finds the
   associated presentation connection.  Matching is done by simply
   comparing local designators for the TCP connection.  Whenever a
   connection remains in or returns to the IDLE state, any associated
   resources, such as an attachment to a local TCP port, are released.

   In the procedures which follow, outgoing PDUs are "placed on the
   input queue for the serializer".  This has a different meaning
   depending on the type of PDU being enqueued.  If the PDU is not an
   abort PDU (user-initiated or provider-initiated), then the PDU is
   simply appended to the input queue regardless of the number of PDUs
   present.  If however, the PDU is an abort PDU, then the provider
   checks the size of the input queue.  If the input queue is non-empty
   or if the serializer is busy transmitting to the network, then the
   abort PDU is discarded, and the serializer is flushed, aborting any
   output to the network in progress.  However, if the input queue is
   empty, then the Abort PDU is appended to the queue, and a small timer
   started.  If the timer expires before the PDU has been serialized and
   transmitted, then the serializer is flushed, aborting any output to
   the network in progress.

   Further, in general, whenever the TCP connection is closed (either
   locally by the provider, or remotely by the network) or has errored,
   the serializer is flushed.  The one exception to this is if a
   ReleaseResponse PDU is being serialized and transmitted to the
   network.  In this case, the provider will not close the TCP
   connection until after the serializer has finished.

<span class="h3"><a class="selflink" id="section-10.2" href="#section-10.2">10.2</a>. Elements of Procedure specific to the udp-based service</span>

   The provider maintains the following information for each
   presentation connection:

      - a local designator for the PS-user

      - the 32-bit IP address and 16-bit UDP port number of the
        initiating host

      - the 32-bit IP address and 16-bit UDP port number of the
        responding host

      - the session connection identifier used to establish the
        presentation connection




<span class="grey">Rose                                                           [Page 16]</span></pre>
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      - a local designator for an UDP endpoint

      - the state of the connection (e.g., IDLE, WAIT1, and so on)

      - a retransmission counter

   Upon receiving an event from the network, the provider finds the
   associated presentation connection.  Matching is done on the basis of
   addresses, ports, and the session connection identifier (i.e., two
   different presentation connections may differ only in their session
   connection identifier).  If no presentation connection can be found,
   then for the purposes of discussion, it may be assumed that a
   "vanilla" presentation connection is created and initialized to the
   IDLE state.  Further, whenever a connection remains in or returns to
   the IDLE state, any associated resources, such as an attachment to a
   local UDP port, are released.

   In the procedures which follow, outgoing PDUs are "placed on the
   input queue for the serializer".  This means that the ASN.1 object is
   serialized and the resulting sequence of octets is sent as a single
   UDP datagram.

<span class="h3"><a class="selflink" id="section-10.3" href="#section-10.3">10.3</a>. State Transitions</span>

   Following are the rules for transitioning states.  If an event
   associated with a user-generated primitive is omitted, then it is an
   interface error for the user to issue that primitive in the given
   state.  Each state considers all possible incoming PDUs.

   We assume that for the tcp-based service, that some entity starts a
   passive TCP open.  When the passive open completes, the entity, using
   some local rule, locates a PS-user to be associated with the incoming
   presentation connection.  This presentation connection is then placed
   in the IDLE state.  The entity then continues listening for other
   passive opens to complete.  The mechanisms associated with this
   entity are entirely a local matter, the concept of this listener is
   introduced solely as a modeling artifact.

   Finally, if the udp-based service is selected, then CL-UserData PDUs
   are exchanged by the provider instead of UserData PDUs.


                                    IDLE state

        Event:     P-CONNECT.REQUEST primitive issued

   Based on the quality of service parameter and the list of network
   addresses in the called presentation address parameter, the provider



<span class="grey">Rose                                                           [Page 17]</span></pre>
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   selects an address for the use of the presentation connection.  The
   method for making this determination is a local matter.  (Appendix C
   discusses a strategy which might be used.)  For the discussion that
   follows, we assume that a network address supporting the desired
   quality of service has been determined.

   Based on the network address chosen from the called presentation
   address parameter, the provider selects a compatible network address
   from the calling presentation address parameter.  The provider
   attaches itself to the port associated with this network address.
   (By local determination, this address need not be used, and an
   "ephemeral" port may be chosen by the provider.)

   For the tcp-based service, the provider attempts to establish a TCP
   connection to the network address listed in the called presentation
   address.  If the connection can not be established, the P-
   CONNECT.CONFIRMATION(-) primitive is issued with a reason of
   provider-rejection, and the provider remains in the IDLE state.

   Regardless, the user data parameter is placed in a ConnectRequest
   PDU, which is put on the input queue for the serializer.

   For the udp-based service, the provider sets the retransmission
   counter to a small value (e.g., 2), and now starts a small timer.

   Regardless, the provider enters the WAIT1 state.


        Event:     ConnectRequest PDU received

   The provider issues the P-CONNECT.INDICATION primitive and enters the
   WAIT2 state.


        Event:     any other PDU received

   If the PDU is not an Abort PDU, the provider constructs a provider-
   initiated Abort PDU, which is put on the input queue for the
   serializer.  Regardless, the provider remains in the IDLE state.


                                    WAIT1 state

        Event:     P-U-ABORT.REQUEST primitive issued

   The user data parameter is placed in an Abort PDU, which is put on
   the input queue for the serializer.  The provider enters the IDLE
   state.



<span class="grey">Rose                                                           [Page 18]</span></pre>
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        Event:     ConnectResponse PDU received

   For the udp-based service, the timer is cancelled.  If the PDU
   indicates rejection, the P-CONNECT.CONFIRMATION(-) primitive is
   issued and the provider enters the IDLE state.  Otherwise, the P-
   CONNECT.CONFIRMATION(+) primitive is issued and the provider enters
   the DATA state.


        Event:     user-initiated Abort PDU received

   The provider issues the P-U-ABORT.INDICATION primitive and enters the
   IDLE state.


        Event:     any other PDU received

   If the PDU not an Abort PDU, the provider constructs a provider-
   initiated Abort PDU, which is put on the input queue for the
   serializer.  Regardless, The provider issues the P-P-ABORT.INDICATION
   primitive and enters the the IDLE state.


        Event:     timer expires

   The provider decrements the retransmission counter.  If the resulting
   value is less than or equal to zero, the provider issues the P-
   CONNECT.CONFIRMATION(-) primitive and enters the IDLE state.
   Otherwise, a ConnectRequest PDU is put on the input queue for the
   serializer, the small timer is started again, and the provider
   remains in the WAIT1 state.


                                    WAIT2 state

        Event:     P-CONNECT.RESPONSE primitive issued

   The user data parameter is placed in a ConnectResponse PDU, which is
   put on the input queue for the serializer.  If the result parameter
   had the value user-rejection, the provider enters the IDLE state.
   Otherwise if the parameter had the value acceptance, the provider
   enters the DATA state.









<span class="grey">Rose                                                           [Page 19]</span></pre>
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        Event:     P-U-ABORT.REQUEST primitive issued

   The user data parameter is placed in an Abort PDU, which is put on
   the input queue for the serializer.  The provider enters the IDLE
   state.


        Event:     user-initiated Abort PDU received

   The provider issues the P-U-ABORT.INDICATION primitive and enters the
   IDLE state.


        Event:     any other PDU received

   If the PDU is not an Abort PDU, the provider constructs a provider-
   initiated Abort PDU, which is put on the input queue for the
   serializer.  Regardless, The provider issues the P-P-ABORT.INDICATION
   primitive and enters the the IDLE state.


                                    DATA state

        Event:     P-DATA.REQUEST primitive issued

   The user data parameter is placed in a UserData PDU, which is put on
   the input queue for the serializer.  The provider remains in the DATA
   state.


        Event:     P-RELEASE.REQUEST primitive issued

   The user data parameter is placed in a ReleaseRequest PDU, which is
   put on the input queue for the serializer.

   For the udp-based service, the provider sets the retransmission
   counter to a small value (e.g., 2), and now starts a small timer.

   Regardless, the provider enters the WAIT3 state.


        Event:     P-U-ABORT.REQUEST primitive issued

   The user data parameter is placed in an Abort PDU, which is put on
   the input queue for the serializer.  The provider enters the IDLE
   state.





<span class="grey">Rose                                                           [Page 20]</span></pre>
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        Event:     UserData PDU received

   The provider issues the P-DATA.INDICATION primitive and remains in
   the DATA state.


        Event:     ReleaseRequest PDU received

   The provider issues the P-RELEASE.INDICATION primitive, and enters
   the WAIT4 state.


        Event:     user-initiated Abort PDU received

   The provider issues the P-U-ABORT.INDICATION primitive and enters
    the IDLE state.


        Event:     any other PDU received

   If the PDU is not an Abort PDU, the provider constructs a provider-
   initiated Abort PDU, which is put on the input queue for the
   serializer.  Regardless, the provider issues the P-P-ABORT.INDICATION
   primitive and enters the the IDLE state.


                                    WAIT3 state

        Event:     P-U-ABORT.REQUEST primitive issued

   The user data parameter is placed in an Abort PDU, which is put on
   the input queue for the serializer.  The provider enters the IDLE
   state.


        Event:     ReleaseResponse PDU received

   For the udp-based service, the timer is cancelled.  The provider
   issues the P-RELEASE.CONFIRMATION primitive and enters the IDLE
   state.


        Event:     user-initiated Abort PDU received

   The provider issues the P-U-ABORT.INDICATION primitive and enters the
   IDLE state.





<span class="grey">Rose                                                           [Page 21]</span></pre>
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        Event:     any other PDU received

   If the PDU is not an Abort PDU, the provider constructs a provider-
   initiated Abort PDU, which is put on the input queue for the
   serializer.  Regardless, the provider issues the P-P-ABORT.INDICATION
   primitive and enters the the IDLE state.


        Event:     timer expires

   The provider decrements the retransmission counter.  If the resulting
   value is less than or equal to zero, the provider constructs a
   provider-initiated Abort PDU, which is put on the input queue for the
   serializer.  It then issues the P-P-ABORT.INDICATION primitive and
   enters the IDLE state.  Otherwise, a ReleaseRequest PDU is put on the
   input queue for the serializer, the small timer is started again, and
   the provider remains in the WAIT3 state.


                                    WAIT4 state

        Event:     P-RELEASE.RESPONSE primitive issued

   The user data parameter is placed in a ReleaseResponse PDU, which is
   put on the input queue for the serializer.  The provider now enters
   the IDLE state.

        Event:     P-U-ABORT.REQUEST primitive issued

   The user data parameter is placed in an Abort PDU, which is put on
   the input queue for the serializer.  The provider now enters the IDLE
   state.


        Event:     user-initiated Abort PDU received

   The provider issues the P-U-ABORT.INDICATION primitive and enters the
   IDLE state.


        Event:     any other PDU received

   If the PDU is not an Abort PDU, the provider constructs a provider-
   initiated Abort PDU, which is put on the input queue for the
   serializer.  Regardless, the provider issues the P-P-ABORT.INDICATION
   primitive and enters the the IDLE state.





<span class="grey">Rose                                                           [Page 22]</span></pre>
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<span class="h2"><a class="selflink" id="section-11" href="#section-11">11</a>. Directory Services</span>

   Although not properly part of the presentation service, this memo
   assumes and specifies a minimal Directory service capability for use
   by the application-entity.

   The function of the Directory Service Element is to provide two
   mappings: first, a service name is mapped into an application entity
   title, which is a global handle on the service; and, second, the
   application-entity title is mapped onto a presentation address.

   The structure of presentation addresses were defined in <a href="#section-5">Section 5</a>.

   The structure of application-entity titles is less solidly agreed
   upon at the present time.  Since objects of this type are not
   interpreted by the presentation service, this memo does not specify
   their structure.  If the DIS version of association control is being
   used, then use of an OBJECT IDENTIFIER will suffice.  If the IS
   version is being employed, then application-entity titles consist of
   two parts: an application-process title and an application-entity
   qualifier.  It is suggested that the AP-Title use an OBJECT
   IDENTIFIER and that the AE-Qualifier use NULL.

   This memo requires the following mapping rules:

      1.  The service name for an OSI application-entity using the
      mechanisms proposed by this memo is:

              &lt;designator&gt; "-" &lt;qualifier&gt;

      where &lt;designator&gt; is a string denoting either domain name or a
      32-bit IP address, and &lt;qualifier&gt; is a string denoting the type
      of application-entity desired, e.g.,

              "gonzo.twg.com-mgmtinfobase"

      2.  Any locally defined mapping rules may be used to map the
      service designation into an application-entity title.

      3.  The application-entity title is then mapped into a
      presentation address, with uninterpreted transport, session, and
      presentation selectors, and one or more network addresses, each
      containing:

         -the 32-bit IP address resolved from the &lt;designator&gt; portion
          of the service name,

         - a set indicating which transport services are available



<span class="grey">Rose                                                           [Page 23]</span></pre>
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           at the IP address,

         - the 16-bit port number resolved from the &lt;qualifier&gt;
           portion of the service name (using the Assigned Numbers
           document), and

         - optionally, a presentation selector, which is an
           uninterpreted sequence of octets.

   The method by which the mappings are obtained are straight-forward.
   The directory services element employs the Domain Name System along
   with a local table which may be used to resolve the address employing
   local rules.

   In the simplest of implementations, the DNS is used to map the
   &lt;designator&gt; to an IP address, and to fill-in the set of transport
   services available at the IP address.  The port number is found in a
   local table derived from the current Assigned Numbers document.
   Finally, the presentation selector is empty.

   A more ambitious implementation would use a local table to perhaps
   provide a presentation selector.  This would be useful, e.g., in
   "proxy" connections.  The network address would resolve to the proxy
   agent for the non-IP device, and the presentation selector would
   indicate to the proxy agent the particular non-IP device desired.
   This implies, of course, that the local table and the proxy agent
   bilaterally agree as to the interpretation of each presentation
   selector.

<span class="h2"><a class="selflink" id="section-12" href="#section-12">12</a>. Remarks</span>

   To begin, if one really wanted to implement ISO applications in a
   TCP/IP-based network, then the method proposed by [<a href="./rfc1006" title="&quot;ISO Transport 1 on Top of the TCP Version: 3&quot;">RFC1006</a>] is the
   preferred method for achieving this.  However, in a constrained
   environment, where it is necessary to host an application layer
   entity with a minimal amount of underlying OSI infrastructure, this
   memo proposes an alternative mechanism.  It should be noted that an
   OSI application realized using this approach can be moved directly to
   an [<a href="./rfc1006" title="&quot;ISO Transport 1 on Top of the TCP Version: 3&quot;">RFC1006</a>]-based environment with no modifications.

   A key motivation therefore is to minimize the size of the alternate
   underling infrastructure specified by this memo.  As more and more
   presentation services functionality is added, the method proposed
   herein would begin to approximate the ISO presentation protocol.
   Since this in contrary to the key motivation, featurism must be
   avoided at all costs.





<span class="grey">Rose                                                           [Page 24]</span></pre>
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<span class="h2"><a class="selflink" id="section-13" href="#section-13">13</a>. Acknowledgements</span>

   Several individuals contributed to the technical quality of this
   memo:

           Karl Auerbach, Epilogue Technologies
           Joseph Bannister, Unisys
           Amatzia Ben-Artzi, Sytek
           Stephen Dunford, Unisys
           Lee Labarre, MITRE
           Keith McCloghrie, The Wollongong Group
           Jim Robertson, Bridge Communications
           Glenn Trewitt, Stanford University

<span class="h2"><a class="selflink" id="section-14" href="#section-14">14</a>. References</span>

     [<a id="ref-ISO7498">ISO7498</a>]  Information Processing Systems - Open Systems
                Interconnection, "Basic Reference Model", October, 1984.

     [<a id="ref-ISO8509">ISO8509</a>]  Information Processing Systems - Open Systems
                Interconnection, " Service Conventions".

     [<a id="ref-ISO8650">ISO8650</a>]  Information Processing Systems - Open Systems
                Interconnection, " Protocol Specification for the
                Association Control Service Element (Final Text
                of DIS 8650)", January, 1988.

     [<a id="ref-ISO8822">ISO8822</a>]  Information Processing Systems - Open Systems
                Interconnection, " Connection Oriented Presentation
                Service Definition (Final Text of DIS 8822)",
                April, 1988.

     [<a id="ref-ISO8823">ISO8823</a>]  Information Processing Systems - Open Systems
                Interconnection, " Connection Oriented Presentation
                Protocol Specification (Final Text of DIS 8822)",
                April, 1988.

     [<a id="ref-ISO8824">ISO8824</a>]  Information Processing Systems - Open Systems
                Interconnection, " Specification of Abstract Syntax
                Notation One (ASN.1)", December, 1987.

     [<a id="ref-ISO8825">ISO8825</a>]  Information Processing Systems - Open Systems
                Interconnection, "Specification of basic encoding rules
                for Abstract Syntax Notation One (ASN.1)",
                December, 1987.

     [ISO9072/2]  Information Processing Systems - Text Communication
                  MOTIS, " Remote Operations Part 2: Protocol



<span class="grey">Rose                                                           [Page 25]</span></pre>
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                  Specification (Working Document for DIS 9072/2)",
                  November, 1987.

     [<a id="ref-RFC768">RFC768</a>]  Postel, J., "User Datagram Protocol", <a href="./rfc768">RFC 768</a>, USC/ISI,
               28 August 1980.

     [<a id="ref-RFC791">RFC791</a>]  Postel, J., "Internet Protocol - DARPA Internet Program
               Protocol Specification", <a href="./rfc791">RFC 791</a>, USC/ISI,
               September 1981.

     [<a id="ref-RFC793">RFC793</a>]  Postel, J., "Transmission Control Protocol - DARPA
               Internet Program Protocol Specification", <a href="./rfc793">RFC 793</a>,
               USC/ISI, September 1981.

     [<a id="ref-RFC1006">RFC1006</a>]  Rose, M., and D. Cass, "ISO Transport 1 on Top of the
                TCP Version: 3", Northrop Research and Technology
                Center, May 1987.

Appendix A:

Abstract Syntax Definitions

   <a href="./rfc1085">RFC1085</a>-PS DEFINITIONS ::=

   BEGIN

   PDUs ::=
           CHOICE {
               connectRequest
                   ConnectRequest-PDU,

               connectResponse
                   ConnectResponse-PDU,

               releaseRequest
                   ReleaseRequest-PDU,

               releaseResponse
                   ReleaseResponse-PDU,

               abort
                   Abort-PDU,

               userData
                   UserData-PDU,

               cL-userData
                   CL-UserData-PDU



<span class="grey">Rose                                                           [Page 26]</span></pre>
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           }



   -- connect request PDU

   ConnectRequest-PDU ::=
       [0]
           IMPLICIT SEQUENCE {
               version[0]          -- version-1 corresponds to to this
                                      memo
                   IMPLICIT INTEGER { version-1(0) },

               reference
                   SessionConnectionIdentifier,

               calling
                   PresentationSelector
                   OPTIONAL,

               called[2]
                   IMPLICIT PresentationSelector
                   OPTIONAL,

               asn[3]              -- the ASN for PCI #1
                   IMPLICIT OBJECT IDENTIFIER,

               user-data
                   UserData-PDU
           }

   SessionConnectionIdentifier ::=
       [0]
           SEQUENCE {
               callingSSUserReference
                   T61String,

               commonReference
                   UTCTime,

               additionalReferenceInformation[0]
                   IMPLICIT T61String
                   OPTIONAL
           }

   PresentationSelector ::=
       [1]
           IMPLICIT OCTET STRING



<span class="grey">Rose                                                           [Page 27]</span></pre>
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   -- connect response PDU

   ConnectResponse-PDU ::=
       [1]
           IMPLICIT SEQUENCE {
               reference           -- present only in the udp-based
                                   -- service
                   SessionConnectionIdentifier
                   OPTIONAL,

               responding
                   PresentationSelector
                   OPTIONAL,

               reason[2]           -- present only if the connection
                                   -- was rejected
                   IMPLICIT Rejection-reason
                   OPTIONAL,

               user-data           -- present only if reason is absent
                                   -- OR has the
                                   -- value rejected-by-responder
                   UserData-PDU
                   OPTIONAL
           }

   Rejection-reason ::=
           INTEGER {
               rejected-by-responder(0)
               called-presentation-address-unknown(1),
               local-limit-exceeded(3),
               protocol-version-not-supported(4),
           }


   -- release request PDU

   ReleaseRequest-PDU ::=
       [2]
           IMPLICIT SEQUENCE {
               reference           -- present only in the udp-based
                                   -- service
                   SessionConnectionIdentifier
                   OPTIONAL,

               user-data
                   UserData-PDU
           }



<span class="grey">Rose                                                           [Page 28]</span></pre>
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<span class="grey"><a href="./rfc1085">RFC 1085</a>               ISO Presentation Services           December 1988</span>


   -- release response PDU

   ReleaseResponse-PDU ::=
       [3]
           IMPLICIT SEQUENCE {
               reference           -- present only in the udp-based
                                   -- service
                   SessionConnectionIdentifier
                   OPTIONAL,

               user-data
                   UserData-PDU
           }

   -- abort PDU

   Abort-PDU ::=
       [4]
           SEQUENCE {
               reference           -- present only in the udp-based
                                   -- service
                   SessionConnectionIdentifier
                   OPTIONAL,

               user-data   -- MAY BE present on user-initiated abort
                   UserData-PDU
                   OPTIONAL,

               reason[1]   -- ALWAYS present on provider-initiated abort
                   IMPLICIT Abort-reason
                   OPTIONAL
           }

   Abort-reason ::=
           INTEGER {
               unspecified(0),
               unrecognized-ppdu(1),
               unexpected-ppdu(2),
               unrecognized-ppdu-parameter(4),
               invalid-ppdu-parameter(5),
               reference-mismatch(9)
           }


   -- data PDU

   UserData-PDU ::=
       [<a href="#ref-5" title=" 5">5</a>]                         -- this is the ASN.1 object



<span class="grey">Rose                                                           [Page 29]</span></pre>
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<span class="grey"><a href="./rfc1085">RFC 1085</a>               ISO Presentation Services           December 1988</span>


           ANY                     -- if it is a top-level PDU, it
                                   -- is in PCI #1, otherwise PCI #3


   -- data PDU for the udp-based service

   CL-UserData-PDU ::=
       [6]
           IMPLICIT SEQUENCE {
               reference
                   SessionConnectionIdentifier,

               user-data[0]                -- this is the ASN.1 object
                   ANY                     -- it is always in PCI #1
           }

   END

Appendix B:

Example of Serialization


   Consider the following call to ROSE:

           RO-INVOKE (operation number      = 5
                      operation class       = synchronous
                      argument              = NONE
                      invocation identifier = 1
                      linked invocation id. = NONE
                      priority              = 0)
               .REQUEST

   Ultimately, ROSE will use the P-DATA service:

           P-DATA (user data = {
                                 1,        -- this is the PCI
                                 {         -- this is the ASN.1 object
                                    invokeID 1,
                                    operation-value 5,
                                    argument {}
                                 }
                               })
               .REQUEST

   The presentation provider will construct a UserData PDU and send this
   via the transport connection:




<span class="grey">Rose                                                           [Page 30]</span></pre>
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<span class="grey"><a href="./rfc1085">RFC 1085</a>               ISO Presentation Services           December 1988</span>


      [<a id="ref-5">5</a>] {
            {
              1,
              5,
              {}
            }
          }

   Applying the basic encoding rules for ASN.1, we have an stream of 12
   octets.

      a5  0a                                       [<a href="#ref-5" title=" 5">5</a>]
      tag len

      a0  08                               [0]
      tag len
      02  01  01           invokeID 1
      tag len value

      02  01  05           operation-value 5
      tag len value

      30  00                       argument NULL
      tag len

   Of course, in actual use, the argument would not be NONE and this
   could be expected to dominate the size of the UserData PDU.  However,
   it is worth nothing that the overhead of the encoding mechanism used
   is on the order of 10 octets, hardly a staggering amount!

Appendix C:

Determination of Network Called Address

   As described in <a href="#section-10">Section 10</a>, when the P-CONNECT.REQUEST primitive is
   issued the presentation provider must determine which of the network
   addresses present in the called presentation address parameter to use
   for the presentation connection.  The first step in this
   determination is to examine the quality of service parameter and
   consider only those network addresses which support the corresponding
   transport service.  In practice, it is likely that each network
   address will support exactly the same transport services, so using
   quality of service as a discriminant will either permit all or none
   or the network addresses present to be selected.  This appendix
   describes a local policy which might be employed when deciding which
   network address to use.

   The policy distinguishes between "underlying failures" and



<span class="grey">Rose                                                           [Page 31]</span></pre>
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<span class="grey"><a href="./rfc1085">RFC 1085</a>               ISO Presentation Services           December 1988</span>


   "connection establishment failures".  An "underlying failure" occurs
   when, using the desired transport service, the initiating
   presentation provider is unable to contact the responding
   presentation provider.  For the tcp-based service, this means that a
   TCP connection could not be established for some reason.  For the
   udp-based service, it means that a response was not received before
   final time-out.  In contrast, a "connection establishment failure"
   occurs when the responding presentation provider can be contacted,
   but the presentation connection is rejected by either the
   presentation provider or the correspondent presentation user.

   The policy is simple: starting with the first network address
   present, attempt the connection procedure.  If the procedure fails
   due to an "underlying failure", then the next network address in the
   list is tried.  This process is repeated until either an underlying
   connection is established or all network addresses are exhausted.
   If, however, a "connection establishment failure" occurs, then the
   presentation provider immediately indicates this failure to the
   presentation user and no further network addresses are considered.

   Note that this is only one conformant policy of many.  For example,
   the presentation provider may wish to order network addresses based
   on the "intensity" associated with the members present in the set of
   transport services for each network address.

Author's Address:

   Marshall Rose
   The Wollongong Group
   1129 San Antonio Road
   Palo Alto, CA 94303

   Phone: (415) 962-7100

   EMail: mrose@TWG.COM
















Rose                                                           [Page 32]
</pre>