rfc1085.txt

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      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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RFC 1085               ISO Presentation Services           December 1988


6. Choice of Transport Service

   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) [RFC768] 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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RFC 1085               ISO Presentation Services           December 1988


   modified accordingly.

7. Connection Establishment

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

7.1. The P-CONNECT Service

   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).

8. Connection Termination

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

8.1. The P-RELEASE Service

   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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RFC 1085               ISO Presentation Services           December 1988


8.2. The P-U-ABORT Service

   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.

8.3. The P-P-ABORT Service

   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: