RFC1085

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
     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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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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  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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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:

     - 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.

9. Information Transfer

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

9.1. The P-DATA Service

  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.

10. Elements of Procedure

  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

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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

10.1. Elements of Procedure specific to the tcp-based service

  The provider maintains the following information for each
  presentation connection:

     - a local designator for the PS-user

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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.

10.2. Elements of Procedure specific to the udp-based service

  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

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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.

10.3. State Transitions

  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

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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.

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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.

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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.

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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.

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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.

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11. Directory Services

  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 Section 5.

  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:

             <designator> "-" <qualifier>

     where <designator> is a string denoting either domain name or a
     32-bit IP address, and <qualifier> 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 <designator> portion
         of the service name,

        - a set indicating which transport services are available

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          at the IP address,

        - the 16-bit port number resolved from the <qualifier>
          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
  <designator> 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.

12. Remarks

  To begin, if one really wanted to implement ISO applications in a
  TCP/IP-based network, then the method proposed by [RFC1006] 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 [RFC1006]-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.

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13. Acknowledgements

  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

14. References

    [ISO7498]  Information Processing Systems - Open Systems
               Interconnection, "Basic Reference Model", October, 1984.

    [ISO8509]  Information Processing Systems - Open Systems
               Interconnection, " Service Conventions".

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

    [ISO8822]  Information Processing Systems - Open Systems
               Interconnection, " Connection Oriented Presentation
               Service Definition (Final Text of DIS 8822)",
               April, 1988.

    [ISO8823]  Information Processing Systems - Open Systems
               Interconnection, " Connection Oriented Presentation
               Protocol Specification (Final Text of DIS 8822)",
               April, 1988.

    [ISO8824]  Information Processing Systems - Open Systems
               Interconnection, " Specification of Abstract Syntax
               Notation One (ASN.1)", December, 1987.

    [ISO8825]  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

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                 Specification (Working Document for DIS 9072/2)",
                 November, 1987.

    [RFC768]  Postel, J., "User Datagram Protocol", RFC 768, USC/ISI,
              28 August 1980.

    [RFC791]  Postel, J., "Internet Protocol - DARPA Internet Program
              Protocol Specification", RFC 791, USC/ISI,
              September 1981.

    [RFC793]  Postel, J., "Transmission Control Protocol - DARPA
              Internet Program Protocol Specification", RFC 793,
              USC/ISI, September 1981.

    [RFC1006]  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

  RFC1085-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

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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

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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
          }

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  -- 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 ::=
      [5]                         -- this is the ASN.1 object

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          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:

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     [5] {
           {
             1,
             5,
             {}
           }
         }

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

     a5  0a                                       [5]
     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 Section 10, 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

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  "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: [email protected]

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