





Network Working Group                                         M. Banan
Request for Comments: 2188                                        Neda
Category: Informational                                      M. Taylor
                                                                   AWS
                                                              J. Cheng
                                                                   AWS
                                                        September 1997


          AT&T/Neda's Efficient Short Remote Operations (ESRO)
                   Protocol Specification Version 1.2


Status of this Memo

   This memo provides information for the Internet community.  It does
   not specify an Internet standard of any kind.  Distribution of this
   memo is unlimited.

IESG Note

   This protocol has not had the benefit of IETF Working Group review,
   but a cursory examination reveals several issues which may be
   significant issues for scalability.  A site considering deployment
   should conduct a careful analysis to ensure they understand the
   potential impacts.

Abstract

  This document specifies the service model, the notation and protocol
  for Efficient Short Remote Operations (ESRO). The ESRO service is
  similar to and is consistent with other Remote Procedure Call
  services.  The emphasis of ESRO service definition and the ESRO
  protocol is on efficiency.  ESRO is designed specifically with
  wireless network (e.g., CDPD) usage in mind.

  ESRO protocol provides reliable connectionless remote operation
  services on top of UDP (or any other non-reliable connectionless
  transport service) with minimum overhead.  ESRO protocol supports
  segmentation and reassembly, concatenation and separation as well as
  multiplexing for service users (applications).

  ESRO allows for trade-offs between efficiency and reliability by
  specifying both 2-way hand-shake and 3-way hand-shake based protocols.

  Encoding mechanisms for presentation of the parameters of remote
  operations are outside the scope of this document.  But,
  identification (tagging) of the encoding mechanism in use (e.g., XDR,



Banan, et. al                Informational                      [Page 1]

RFC 2188                          ESRO                    September 1997


  BER, PER) is supported by ESRO protocol.

  A variety of applications can use the ESRO protocol.  Some early
  applications using ESRO include efficient short message submission and
  delivery, credit card authorization and white pages lookup.














































Banan, et. al                Informational                      [Page 2]

RFC 2188                          ESRO                    September 1997


Contents

  1  INTRODUCTION                                                      4
     1.1 Relationship To Existing Remote Operation Services    .       5
         1.1.1 ESRO and RPC    .   .   .   .   .   .   .   .   .       5
         1.1.2 ESRO and ROSE   .   .   .   .   .   .   .   .   .       5
     1.2 Overview of ESROS     .   .   .   .   .   .   .   .   .       5
     1.3 The Remote Operation Model    .   .   .   .   .   .   .       6
  2  ESRO SERVICE DEFINITIONS                                          8
     2.1 Acknowledged Result Service Mode  .   .   .   .   .   .       9
         2.1.1 Performer side .   .   .   .   .   .   .   .   .        9
         2.1.2 Invoker side    .   .   .   .   .   .   .   .   .      11
     2.2 Non-acknowledged Result   .   .   .   .   .   .   .   .      11
         2.2.1 Performer side .   .   .   .   .   .   .   .   .       12
         2.2.2 Invoker side    .   .   .   .   .   .   .   .   .      12
     2.3 Serialized Use of ESRO Services   .   .   .   .   .   .      12
         2.3.1 Invoker     .   .   .   .   .   .   .   .   .   .      12
         2.3.2 Performer   .   .   .   .   .   .   .   .   .   .      12
     2.4 ESROS-INVOKE Service  .   .   .   .   .   .   .   .   .      13
         2.4.1 Operation-value     .   .   .   .   .   .   .   .      13
         2.4.2 Performer-address   .   .   .   .   .   .   .   .      14
         2.4.3 Invoker-address     .   .   .   .   .   .   .   .      14
         2.4.4 Invoke-argument-encoding-type   .   .   .   .   .      15
         2.4.5 Invoke-argument     .   .   .   .   .   .   .   .      15
         2.4.6 Invoke-ID   .   .   .   .   .   .   .   .   .   .      15
         2.4.7 Failure-value   .   .   .   .   .   .   .   .   .      16
     2.5 ESROS-RESULT Service  .   .   .   .   .   .   .   .   .      16
         2.5.1 Result-argument-encoding-type   .   .   .   .   .      16
         2.5.2 Result-argument     .   .   .   .   .   .   .   .      17
         2.5.3 Invoke-ID   .   .   .   .   .   .   .   .   .   .      17
         2.5.4 Failure-value   .   .   .   .   .   .   .   .   .      18
     2.6 ESROS-ERROR Service   .   .   .   .   .   .   .   .   .      18
         2.6.1 Error-value     .   .   .   .   .   .   .   .   .      18
         2.6.2 Error-argument-encoding-type    .   .   .   .   .      19
         2.6.3 Error-argument .   .   .   .   .   .   .   .   .       19
         2.6.4 Invoke-ID   .   .   .   .   .   .   .   .   .   .      20
         2.6.5 Failure-value   .   .   .   .   .   .   .   .   .      20
     2.7 ESROS-FAILURE Service     .   .   .   .   .   .   .   .      20
         2.7.1 Failure-value   .   .   .   .   .   .   .   .   .      21
         2.7.2 Invoke-ID   .   .   .   .   .   .   .   .   .   .      21
  3  ESRO SERVICE NOTATION                                            21
     3.1 ES-OPERATION Notation     .   .   .   .   .   .   .   .      22
     3.2 Mapping of ESROS Notation     .   .   .   .   .   .   .      22
         3.2.1 Invocation of an Operation .   .   .   .   .   .       22
         3.2.2 Reply of an Operation   .   .   .   .   .   .   .      22
  4  REMOTE OPERATIONS PROTOCOL                                       23
     4.1 Overview of the Protocol  .   .   .   .   .   .   .   .      23
         4.1.1 Service Provision (Invoker User)    .   .   .   .      24



Banan, et. al                Informational                      [Page 3]

RFC 2188                          ESRO                    September 1997


         4.1.2 Service Provision (Performer User) .   .   .   .       24
     4.2 Protocol Procedures   .   .   .   .   .   .   .   .   .      25
         4.2.1 Service Access Point (SAP) Bind Procedure   .   .      25
         4.2.2 Invoke Service Procedure    .   .   .   .   .   .      25
         4.2.3 Invoke ID Assignment Procedure .   .   .   .   .       25
         4.2.4 Functional Unit Selection Procedure     .   .   .      26
     4.3 Connectionless PDU Transfer For Small PDUs    .   .   .      26
         4.3.1 Overview    .   .   .   .   .   .   .   .   .   .      26
         4.3.2 3-Way Handshake Functional Unit     .   .   .   .      28
         4.3.3 2-Way Handshake Functional Unit     .   .   .   .      35
         4.3.4 Segmentation and Reassembly     .   .   .   .   .      40
     4.4 Structure and Encoding of ESROS PDUs  .   .   .   .   .      43
         4.4.1 ESRO-INVOKE-PDU Format .   .   .   .   .   .   .       43
         4.4.2 ESRO-RESULT-PDU Format .   .   .   .   .   .   .       45
         4.4.3 ESRO-ERROR-PDU Format   .   .   .   .   .   .   .      46
         4.4.4 ESRO-ACK-PDU Format     .   .   .   .   .   .   .      47
         4.4.5 ESRO-FAILURE-PDU Format     .   .   .   .   .   .      47
         4.4.6 ESRO-INVOKE-SEGMENTED-PDU Format    .   .   .   .      48
         4.4.7 ESRO-RESULT-SEGMENTED-PDU Format    .   .   .   .      50
         4.4.8 ESRO-ERROR-SEGMENTED-PDU Format     .   .   .   .      51
     4.5 Concatenation and Separation  .   .   .   .   .   .   .      52
         4.5.1 Procedures .   .   .   .   .   .   .   .   .   .       53
         4.5.2 ESRO-CONCATENATED-PDU format    .   .   .   .   .      53
     4.6 ES Remote Operations Protocol Parameters  .   .   .   .      54
         4.6.1 PDU size    .   .   .   .   .   .   .   .   .   .      54
         4.6.2 Timers .   .   .   .   .   .   .   .   .   .   .       55
         4.6.3 Use of lower layers     .   .   .   .   .   .   .      56
  5  ACKNOWLEDGMENTS  .   .   .   .   .   .   .   .   .   .   .   .   56
  6  SECURITY CONSIDERATIONS  .   .   .   .   .   .   .   .   .   .   56
  7  AUTHORS' ADDRESSES   .   .   .   .   .   .   .   .   .   .   .   56

1  INTRODUCTION

   Efficient Short Remote Operations (ESRO) provide an efficent
   mechanism for realization of Remote Procedure Call.  This document
   specifies many aspects of ESRO including:

     o Service Model

     o Service Primitves

     o A Notation for user of the Service

     o Confirmed Connectionless Protocol (based on a 3-way hand-shake)

     o Unconfirmed Connectionless Protocol (based on a 2-way hand-shake)





Banan, et. al                Informational                      [Page 4]

RFC 2188                          ESRO                    September 1997


1.1  Relationship To Existing Remote Operation Services

   The overall model of ESRO is similar to and consistent with many
   existing protocols.  ESRO's distinguishing characteristic is
   efficiency.

   A brief comparison of ESRO and Remote Procedure Calls [7] and Remote
   Operation Service Elements [1] follows.

1.1.1  ESRO and RPC

   Remote Procedure Call (RPC) is specified in [7] (RFC-1831) and [6]
   (RFC-1833).

   RPC specifications define a remote procedure model that is
   essentially same as ESRO. RPC's notation uses a syntax quite
   different from that of ESRO. RPC can rely on a connection oriented or
   connectionless transport mechanism.  When using the connectionless
   mechanism, the retransmission and reliability issues are considered
   beyond the scope of the RPC specification.  RPC is usually used in
   combination with External Data Representation, XDR [8] (RFC-1832).

1.1.2  ESRO and ROSE

   ROSE is specified in [1] and [2].  The service definition for ESRO
   Service (ESROS) specified in this document is similar ROSE's
   Notation.  The Notation specified in this document for ESROS is
   similar ROSE's Notation.  The ESRO protocol specified in this
   document is very different from the ROSE protocol [2].

   The operation model for ESRO Service (ESROS) is based on Remote
   Operations Services Element (ROSE) in [1].  In ESROS model both
   entities can invoke operations.

   ESRO protocols can accomplish short operations with much less
   overhead than ROSE.

1.2  Overview of ESROS

   ESROS provides a service which supports interaction of applications
   based on a remote operation model.  A Remote Operation is invoked by
   one entity; the other entity attempts to perform the Remote Operation
   and then reports the outcome of the attempt.  The ESROS protocol is
   designed such that it could support many applications.







Banan, et. al                Informational                      [Page 5]

RFC 2188                          ESRO                    September 1997


1.3  The Remote Operation Model

   ESROS provides for performance of operations between two peer
   sublayers.  Users of the ESROS assume the roles of invoker and
   performer which invoke and perform the operations respectively.  An
   ESROS-User can assume both roles and be an invoker for some
   operations and be a performer for other operations.  The performer is
   expected to report either the result of the operation or an error.  A
   result reply is sent to the invoker if the operation is successful,
   and an error reply is sent if the operation is unsuccessful.  If the
   performer is unreachable, the ESROS sends a failure indication
   primitive to the invoker.

   Operations are asynchronous and the invoker may continue to invoke
   further operations without waiting for a reply.  Synchronous or
   serialized operations are also supported as a subset and a special
   case of asynchronous service.  By default the ESRO service provider
   on both invoker and performer sides supports the asynchronous
   operation invocation.  However, if one side is to support only
   serialized (synchronous) mode, it should be in agreement with the
   peer side.

   ESROS has no authentication mechanism.  Authentication is the
   responsibility of the performer (which is outside of the scope of
   ESROS) and the performer is not expected to honor the invoker when it
   is not authenticated.

   The ESROS operation model is represented in Figure 1.  In this
   example, the ESROS User on the left is the Invoker and the ESROS User
   on the right is the Performer.  The Provider is the entity providing
   a service to the layer above it.




















Banan, et. al                Informational                      [Page 6]

RFC 2188                          ESRO                    September 1997


   ESROS      -------------------    -------------------     ESROS
   User      | Layer above ESROS |  | Layer above ESROS |    User
   (Invoker) |                   |  |                   |  (Performer)
              -------------------    -------------------
      ^                |                      |                ^
      |                |                      |                |
      v                |                      |                v
   ESROS     -------------------    -------------------      ESROS
   Provider |       ESROS       |  |       ESROS       |    Provider
             -------------------    -------------------
                       |                      |
                       |                      |
                       |                      |
             -------------------    -------------------
            |    UDP           |   |   UDP             |
             -------------------    -------------------
                     _                    _/
                       _                _/
                         _    .       _/
                           _ . .* . _/
                             . * .* .
                               * . *


                    Figure 1:  ES Remote Operation Model


























Banan, et. al                Informational                      [Page 7]

RFC 2188                          ESRO                    September 1997


                  Invoker        Performer
                 ESRO SAP         ESRO SAP
                       |               |
                       |               |
   ESROS-INVOKE.req.   |               | ESROS-INVOKE.ind.
   -------->-----------|               |-------->---------
                       |               |
   ESROS-INVOKE-P.conf.|               |
   --------<-----------|               |
                       |               |
                       |               |
                       |               |
   ESROS-RESULT.ind.   |               | ESROS-RESULT.req.
   --------<-----------|               |--------<---------
                       |               |
                       |               | ESROS-RESULT.conf.
                       |               |-------->---------
                       |               |
                       |               |
   ESROS-ERROR.ind.    |               | ESROS-ERROR.req.
   --------<-----------|               |--------<---------
                       |               |
                       |               | ESROS-ERROR.conf.
                       |               |-------->---------
                       |               |
                       |               |
                       |               |
                       |               |
   ESROS-FAILURE.ind.  |               | ESROS-FAILURE.ind.
   --------<-----------|               |-------->---------
                       |               |


             Figure 2:  Time sequence diagram for ESRO services



2  ESRO SERVICE DEFINITIONS

   ESRO service primitives are illustrated in Figure 2, Table 1 and
   Table 2.  The description of services and primitives comes in the
   following sections.

   ESROS-User accesses ESRO services through Efficient Short Remote
   Operations Service Access Point (ESRO-SAP) as shown in Figure 2.

   The RESULT.request, ERROR.request and FAILURE.indication service
   primitives can be implemented in two different modes:



Banan, et. al                Informational                      [Page 8]

RFC 2188                          ESRO                    September 1997


    1. Acknowledged Result, and

    2. Non-Acknowledged Result

                 _____________________________________________
                 | ESRO Service   |Type                      |
                 |________________|__________________________|
                 | ESROS-INVOKE   |Non-confirmed             |
                 | ESROS-INVOKE-P |Provider-initiated        |
                 | ESROS-RESULT   |Confirmed / Non-confirmed |
                 | ESROS-ERROR    |Confirmed / Non-confirmed |
                 | ESROS-FAILURE  |Provider initiated        |
                 |________________|__________________________|

                           Table 1:  ESRO Services

   as described below.  The difference between different modes is in
   their reliability of service and efficiency.  Reliability of service
   is defined based on the understanding of invoker and performer about
   the success or failure of the operation on the peer side.  Table 3
   and Table 4 summarize understanding of performer about success or
   failure on invoker side in different situations.  In these tables the
   FAILURE.indication refers to the primitive generated by protocol and
   not the failure of local provider.

2.1  Acknowledged Result Service Mode

   In this service mode, the result is acknowledged by invoker, but the
   mechanism by which the acknowledgment is accomplished may not be
   reliable.  Table 3 summarizes the relationship between performer and
   invoker in success and failure cases.

2.1.1  Performer side

   In this type of service, the RESULT.confirm and ERROR.confirm
   primitives on performer side are generated if the result/error is
   acknowledged by invoker.

   The FAILURE.indication on performer side is generated if result/error
   is not acknowledged by invoker or if there is a local failure on
   performer side.

   >From the protocol point of view, the FAILURE.indication might be
   because either the result/error PDU or the ack PDU is lost.  The
   outcome of this is that a FAILURE.indication is not robust as the
   operation may have been successful from the invoker's perspective.
   One method of compensating for this shortcoming is having the
   performer verify the FAILURE.indication in a separate operation.



Banan, et. al                Informational                      [Page 9]

RFC 2188                          ESRO                    September 1997


   ____________________________________________________________
   | Primitive                |Parameters                     |
   |--------------------------+-------------------------------|
   |                          |Operation-value                |
   |                          |Performer-address              |
   | ESROS-INVOKE.request     |Invoke-argument-encoding-type  |
   |                          |Invoke-argument                |
   |--------------------------+-------------------------------|
   |                          |Operation-value                |
   |                          |Invoker-address                |
   | ESROS-INVOKE.indication  |Invoke-argument-encoding-type  |
   |                          |Invoke-argument                |
   |                          |Invoke-ID                      |
   |--------------------------+-------------------------------|
   | ESROS-INVOKE-P.confirm   |Invoke-ID                      |
   |==========================================================|
   |                          |                               |
   |                          |Result-argument-encoding-type  |
   | ESROS-RESULT.request     |Result-argument                |
   |                          |Invoke-ID                      |
   |--------------------------+-------------------------------|
   |                          |Result-argument-encoding-type  |
   | ESROS-RESULT.indication  |Result-argument                |
   |                          |Invoke-ID                      |
   |--------------------------+-------------------------------|
   | ESROS-RESULT.confirm     |Invoke-ID                      |
   |==========================================================|
   |                          |                               |
   |                          |Error-value                    |
   |                          |Error-argument-encoding-type   |
   | ESROS-ERROR.request      |Error-argument                 |
   |--------------------------+-------------------------------|
   |                          |Error-value                    |
   |                          |Error-argument-encoding-type   |
   | ESROS-ERROR.indication   |Error-argument                 |
   |                          |Invoke-ID                      |
   |--------------------------+-------------------------------|
   | ESROS-ERROR.confirm      |Invoke-ID                      |
   |==========================================================|
   |                          |                               |
   |                          |Failure-value                  |
   | ESROS-FAILURE.indication |Invoke-ID                      |
   |__________________________|_______________________________|


    Table 2:  ESRO service primitives and associated parameters





Banan, et. al                Informational                     [Page 10]

RFC 2188                          ESRO                    September 1997


   ______________________________________________________________
   |Service Mode        |Performer          |Invoker            |
   |--------------------+-------------------+-------------------|
   |Acknowledged Result |RESULT.confirm     |RESULT.indication  |
   |                    |-------------------+-------------------|
   |                    |FAILURE.indication |RESULT.indication  |
   |                    | (protocol)        |                   |
   |                    |-------------------+-------------------|
   |                    |FAILURE.indication |FAILURE.indication |
   |                    | (protocol)        | (protocol)        |
   |____________________|___________________|___________________|


    Table 3:  Success and Failure in Acknowledged Result Mode


   __________________________________________________________________
   |Service Mode            |Performer           |Invoker            |
   |------------------------+--------------------+-------------------|
   |Non-acknowledged Result |RESULT.confirm      |RESULT.indication  |
   |                        +--------------------+-------------------|
   |                        |RESULT.confirm      |FAILURE.indication |
   |                        |                    | (protocol)        |
   |                        +--------------------+-------------------|
   |                        |FAILURE.indication  |                   |
   |                        |(protocol)          |                   |
   |                        |does not            |---                |
   |                        |exist               |                   |
   |________________________|____________________|___________________|


    Table 4:  Success and Failure in Non-acknowledged Result Mode



2.1.2  Invoker side

   When invoker receives failure indication, the performer has the
   failure indication too.

   This type of service can be implemented by protocols based on 3-Way
   handshaking.

2.2  Non-acknowledged Result

   In this service mode the result is not acknowledged.  Table 4
   summarizes the relationship between performer and invoker in success
   and failure cases.



Banan, et. al                Informational                     [Page 11]

RFC 2188                          ESRO                    September 1997


2.2.1  Performer side

   In this type of service, the RESULT.confirm and ERROR.confirm
   primitives on performer side are generated without receiving
   additional information from the invoker peer.  In other words, these
   Primitives have no protocol-related meaning and convey no
   information, other than end-of-operation.

   The FAILURE.indication on performer side is not generated by
   protocol.  The only case that can generate FAILURE.indication on
   performer side is local failure in service provider on performer
   side.

2.2.2  Invoker side

   The FAILURE.indication on invoker side can be the resultof not
   receiving result/error/failure from peer performer or it can result
   from failure in local service provider.

   This type of service can be implemented by protocols based on 2-Way
   handshaking.

2.3  Serialized Use of ESRO Services

   Although the ESRO Services are defined to support asynchronous
   operation invocation in general, they can be used in the special case
   of synchronous (serialized) mode too.  The serialized use of ESRO
   Services is implementation specific.  However, one of the possible
   scenarios is as follows:

2.3.1  Invoker

   Invokes an operation after it receives either RESULT.indication,
   ERROR.indication, or FAILURE.indication for the previous operation.

2.3.2  Performer

   Considers an operation to be complete and accepts the next operation
   after it receives RESULT.confirm, ERROR.confirm, or
   FAILURE.indication.











Banan, et. al                Informational                     [Page 12]

RFC 2188                          ESRO                    September 1997


                  Invoker        Performer
                 ESROS AP         ESROS AP
                       |               |
                       |               |
   ESROS-INVOKE.req.   |               | ESROS-INVOKE.ind.
   -------->-----------|               |-------->---------
                       |               |
   ESROS-INVOKE-P.conf.|               |
   --------<-----------|               |
                       |               |
   ESROS-FAILURE.ind.  |               |
   --------<-----------|               |
                       |               |


   Figure 3:  Time sequence diagram for ESROS-INVOKE service




2.4  ESROS-INVOKE Service

   The ESROS-INVOKE service is used by an ESROS-User (the invoker) to
   cause the invocation of an OPERATION to be performed by the other
   ESROS-User (the performer).

   ESROS Invoker User issues ESROS-INVOKE.request primitive to invoke an
   operation.

   ESROS-INVOKE.indication primitive provides the ESROS Performer User
   with the parameters of the invoked operation.

   ESRO Service Provider issues the ESROS-INVOKE-P.confirm primitive to
   provide the ESROS Invoker User with Invoke-ID of the invoked
   operation.

   The related service structure consists of three service primitives as
   illustrated in Figure 3 and Table 5.

2.4.1  Operation-value

   This value is the identifier of the operation to be invoked.  The
   value is agreed upon between the ESROS Users.  This parameter has to
   be supplied by the invoker of the service.

   ESROS Invoker User provides the Operation-value parameter for the
   ESROS-INVOKE.request primitive.  The Operation-value parameter of
   ESROS-INVOKE.indication is provided to the ESROS Performer User.



Banan, et. al                Informational                     [Page 13]

RFC 2188                          ESRO                    September 1997


          _____________________________________________________________
          | Primitive                |Parameters                       |
          |__________________________|_________________________________|
          |                          |Operation-value                  |
          |                          |Performer-address                |
          | ESROS-INVOKE.request     |Invoke-argument-encoding-type    |
          |                          |Invoke-argument                  |
          |__________________________|_________________________________|
          |                          |Operation-value                  |
          |                          |Invoker-address                  |
          |                          |Invoke-argument-encoding-type    |
          | ESROS-INVOKE.indication  |Invoke-argument                  |
          |                          |Invoke-ID                        |
          |__________________________|_________________________________|
          | ESROS-INVOKE-P.confirm   |Invoke-ID                        |
          |                          |Failure-value                    |
          |__________________________|_________________________________|
          | ESROS-FAILURE.indication |Invoke-ID                        |
          |__________________________|_________________________________|


     Table 5:  ESROS-INVOKE service primitives and associated parameters




2.4.2  Performer-address

   This parameter is the address of the ESROS Performer User which
   consists of ESRO Service Access Point (SAP) Selector, Transport
   Service Access Point (TSAP) Selector (e.g., port number), and Network
   Service Access Point (NSAP) address (e.g., IP address).  This
   parameter has to be supplied by the invoker of the service.

   ESROS Invoker User provides the Performer-address parameter for the
   ESROS-INVOKE.request primitive.

2.4.3  Invoker-address

   This parameter is the address of the ESROS Invoker User which
   consists of ESRO Service Access Point (SAP) Selector, Transport
   Service Access Point (TSAP) Selector (e.g.  port number), and Network
   Service Access Point (NSAP) address (e.g.  IP address).

   The Invoker-address parameter of ESROS-INVOKE.indication is provided
   to the ESROS Performer User.





Banan, et. al                Informational                     [Page 14]

RFC 2188                          ESRO                    September 1997


2.4.4  Invoke-argument-encoding-type

   This parameter identifies the encoding type of the Invoke-argument
   (see next subsection).  The encoding type has to be agreed upon
   between ESROS Users.  This parameter has to be supplied by the
   invoker of the service.

   ESROS Invoker User provides the Invoke-argument-encoding-type
   parameter for the ESROS-INVOKE.request primitive.  The Invoke-
   argument-encoding-type parameter of ESROS-INVOKE.indication is
   provided to the ESROS Performer User.

2.4.5  Invoke-argument

   This parameter is the argument of the invoked operation.  The type
   has to be agreed between the ESROS Users.  This parameter has to be
   supplied by the invoker of the service.  Encoding type of the
   Invoke-argument is specified through the Invoke-argument-encoding-
   type parameter (see previous subsection).

   ESROS Invoker User provides the Invoke-argument parameter for the
   ESROS-INVOKE.request primitive.  The Invoke-argument parameter of
   ESROS-INVOKE.indication is provided to the ESROS Performer User.

2.4.6  Invoke-ID

   This parameter identifies the invocation of an ESROS-INVOKE service
   and is used to correlate this invocation with the corresponding
   replies (ESROS-RESULT, ESROS-ERROR, and ESROS-FAILURE services.)
   This parameter has to be supplied by the ESROS provider.

   This parameter distinguishes several invocations of the service in
   progress (asynchronous operations).  The ESROS provider may begin to
   reuse Invoke-ID values whenever it chooses, subject to the constraint
   that it may not reuse an Invoke-ID value that was previously assigned
   to an invocation of the service for which it expects, but has not yet
   received a reply.  In other words, the provider does not reuse a
   previously used Invoke-ID unless the corresponding service is fully
   completed.












Banan, et. al                Informational                     [Page 15]

RFC 2188                          ESRO                    September 1997


2.4.7  Failure-value

   This parameter identifies the failure that occurred during the
   processing or transmission of any of the service primitives of ESROS.

                  Invoker        Performer
                 ESROS AP         ESROS AP
                       |               |
                       |               |
   ESROS-RESULT.ind.   |               | ESROS-RESULT.req.
   --------<-----------|               |--------<---------
                       |               |
                       |               | ESROS-RESULT.conf.
                       |               |-------->---------
                       |               |
                       |               | ESROS-FAILURE.ind.
                       |               |-------->---------
                       |               |


   Figure 4:  Time sequence diagram for ESROS-RESULT service



   This parameter has to be supplied by the ESROS provider (see also
   Section 2.7).

2.5  ESROS-RESULT Service

   The ESROS-RESULT service is used by an ESROS User to reply to a
   previous ESROS-INVOKE.indication in the case of a successfully
   performed operation.  This service is either confirmed or non-
   confirmed based on the service mode (see Section 2).

   The related service structure consists of three service primitives as
   illustrated in Figure 4 and Table 6.

2.5.1  Result-argument-encoding-type

   This parameter identifies the encoding type of the Result-argument
   (see next subsection).  The encoding type has to be agreed upon
   between the ESROS Users.  This parameter has to be supplied by the
   ESROS Performer User.

   ESROS Performer User provides the Result-argument-encoding-type
   parameter for the ESROS-RESULT.request primitive.  The Result-
   argument-encoding-type parameter of ESROS-RESULT.indication is
   provided to the ESROS Invoker User.



Banan, et. al                Informational                     [Page 16]

RFC 2188                          ESRO                    September 1997


   ______________________________________________________________
   | Primitive                |Parameters                       |
   |__________________________|_________________________________|
   |                          |Result-argument-encoding-type    |
   |                          |Result-argument                  |
   | ESROS-RESULT.request     |Invoke-ID                        |
   |__________________________|_________________________________|
   |                          |                                 |
   |                          |Result-argument-encoding-type    |
   |                          |Result-argument                  |
   | ESROS-RESULT.indication  |Invoke-ID                        |
   |__________________________|_________________________________|
   |                          |                                 |
   | ESROS-RESULT.confirm     |Invoke-ID                        |
   |                          |Failure-value                    |
   |                          |                                 |
   |__________________________|_________________________________|
   | ESROS-FAILURE.indication |Invoke-ID                        |
   |__________________________|_________________________________|


   Table 6:  ESROS-RESULT service primitives and associated parameters




2.5.2  Result-argument

   This parameter is the result of an invoked and successfully performed
   operation.  The type has to be agreed between the ESROS Users.  This
   parameter has to be supplied by the invoker of the service.  Encoding
   type of the Result-argument is specified through the Result-
   argument-encoding-type parameter (see previous subsection).

   ESROS Performer User provides the Result-argument parameter for the
   ESROS-RESULT.request primitive.  The Result-argument parameter of
   ESROS-RESULT.indication is provided to the ESROS Invoker User.

2.5.3  Invoke-ID

   This parameter identifies the corresponding invocation.  This
   Invoke-ID, which is originally generated by the ESROS provider at the
   time of ESROS-INVOKE indication, is extracted from the Invoke ID that
   has to be supplied by the ESROS performer User.  The value is that of
   the corresponding ESROS-INVOKE.indication primitive.






Banan, et. al                Informational                     [Page 17]

RFC 2188                          ESRO                    September 1997


                  Invoker        Performer
                 ESROS AP         ESROS AP
                       |               |
                       |               |
   ESROS-ERROR.ind.    |               | ESROS-ERROR.req.
   --------<-----------|               |--------<---------
                       |               |
                       |               | ESROS-ERROR.conf.
                       |               |-------->---------
                       |               |
                       |               | ESROS-FAILURE.ind.
                       |               |-------->---------


   Figure 5:  Time sequence diagram for ESROS-ERROR service




2.5.4  Failure-value

   This parameter identifies the failure that occurred during the
   processing or transmission of any of the service primitives of ESROS.
   This parameter has to be supplied by the ESROS provider (see also
   Section 2.7).

2.6  ESROS-ERROR Service

   The ESROS-ERROR service is used by an ESROS User to reply to a
   previous ESROS-INVOKE.indication in the case of an unsuccessfully
   performed operation.  This service is either confirmed or non-
   confirmed based on the service mode (see Section 2).

   The related service structure consists of three service primitives as
   illustrated in Figure 5 and Table 7.

2.6.1  Error-value

   This parameter identifies the error in reply to a previous ESROS-
   INVOKE.indication in the case of an unsuccessfully performed
   operation.  The value has to be agreed between the ESROS-Users.  This
   parameter has to be supplied by the ESROS Performer User.

   ESROS Performer User provides the Error-argument parameter for the
   ESROS-ERROR.request primitive.  The Error-argument parameter of
   ESROS-ERROR.indication is provided to the ESROS Invoker User.





Banan, et. al                Informational                     [Page 18]

RFC 2188                          ESRO                    September 1997


          ________________________________________________________
          | Primitive                |Parameters                  |
          |__________________________|____________________________|
          |                          |Error-value                 |
          |                          |Error-argument-encoding-type|
          | ESROS-ERROR.request      |Error-argument              |
          |__________________________|____________________________|
          |                          |                            |
          |                          |Error-value                 |
          |                          |Error-argument-encoding-type|
          | ESROS-ERROR.indication   |Error-argument              |
          |                          |Invoke-ID                   |
          |                          |                            |
          |__________________________|____________________________|
          | ESROS-ERROR.confirm      |Invoke-ID                   |
          |                          |Failure-value               |
          |                          |                            |
          |__________________________|____________________________|
          | ESROS-FAILURE.indication |Invoke-ID                   |
          |__________________________|____________________________|



    Table 7:  ESROS-ERROR service primitives and associated parameters

2.6.2  Error-argument-encoding-type

   This parameter identifies the encoding type of the Error-argument
   (see next subsection).  The encoding type has to be agreed upon
   between the ESROS Users.  This parameter has to be supplied by the
   ESROS Performer User.

   ESROS Performer User provides the Error-argument-encoding-type
   parameter for the ESROS-ERROR.request primitive.  The Error-
   argument-encoding-type parameter of ESROS-ERROR.indication is
   provided to the ESROS Invoker User.

2.6.3  Error-argument

   This parameter provides additional information about the error in
   reply to a previous ESROS-INVOKE.indication in the case of an
   unsuccessfully performed operation.  The type (if any) has to be
   agreed between the ESROS users.  This parameter has to be supplied by
   the ESROS Performer User.  Encoding type of the Error-argument is
   specified through the Error-argument-encoding-type parameter (see
   previous subsection).





Banan, et. al                Informational                     [Page 19]

RFC 2188                          ESRO                    September 1997


                  Invoker        Performer
                 ESROS AP         ESROS AP
                       |               |
                       |               |
   ESROS-FAILURE.ind.  |               |
   --------<-----------|               |
                       |               |
                       |               | ESROS-FAILURE.ind.
                       |               |--------->---------
                       |               |


   Figure 6:  Time sequence diagram for ESROS-FAILURE service




   ESROS Performer User provides the Error-argument parameter for the
   ESROS-ERROR.request primitive.  The Error-argument parameter of
   ESROS-ERROR.indication is provided to the ESROS Invoker User.

2.6.4  Invoke-ID

   This parameter identifies the corresponding invocation.  This
   Invoke-ID, which is originally generated by the ESROS provider at the
   time of the ESROS-INVOKE.indication, is extracted from the Invoke ID
   which has to be supplied by the ESROS performer User.  The value is
   that of the corresponding ESROS-INVOKE.indication primitive.

2.6.5  Failure-value

   This parameter identifies the failure that occurred during the
   processing or transmission of any of the service primitives of ESROS.
   This parameter has to be supplied by the ESROS provider (see also
   Section 2.7).

2.7  ESROS-FAILURE Service

   The ESROS-FAILURE service is used by ESROS provider to indicate the
   failure in providing an ESROS-INVOKE, ESROS-RESULT, or ESROS-ERROR
   service.

   The related service structure consists of one service primitive as
   illustrated in Figure 6 and Table 8.







Banan, et. al                Informational                     [Page 20]

RFC 2188                          ESRO                    September 1997


          _____________________________________________
          | Primitive                |Parameters       |
          |__________________________|_________________|
          |                          |Failure-value    |
          | ESROS-FAILURE.indication |Invoke-ID        |
          |__________________________|_________________|



   Table 8:  ESROS-FAILURE service primitives and associated parameters


          _________________________________________
          | Failure Value |Meaning                 |
          |_______________|________________________|
          | 0             |Transmission failure    |
          | 1             |Out of local resources  |
          | 2             |User not responding     |
          | 3             |Out of remote resources |
          | 4             |Reassembly failure      |
          |_______________|________________________|


             Table 9:  Encoding of Failure-value

2.7.1  Failure-value

   This parameter identifies the failure that occurred during the
   processing or transmission of any of the service primitives of ESROS.
   This parameter has to be supplied by the ESROS provider.

   The values for encoding of Failure-value are presented in Table 9.

2.7.2  Invoke-ID

   This parameter identifies the corresponding invocation.  This
   Invoke-ID, which is originally generated by ESROS provider at the
   time of the ESROS-INVOKE.indication, is extracted from the Invoke ID
   which has to be supplied by ESROS performer User.  The value is that
   of the corresponding ESROS-INVOKE.indication primitive.

3  ESRO SERVICE NOTATION

   Users of ESRO services (invoker and performer) need to agree on a
   well defined set of parameters which are enumerated below.

    1. The operation's Argument data type.




Banan, et. al                Informational                     [Page 21]

RFC 2188                          ESRO                    September 1997


    2. The operation's Result data type.

    3. The operation's Error data type.

    4. The operation's value.  A specific tag which uniquely identifies
        the operation.

   The invoker and the performer can specify these parameters using a
   variety of mechanisms.  The notation specified in this section is one
   such mechanism.  It is not the only machanism and ESRO protocol can
   be used independent of this notation.

3.1  ES-OPERATION Notation

   The Remote Operations and Operation Errors are specified in this
   section.  The notation is defined by means of the macro facility
   defined in [3].

   The macros enabling the specification of operations and errors are
   listed in Figure 7.

   Note that this notation is very similar to the abstract operation
   defined in [1].  The value form of ES-OPERATION is always an integer.

3.2  Mapping of ESROS Notation

3.2.1  Invocation of an Operation

   An operation is mapped onto the ESRO Services.

   The invocation of an operation is mapped on the ESRO-INVOKE service.

   The value assigned to the operation is mapped on the Operation-value
   parameter of that service.  The value of the Named-Type in the
   ARGUMENT clause of the OPERATION Macro is mapped on the Argument
   parameter of that service.

3.2.2  Reply of an Operation

   If an operation was successfully performed, the reply is mapped on
   the ESRO-RESULT service.

   The value of the Named-Type in the RESULT clause of the OPERATION

   DEFINITIONS ::=
   BEGIN
     ES-OPERATION, ERROR;
     -- macro definition for operations



Banan, et. al                Informational                     [Page 22]

RFC 2188                          ESRO                    September 1997


     ES-OPERATION MACRO ::=
     BEGIN
           TYPE NOTATION   ::=     Argument Result Errors
           VALUE NOTATION  ::=     value (localValue INTEGER)
           Argument        ::=     "ARGUMENT" NamedType | empty
           Result          ::=     "RESULT" ResultType | empty
           ResultType      ::=     NamedType | empty
           Errors          ::=     "ERRORS" "{"ErrorNames"}" | empty
           ErrorNames      ::=     ErrorList | empty
           ErrorList       ::=     Error | ErrorList "," Error
           Error           ::=     value (ERROR) | type
           NamedType       ::=     identifier type | type
     END
     -- macro definition for operations errors
     ERROR MACRO   ::=
     BEGIN
           TYPE NOTATION   ::=     Parameter
           VALUE NOTATION  ::=     value (localValue INTEGER)
           Parameter       ::=     "PARAMETER" NamedType | empty
           NamedType       ::=     identifier type | type
     END
   END


            Figure 7:  ES Remote Operation Notation

   macro is mapped on the Result parameter of that service.

   If an operation was not successfully performed, the reply is mapped
   on the ESRO-ERROR service.

   In this case one of the errors in the Identifier List of Error Names
   in the ERROR clause of the OPERATION macro may be applied.  The value
   assigned to the applied error is mapped onto the Error parameter of
   that service.  The value of the Named-Type in the PARAMETER clause of
   the ERROR macro of the applied error is mapped on the Error-
   parameter of that service.

4  REMOTE OPERATIONS PROTOCOL

4.1  Overview of the Protocol

   The ESROS protocol realizes the services defined in the section
   entitled ESROS Service Definitions.  Short operations are performed
   in a highly efficient manner.  The protocol operation is summarized
   below and is described in detail in the following sections.





Banan, et. al                Informational                     [Page 23]

RFC 2188                          ESRO                    September 1997


   Two Functional Units are defined which realize the services with 2-
   Way handshake and 3-Way handshake, called 2-Way Handshake Functional
   Unit and 3-Way Handshake Functional Unit respectively.

   The procedures specified in this section refer to Protocol Data Units
   (PDUs) which are defined in Section 4.4.

4.1.1  Service Provision (Invoker User)

      o An ESROS user binds to an ESRO Service Access Point (SAP) and
        specifies whether 3-Way or 2-Way handshake Functional Unit is to
        be associated with the SAP.

      o An ESROS user initiates the transfer of a PDU using the INVOKE
        service.

      o On receipt of an ESROS-INVOKE.request service primitive from the
        ESROS user:

            -- The ESROS provider generates an Invoke ID,

            -- Communicates the Invoke-ID to the invoker of the service
                   through the ESROS-INVOKE-P.confirm primitive,

4.1.2  Service Provision (Performer User)

      o An ESROS user binds to an ESRO Service Access Point (SAP) and
        specifies whether 3-Way or 2-Way handshake Functional Unit is to
        be associated with the SAP.

      o On receipt of an ESRO-INVOKE-PDU, the ESROS provider issues an
        ESROS-INVOKE.indication to the ESROS performer user.

      o On receipt of ESROS-RESULT.request or ESROS-ERROR.request from
        the performer, the provider creates the ESRO-RESULT-PDU or
        ESRO-ERROR-PDU.

      o In the case that the provider receives an ESRO-ACK-PDU for the
        transmitted ESRO-RESULT-PDU or ESRO-ERROR-PDU, if the
        corresponding SAP is associated with the 3-Way Handshake
        Functional Unit, it passes an ESROS-RESULT.confirm or ESROS-
        ERROR.confirm to the performer user.  If the corresponding SAP
        is associated with the 2-Way handshake Functional Unit, the
        ESRO-ACK-PDU is dropped as an invalid PDU.

      o In the case that the provider is not able to deliver the
        ESRO-RESULT-PDU or ESRO-ERROR-PDU, it issues an ESROS-
        FAILURE.indication to the performer user.  In the case that the



Banan, et. al                Informational                     [Page 24]

RFC 2188                          ESRO                    September 1997


        performer's SAP is associated with the 3-Way handshake
        Functional Unit and provider doesn't receive the ESRO-ACK-PDU
        for a transmitted ESRO-RESULT-PDU or an ESRO-ERROR-PDU, it
        passes an ESROS- FAILURE.indication to the performer user.

      o In the case that the performer's SAP is associated with the
        3-Way handshake Functional Unit and provider receives an ESRO-
        ACK-PDU for the operation, it passes an ESROS-RESULT.confirm or
        ESROS-ERROR.confirm.  In the case that the performer's SAP is
        associated with a 2-Way handshake Functional Unit and provider
        doesn't receive duplicate ESROS-INVOKE-PDUs from the invoker, it
        passes an ESROS-RESULT.confirm or ESROS-ERROR.confirm.

      o On receipt of an ESRO-FAILURE-PDU, the ESROS provider issues an
        ESROS-FAILURE.indication to the ESROS performer user.

4.2  Protocol Procedures

4.2.1  Service Access Point (SAP) Bind Procedure

   To access the ESRO Services, an ESROS user binds to an ESRO Service
   Access Point and specifies the SAP to be associated with 3-Way
   handshake Functional Unit or 2-Way handshake Functional Unit.  ESROS
   provider generates a SAP descriptor which is passed to the user.  The
   handshaking for all Invoke.requests addressed to that SAP and all
   PDUs addressed to that SAP will be either 3-Way or 2-Way based on the
   Functional Unit associated with SAP and specified by user at SAP bind
   time.

   It is the responsibility of the ESROS peer users (invoker and
   performer) to address their operations to the appropriate SAP (3-Way
   or 2-Way) based on the agreement between users.

4.2.2  Invoke Service Procedure

   An ESROS user initiates the transfer of a PDU using the INVOKE
   service.

   On receipt of an ESRO-INVOKE-PDU, the ESROS provider sends an ESROS-
   INVOKE.indication primitive to the ESROS performer user.

4.2.3  Invoke ID Assignment Procedure

   On receipt of an ESROS-INVOKE.request primitive from the ESROS user,
   the ESROS provider generates two invoke identifiers:






Banan, et. al                Informational                     [Page 25]

RFC 2188                          ESRO                    September 1997


      o Invoke-Reference-Number:  Uniquely identifies the invocation
        between the two peers.  This is a PDU field with a length of 8
        bits (see section 4.4).

      o Invoke-ID-Parameter:  Uniquely identifies the invocation to the
        service user.  This Invoke-ID-Parameter is a combination of the
        Invoke-Reference-Number described above and the invoker address,
        performer address, and the SAP Selector.

   The provider communicates the Invoke-ID-Parameter to the invoker of
   the INVOKE service through the ESROS-INVOKE-P.confirm primitive.

   The Invoke-Reference-Number distinguishes several invocations of the
   service in progress (asynchronous operations).  It is also used as
   segment identifier when a Service Data Unit (SDU) is transferred
   using segmentation and reassembly.  The ESROS provider may begin to
   reuse the Invoke-Reference-Number values whenever it chooses, subject
   to the constraint that it may not reuse an Invoke-Reference-Number
   value that was previously assigned to an invocation of the service
   for which it expects, but has not yet received, a reply.  In other
   words the provider does not reuse a previously used Invoke-
   Reference-Number unless the corresponding service is fully completed.
   The same value of the Invoke-Reference-Number can be reused to
   identify the invocation between different peer entities.  In that
   case, the combination of the peer entity's address and the Invoke-
   Reference-Number guarantees unique identification of each invocation.

4.2.4  Functional Unit Selection Procedure

   When an ESRO Services user binds to an ESRO SAP, it associates its
   SAP descriptor to 3-Way Handshake Functional Unit or 2-Way Handshake
   Functional Unit.

   Based on the Functional Unit associated with SAP, provider selects
   the corresponding Functional Unit for all Invoke Requests or PDUs
   addressed to that SAP.

4.3  Connectionless PDU Transfer For Small PDUs

4.3.1  Overview

   PDUs sent by UDP use port ESRO_CL_PORT. PDUs carried by UDP are
   restricted to CLRO_SMALL_PDU_MAX_SIZE bytes (see 4.6.1)

   Each PDU is encapsulated in a single UDP datagram.






Banan, et. al                Informational                     [Page 26]

RFC 2188                          ESRO                    September 1997


   For PDUs larger than CLRO_SMALL_PDU_MAX_SIZE but smaller than
   CLRO_SEGMENTED_PDU_MAX_SIZE bytes (see 4.6.1), segmentation and
   reassembly is used and each segment is transmitted in a UDP datagram.

   PDUs sent using UDP may be lost, and hence a retransmission strategy
   is defined.  When a PDU is segmented, the retransmission strategy is
   not applied to individual segments (i.e., loss of one segment results
   in retransmission of the whole SDU).

   The optimal UDP retransmission policy will vary with the performance
   of the network and the needs of the transmitter, but the following
   are considered:

   The retransmission interval should be based on prior statistics if
   possible.  Too aggressive retransmission can easily slow response
   time of the network at large.  Depending on how well connected the
   invoker is to its performer, the minimum retransmission interval
   should be RETRANSMISSION_INTERVAL (see 4.6.2) seconds.

   Delivery of PDUs is asynchronous which means the ESROS does not wait
   for the result of a transmitted PDU and continues delivering the next
   PDUs.

       ______________________________________________________
       |From Idle to:                      |Event            |
       |___________________________________|_________________|
       |CL-Invoker Transition Diagram      |ESRO-INVOKE.req  |
       | 2-way Handshake (Connectionless)  |                 |
       |___________________________________|_________________|
       |CL-Invoker Transition Diagram      |ESRO-INVOKE.req  |
       | 3-way Handshake (Connectionless)  |                 |
       |___________________________________|_________________|
       |CL-Performer Transition Diagram    |INVOKE-PDU       |
       | 3-way Handshake (Connectionless)  |                 |
       |___________________________________|_________________|
       |CL-Performer Transition Diagram    |INVOKE-PDU       |
       | 2-way Handshake  (Connectionless) |                 |
       |___________________________________|_________________|

                Table 10:  ESROS Finite State Machine




   This section describes the ESROS protocols in terms of state
   diagrams.  The ESROS Finite State Machine is expressed as four
   separate transition diagrams.  This is illustrated in Table 10.




Banan, et. al                Informational                     [Page 27]

RFC 2188                          ESRO                    September 1997


   Details of each of the two transition diagrams for connectionless
   transmission and different handshakings are described in the
   following sections.  The state diagrams show the state, the events,
   the actions taken and the resultant state.The ESROS state transition
   diagrams for connectionless data transmission are presented in Table
   11, Table 12, Table 13, and Table 14.

   Transitions are identified by numbers on the state diagrams.  The
   corresponding actions are listed next to each table.

4.3.2  3-Way Handshake Functional Unit

   This unit implements the Acknowledged Result model of ESRO Services.
   3-Way handshaking is used in this unit.

   The RESULT.confirm and ERROR.confirm primitives on performer are
   generated when ESRO-ACK-PDU is received.

   The FAILURE.indication on performer side is resulted from remote or
   local failures.  Not receiving ESRO-ACK-PDU or local failure can
   generate FAILURE.indication primitive.

   The FAILURE.indication on invoker side is generated if a local
   failure happens or a ESRO-FAILURE-PDU is received.



























Banan, et. al                Informational                     [Page 28]

RFC 2188                          ESRO                    September 1997


   _______________________________________________________________
   |      State      |STA01     |STA02      |STA03    |STA04     |
   |                 |CL Invoker|Invoke PDU |ACK-PDU  |Invoker   |
   |Event            |Start     |Send       |Send     |RefNu Wait|
   |-----------------+----------+-----------+---------+----------+
   |U: INVOKE.request|(1) STA02 |           |         |          |
   |-----------------+----------+-----------+---------+----------+
   |T: INVOKE PDU    |          |(2) STA02  |         |          |
   | Retransmit      |          |           |         |          |
   |-----------------+----------+-----------+---------+----------+
   |T: Last Timer    |          |(3) STA04  |         |          |
   |-----------------+----------+-----------+---------+----------+
   |P: Result-PDU    |          |           |         |(9) STA04 |
   |-----------------+----------+-----------+---------+----------+
   |P: Failure-PDU   |          |(5) STA04  |         |          |
   |-----------------+----------+-----------+---------+----------+
   |P: ACK-PDU       |          |(6) STA02  |         |          |
   | (Hold On)       |          |           |         |          |
   |-----------------+----------+-----------+---------+----------+
   |P: Duplicate     |          |           |(7) STA03|          |
   |  Result-PDU     |          |           |         |          |
   |-----------------+----------+-----------+---------+----------+
   |T: RefNu Timer   |          |           |         |(8) STA01 |
   |-----------------+----------+-----------+---------+----------+
   |P: Result-PDU    |          |(4) STA03  |         |          |
   |-----------------+----------+-----------+---------+----------+
   |T: Inactivity    |          |           |(10)     |          |
   | Timer           |          |           |STA04    |          |
   |_________________|__________|___________|_________|__________|



   Table 11:  ESROS State Transition Diagram-Connectionless Transmission,
   3-Way HS. P = Protocol, T = Timer, U = User, I = Internal.

   The transmission of INVOKE, RESULT, and ERROR SDUs can be in a single
   PDU (when it fits in one UDP) or a sequence of segment PDUs.


   3-Way Handshake Connectionless Transmission:  Invoker


   For each transition number in the state diagram Table 11, the
   corresponding actions are listed below:

    1. INVOKE.request:

      o Assign Invoke-ID.



Banan, et. al                Informational                     [Page 29]

RFC 2188                          ESRO                    September 1997


      o Issue ESROS-INVOKE-P.confirm primitive.

      o Assign invoke reference number.

      o Send operation in one ESRO-INVOKE-PDU or in segmented INVOKE-
        PDUs depending on the size of the operation.

      o Initialize retransmission counter.

      o Initialize retransmission timer.

    2. Invoke PDU Retransmit:

      o Retransmit operation in one ESRO-INVOKE-PDU or segmented PDUs
        while number of retransmissions is less than
        MAX_RETRANSMISSIONS.

      o Increment the retransmission counter.  When MAX_RETRANSMISSIONS
        reached, start LAST_TIMER, otherwise initialize retransmission
        timer.

    3. Last Timer:

      o Issue ESROS-FAILURE.indication primitive.

      o Initialize reference number timer.


    4. ESRO-RESULT-PDU or ESRO-ERROR-PDU (or reassembled ESRO-
       RESULT-SEGMENTED-PDU or ESRO-ERROR-SEGMENTED-PDU when the PDU is
       received in segmented format):

      o Send ESRO-ACK-PDU.

      o Issue ESROS-RESULT.indication or ESROS-ERROR.indication
        primitive.

      o Initialize inactivity timer.

    5. ESRO-FAILURE-PDU:

      o Issue ESROS-FAILURE.indication primitive with User not
        Responding failure cause.

      o Initialize reference number timer.






Banan, et. al                Informational                     [Page 30]

RFC 2188                          ESRO                    September 1997


    6. ESRO-ACK-PDU (Hold on):

      o For future use (no action).

    7. Duplicate ESRO-RESULT-PDU or ESRO-ERROR-PDU:

      o Initialize inactivity timer (Ignore PDU).

      o Send ESRO-ACK-PDU.

    8. Invoke reference number timer:

      o Release the invoke reference number.

    9. ESRO-RESULT-PDU or ESRO-ERROR-PDU:

      o Reset Invoke reference number timer.

    10. Inactivity timer:

      o Initialize reference number timer.

   On receipt of an ESROS-INVOKE.request, ESROS provider generates an
   Invoke- Reference-Number and an Invoke-ID (see Section 4.2.3).  The
   provider issues an ESROS-INVOKE-P.confirm primitive and passes the
   Invoke-ID to the invoker.

   The ESROS provider initiates the timer for the Invoke-ID and
   transmits the PDU. Based on the size of SDU, if segmentation is
   required, the SDU is segmented and transmitted in a sequence of
   segmented PDUs.  If the ESRO-RESULT-PDU or ESRO-ERROR-PDU associated
   with the invoke ID is not received within the
   INVOKE_PDU_RETRANSMISSION_INTERVAL (see 4.6.2) period, the SDU is
   retransmitted (in one PDU or segmented and transmitted in a sequence
   of segment PDUs).  The retransmission is repeated for a maximum of
   MAX_RETRANSMISSIONS unless an ESRO-RESULT-PDU or ESRO-ERROR-PDU is
   received.

   If the ESRO-RESULT-PDU or ESRO-ERROR-PDU is received in a segmented
   format, the reassembly process reassembles the sequence of segment
   PDUs.

   In the case that the Hold-on ESRO-ACK-PDU is received from the
   performer, the provider stops retransmitting the ESRO-INVOKE-PDU and
   waits for the ESRO- RESULT-PDU or ESRO-ERROR-PDU for a period equal
   to the multiplication of INVOKE_PDU_RETRANSMISSION_INTERVAL (see
   4.6.2) and MAX_RETRANSMISSIONS (see 4.6.2, for future use).




Banan, et. al                Informational                     [Page 31]

RFC 2188                          ESRO                    September 1997


   In the case that the ESRO-INVOKE-PDU is sent MAX_RETRANSMISSIONS (see
   4.6.2) times and no ESRO-RESULT-PDU or ESRO-ERROR-PDU is received,
   the ESROS provider sends an ESROS-FAILURE.indication primitive, with
   the Invoke-ID of the failed PDU and the Failure-value as parameters,
   to the invoker.

   When an ESRO-RESULT-PDU or ESRO-ERROR-PDU is received (whether in one
   PDU or reassembled from a sequence of segmented PDUs), the provider
   issues an ESROS-RESULT.indication or ESROS-ERROR.indication to the
   invoker user, sends an ESRO-ACK-PDU and initializes the inactivity
   timer.  In the case that duplicate ESRO- RESULT-PDU or ESRO-ERROR-PDU

   ____________________________________________________________________
   |      State      |STA01        |STA02      |STA03     |STA04      |
   |                 |CL Performer |Invoke PDU |ACK-PDU   |Performer  |
   |Event            |Start        |Received   |Wait      |RefNu Wait |
   |-----------------+-------------+-----------+----------+-----------|
   |P: Invoke-PDU    |(1) STA02    |           |          |           |
   |-----------------+-------------+-----------+----------+-----------|
   |U: RESULT.req.   |             |(2) STA03  |          |           |
   |-----------------+-------------+-----------+----------+-----------|
   |P: ACK-PDU       |             |           |(3) STA04 |           |
   |-----------------+-------------+-----------+----------+-----------|
   |P: Invoke-PDU    |             |(4) STA02  |(6) STA03 |(7) STA04  |
   | Duplicate       |             |           |          |           |
   |-----------------+-------------+-----------+----------+-----------|
   |T: Result-PDU    |             |           |(5) STA03 |           |
   | Retransmission  |             |           |          |           |
   | Timer           |             |           |          |           |
   |-----------------+-------------+-----------+----------+-----------|
   |I: Failure       |             |(8) STA01  |          |           |
   |-----------------+-------------+-----------+----------+-----------|
   |T: Last Time     |             |           |(9) STA04 |           |
   |-----------------+-------------+-----------+----------+-----------|
   |T: RefNu Timer   |             |           |          |(10) STA01 |
   |-----------------+-------------+-----------+----------+-----------|
   |P: ACK-PDU       |             |           |          |(11) STA04 |
   | Duplicate       |             |           |          |           |
   |-----------------+-------------+-----------+----------+-----------|
   |U/P: Hold On ACK |             |(12) STA02 |          |           |
   ____________________________________________________________________



   Table 12:  ESROS State Transition Diagram-Connectionless Transmission,
   3-Way HS: Performer.  P = Protocol, T = Timer, U = User, I = Internal.





Banan, et. al                Informational                     [Page 32]

RFC 2188                          ESRO                    September 1997


   are received, they are ignored, the inactivity timer is reset, and an
   ESRO-ACK-PDU is retransmitted.

   When no duplicate ESRO-RESULT-PDU or ESRO-ERROR-PDU is received for a
   period equal to INACTIVITY_TIME (see 4.6.2), or in the case of ESRO-
   INVOKE- PDU retransmission time-out, or in the case of internal
   failure, the provider initializes the reference number timer.  After
   REFERENCE_NUMBER_TIME (see 4.6.2), the reference number is released.

   3-Way Handshake Connectionless Transmission:  Performer

   For each transition number in the state diagram above, the
   corresponding actions are listed below:

     1. ESRO-INVOKE-PDU (as a single PDU or a sequence of segment PDUs):

      o Issue ESROS-INVOKE.indication primitive.

    2. ESROS-RESULT.request or ESROS-ERROR.request:

      o Add invoke reference number to the active list.

      o Transmit ESRO-RESULT-PDU or ESRO-ERROR-PDU (in a single PDU or
        a sequence of segment PDUs).

      o Set ESRO-RESULT-PDU or ESRO-ERROR-PDU retransmission timer.

    3. ESRO-ACK-PDU:

      o Initialize invoke reference number timer.

      o Issue ESROS-RESULT.confirm or ESROS-ERROR.confirm.

    4. Duplicate ESRO-INVOKE-PDU:

      o No action (ignore the duplicate ESRO-INVOKE-PDU).

    5. ESRO-RESULT-PDU or ESRO-ERROR-PDU retransmission timer:

      o Retransmit ESRO-RESULT-PDU or ESRO-ERROR-PDU (in a single PDU
        or in a segmented format) while number of retransmissions is
        less than MAX_RETRANSMISSIONS.

      o Increment the transmission counter.







Banan, et. al                Informational                     [Page 33]

RFC 2188                          ESRO                    September 1997


    6. Duplicate ESRO-INVOKE-PDU:

      o Retransmit ESRO-RESULT-PDU or ESRO-ERROR-PDU.

      o Reset ESRO-RESULT-PDU or ESRO-ERROR-PDU retransmission timer.

      o Re-initialize the number of retransmissions counter to 1.

    7. Duplicate ESRO-INVOKE-PDU:

      o Reset invoke reference number timer.

    8. Internal failure:

      o Send ESRO-FAILURE-PDU.

      o Release the invoke reference number.

    9. Last time:

      o Issue ESROS-FAILURE.indication.

      o Initialize invoke reference number timer.

    10. Invoke reference number timer:

      o Release the invoke reference number.

    11. Duplicate ESRO-ACK-PDU:

      o Reset invoke reference number timer.

    12. Hold-on ACK request:

      o Send hold-on ESRO-ACK-PDU (for future use).

   On receipt of an ESRO-INVOKE-PDU, the ESROS provider issues an
   ESROS-INVOKE.indication to the ESROS performer user.  The provider
   ignores the duplicate ESRO-INVOKE-PDUs.

   In the case of internal failure or no response from performer user,
   the provider sends an ESRO-FAILURE-PDU and releases the invoke
   reference number.

   On receipt of a Hold-on request from the performer user, or based on
   other information, provider sends a Hold-on ESRO-ACK-PDU (future
   use).




Banan, et. al                Informational                     [Page 34]

RFC 2188                          ESRO                    September 1997


   On receipt of either ESROS-RESULT.request or ESROS-ERROR.request from
   the ESROS performer user, the ESROS provider initiates the
   retransmission timer for the ESRO-RESULT-PDU or ESRO-ERROR-PDU and
   transmits the ESRO-RESULT-PDU or ESRO-ERROR-PDU in a single PDU or in
   a sequence of segment PDUs.  If the ESRO-ACK-PDU associated with the
   Invoke-ID is not received within
   RESULT_ERROR_PDU_RETRANSMISSION_INTERVAL (see 4.6.2), the PDU is
   retransmitted.

   When provider is waiting for ESRO-ACK-PDU and a duplicate ESRO-
   INVOKE-PDU arrives, ESRO-RESULT-PDU or ESRO-ERROR-PDU is
   retransmitted (in a single PDU or in a sequence of segment PDUs), the
   retransmission timer is reset and counter for number of
   retransmissions is re-initialized to 1.

   If after MAX_TRANSMISSIONS (see 4.6.2) no ESRO-ACK-PDU is received,
   the provider issues an ESROS-FAILURE.indication primitive, with the
   Invoke-ID of the failed PDU and the Failure-value as parameters, to
   the performer user.  Then the provider sets the reference number
   timer and releases the reference number after REFERENCE_NUMBER_TIME
   (see 4.6.2).

   On receipt of ESRO-ACK-PDU associated with the Invoke-ID before
   MAX_TRANSMISSIONS (see 4.6.2), the provider issues a ESROS-
   RESULT.confirm or ESROS-ERROR.confirm primitive and sets the
   reference number timer and releases the reference number after
   REFERENCE_NUMBER_TIME (see 4.6.2).

   The duplicate ESRO-ACK-PDU and duplicate ESRO-INVOKE-PDUs are ignored
   while provider waits for the reference number timer to expire.

4.3.3  2-Way Handshake Functional Unit

   This Functional Unit implements the Not-Acknowledged Result model of
   ESRO Services.  2-Way handshaking is used in this unit.

   The RESULT.confirm and ERROR.confirm primitives on performer side are
   generated based on time-out, i.e.  when no duplicate ESRO-INVOKE-PDU
   is received in a specified period of time, provider issues
   RESULT.confirm or ERROR.confirm primitive.

   The FAILURE.indication on performer side is generated as a result of
   local failure or after time-out of retransmission of ESRO-RESULT-PDU
   or ESRO-ERROR-PDU.

   The FAILURE.indication on invoker side is generated if a local
   failure happens or a ESRO-FAILURE-PDU is received.




Banan, et. al                Informational                     [Page 35]

RFC 2188                          ESRO                    September 1997


   The transmission of INVOKE, RESULT, and ERROR PDUs can be in a single
   PDU (when it fits in one PDU) or a sequence of segmented PDUs.


   2-Way Handshake Connectionless Transmission:  Invoker


   For each transition number in the state diagram above, the
   corresponding actions are listed below:

    1. INVOKE.request:

      o Assign Invoke-ID.

      o Issue ESROS-INVOKE-P.confirm primitive.

      o Assign invoke reference number.

      o Send ESRO-INVOKE-PDU in a single PDU or as a sequence of
        segment PDUs.

      o Initialize retransmission counter.

      ______________________________________________________________
      |      State              |STA01      |STA02      |STA03     |
      |                         |2-Way HS   |Invoke PDU |Invoker   |
      |                         |CL Invoker |Send       |RefNu Wait|
      |Event                    |Start      |           |          |
      |-------------------------+-----------+-----------+----------|
      |U: INVOKE.req.           |(1) STA02  |           |          |
      |-------------------------+-----------+-----------+----------|
      |T: Invoke PDU Retransmit |           |(2) STA02  |          |
      |-------------------------+-----------+-----------+----------|
      |T: Last Timer            |           |(3) STA03  |          |
      |-------------------------+-----------+-----------+----------|
      |P: Result/Error PDU      |           |(4) STA03  |          |
      |-------------------------+-----------+-----------+----------|
      |P: Failure-PDU           |           |(5) STA03  |          |
      |-------------------------+-----------+-----------+----------|
      |P: Duplicate Result PDU  |           |           |(6) STA03 |
      |-------------------------+-----------+-----------+----------|
      |T: RefNu Timer           |           |           |(7) STA01 |
      |_________________________|___________|___________|__________|


      Table 13:  ESROS State Transition Diagram-Connectionless Transmission,
      2-Way HS: Invoker p = Protocol, T = Timer, U = User, I = Internal.




Banan, et. al                Informational                     [Page 36]

RFC 2188                          ESRO                    September 1997


    2. Invoke PDU Retransmit:

      o Retransmit ESRO-INVOKE-PDU (in a single PDU or in a sequence of
        segment PDUs) while number of retransmissions is less than
        MAX_RETRANSMISSIONS.

      o Increment the transmission counter.  When MAX_RETRANSMISSIONS
        reached, start LAST_TIMER.

    3. Last Timer:

      o Issue ESROS-FAILURE.indication primitive.

      o Initialize reference number timer.

    4. ESRO-RESULT-PDU or ESRO-ERROR-PDU:

      o Issue ESROS-RESULT.indication or ESROS-ERROR.indication
        primitive.

      o Initialize reference number timer.

    5. ESRO-FAILURE-PDU:

      o Issue ESROS-FAILURE.indication primitive with User not
        Responding failure cause.

      o Initialize reference number timer.

    6. Duplicate ESRO-RESULT-PDU or ESRO-ERROR-PDU:

      o Reset Invoke reference number timer.

    7. Invoke reference number timer:

      o Release the invoke reference number.

   On receipt of an ESROS-INVOKE.request, ESROS provider generates an
   Invoke- Reference-Number and an Invoke-ID (see 4.2.3).  The provider
   issues an ESROS-INVOKE-P.confirm primitive and passes the Invoke-ID
   to the invoker.

   The ESROS provider initiates the timer for the Invoke-ID and
   transmits the PDU. The PDU is transmitted as a single PDU or a
   sequence of segment PDUs.  If the ESRO- RESULT-PDU or ESRO-ERROR-PDU
   associated with the invoke ID is not received within the





Banan, et. al                Informational                     [Page 37]

RFC 2188                          ESRO                    September 1997


   INVOKE_PDU_RETRANSMISSION_INTERVAL (see 4.6.2) period, the PDU is
   retransmitted.  The retransmission is repeated for a maximum of
   MAX_RETRANSMISSIONS unless an ESRO-RESULT-PDU or ESRO-ERROR-PDU is
   received.

   In the case that the ESRO-INVOKE-PDU is sent MAX_RETRANSMISSIONS (see
   4.6.2) times and no ESRO-RESULT-PDU or ESRO-ERROR-PDU is received,
   the ESROS provider sends an ESROS-FAILURE.indication primitive, with
   the Invoke-ID of the failed PDU and the Failure-value as parameters,
   to the invoker.  If ESRO- FAILURE-PDU is received, the ESROS provider
   sends and ESROS-FAILURE.indication primitive, with the Invoke-Id of
   the failed PDU and the Failure- value as parameters to the invoker.

   When an ESRO-RESULT-PDU or ESRO-ERROR-PDU is received, the provider
   issues an ESROS-RESULT.indication or ESROS-ERROR.indication to the
   invoker user, and initializes the Reference-Number timer.  In the
   case that duplicate ESRO-RESULT-PDU or ESRO-ERROR-PDU are received,
   they are ignored.  In the case of internal failure, the provider
   initializes the reference number timer.  After REFERENCE_NUMBER_TIME
   (see 4.6.2), the reference number is released.


   2-Way Handshake Connectionless Transmission:  Performer

   ___________________________________________________________________
   |      State        |STA01      |STA02     |STA03     |STA04      |
   |                   |2-Way HS CL|Invoke PDU|Result PDU|Performer  |
   |Event              |Performer  |Received  |Retransmit|RefNu Wait |
   |                   |Start      |          |          |           |
   |-------------------+-----------+----------+----------+-----------|
   |P: Invoke-PDU      |(1) STA02  |          |          |           |
   |-------------------+-----------+----------+----------+-----------|
   |P: Invoke-PDU      |           |(2) STA02 |(5) STA03 |(7) STA04  |
   | Duplicate         |           |          |          |           |
   |-------------------+-----------+----------+----------+-----------|
   |U: RESULT.req.     |           |(3) STA03 |          |           |
   |-------------------+-----------+----------+----------+-----------|
   |I: Failure         |           |(4) STA01 |          |           |
   |-------------------+-----------+----------+----------+-----------|
   |T: Inactivity Timer|           |          |(6) STA04 |           |
   |-------------------+-----------+----------+----------+-----------|
   |T: RefNu Timer     |           |          |          |(8) STA01  |
   ___________________________________________________________________



   Table 14:  ESROS State Transition Diagram-Connectionless Transmission,
   2-Way HS: Performer.  P = Protocol, T = Timer, U = User, I = Internal.



Banan, et. al                Informational                     [Page 38]

RFC 2188                          ESRO                    September 1997


   For each transition number in the state diagram above, the
   corresponding actions are listed below:

    1. ESRO-INVOKE-PDU (received in a single PDU or reassembled from a
       sequence of segment PDUs):

      o Issue ESROS-INVOKE.indication primitive.

    2. Duplicate ESRO-INVOKE-PDU:

      o No action (ignore the duplicate ESRO-INVOKE-PDU).

    3. EROS-RESULT.request or ESROS-ERROR.request:

      o Add invoke reference number to the active list.

      o Transmit ESRO-RESULT-PDU or ESRO-ERROR-PDU (as a single PDU or
        as a sequence of segment PDUs.)

      o Set Inactivity timer.

    4. Internal failure:

      o Send ESRO-FAILURE-PDU.

      o Release the invoke reference number.

    5. Duplicate ESRO-INVOKE-PDU:

      o Retransmit ESRO-RESULT-PDU or ESRO-ERROR-PDU (as a single PDU
        or as a sequence of segment PDUs.)

      o Set Inactivity timer.

    6. Inactivity Timer:

      o Issue ESROS-RESULT.confirm.

      o Initialize invoke reference number timer.

    7. Dplicate ESRO-INVOKE-PDU:

      o Reset invoke reference number timer.

    8. Invoke reference number timer:

      o Release the invoke reference number.




Banan, et. al                Informational                     [Page 39]

RFC 2188                          ESRO                    September 1997


   On receipt of an ESRO-INVOKE-PDU (as a single PDU or reassembled from
   a sequence of segment PDUs), the ESROS provider issues an ESROS-
   INVOKE.indication to the ESROS performer user.  The provider ignores
   the duplicate ESRO-INVOKE-PDUs.

   In the case of internal failure or no response from performer user,
   the provider sends an ESRO-FAILURE-PDU and releases the invoke
   reference number.

   On receipt of either ESROS-RESULT.request or ESROS-ERROR.request from
   the ESROS performer user, the ESROS provider initiates the inactivity
   timer for the ESRO- RESULT-PDU or ESRO-ERROR-PDU and transmits the
   ESRO-RESULT-PDU or ESRO-ERROR-PDU (in a single PDU or as a sequence
   of segment PDUs.)  If a duplicate ESRO-INVOKE-PDU associated with the
   Invoke-ID is received within INACTIVITY_TIME interval (see 4.6.2),
   the PDU is retransmitted.

   If no duplicate ESRO-INVOKE-PDU is received within the
   INACTIVITY_TIME interval (see 4.6.2), provider issues a ESROS-
   RESULT.confirm or ESROS-ERROR.confirm primitive and sets the
   reference number timer and releases the reference number after
   REFERENCE_NUMBER_TIME (see 4.6.2).

   The duplicate ESRO-INVOKE-PDUs are ignored while provider waits for
   the reference number timer to expire.

4.3.4  Segmentation and Reassembly

   Small ESRO Service Data Units (ESRO-SDUs) can benefit from the
   efficiencies of connectionless feature of ESROS (See Section 4.3.1).

   When an ESRO-SDU is too large to fit in a single connectionless PDU
   it is segmented and reassembled.  There might be similar mechanisms
   in the upper layers with different levels of efficiency.  When in
   addition to the ESROS segmentation/reassembly, the upper layers are
   capable of segmentation/reassembly services, then the ESROS user can
   decide whether to use ESROS segmenting/reassembly mechanism depending
   on the factors such as reliability of the underlying network.

   In the case of segmentation/reassembly in ESROS layer, transmission
   of operation segments is not acknowledged.  This results in an
   efficient transmission over a reliable underlying network.  However
   failure of one segment results in retransmission of all segments.

   When acknowledged segments are desired, the ESROS user should
   implement it using the acknowledged result service of ESROS.





Banan, et. al                Informational                     [Page 40]

RFC 2188                          ESRO                    September 1997


   The ESROS segmentation/reassembly is accommodated by:

      o Use of two additional PDU codes for segmented INVOKE PDU.

      o Use of one byte segmentation information, which contains
        First/Other flag and segment number.

      o Use of unused bits of RESULT and ERROR PDUs to identify a
        segmented RESULT or ERROR PDU.

   Segmentation and Assembly applies to INVOKE, RESULT, and ERROR SDUs.

   The sender of the message is responsible for segmenting the ESRO-SDU
   into segments that fit in CL PDUs.  The segmented ESRO-SDU is sent in
   a sequence of segments each carrying a segment of the SDU. The
   Invoke-Reference-Number is a unique identifier that is used as the
   segment identifier which relates all segments of an ESRO-SDU. In
   addition to this identifier, the first segment specifies the total
   number of segments (number-of-segments).  Other segments have a
   segment sequence number (segment- number).  The receiver is
   responsible for sequencing (based on segment-number) and reassembling
   the entire ESRO-SDU.

   Segmenting/Reassembling over the Connectionless ESRO Service

   The sender maps the original ESRO-SDU into an ordered sequence of
   segments.  Several ESRO-SDU segment sequences can exist over the same
   ESROS association, distinguished by their Invoke-Reference-Number
   (used as segment identifier.)

   All segments in the sequence have the same Invoke-Reference-Number
   assigned by sender.

   The first segment specifies the total number of segments.  All
   segments in the sequence except the first one shall be sequentially
   numbered, starting at 1 (first segment has an implicit segment number
   of 0).

   Each segment is transmitted in one UDP PDU and is sent by sender.
   All segments of a segmented ESRO-SDU are identified by the same
   Invoke-Reference-Number.  For a given operation, the receiver should
   not impose any restrictions on the order of arrival of segments.

   There is no requirement that any segment content be of
   CLRO_SMALL_PDU_MAX_SIZE for connectionless transmission; however, no
   more than CLRO_MAX_PDU_SEGMENTS segments can be derived from a single
   ESRO-SDU.




Banan, et. al                Informational                     [Page 41]

RFC 2188                          ESRO                    September 1997


   The receiver reassembles a sequence of segments into a single ESRO-
   SDU. An ESRO-SDU shall not be further processed unless all segments
   of the ESRO-SDU are received.  Failure to receive the SDU shall be
   determined by the following event:

      o Expiration of Reassembly Timer (see Section 4.3.4).

   In the event of the above mentioned failure, the receiver shall
   discard a partially assembled sequence.

   The reassembly is done as described below:

      o In the case of segmented Invoke ESRO-SDU, the encoding type and
        operation-value fields are carried in the first segment used for
        the whole operation.  These three fields are ignored in the
        segments other than the first one.

      o In the case of segmented Result ESRO-SDU, the encoding type of
        the first segment is used for all segments.  The encoding type
        field of segments other than the first one are ignored.

      o In the case of segmented Error ESRO-SDU, the encoding type and
        Error-value field of the first segment are used for all
        segments.  These two fields are ignored in segments other than
        the first one.

   Sender sends all segments of a segmented ESRO-SDU one after the
   other.  There is no mechanism for retransmission of a single segment.
   In the case that the sender receives a failure indication for a
   segment, it means that receiver has failed in reassembly process, and
   the sender retransmits the whole ESRO-SDU (all segments).


   Reassembly Timer


   The Reassembly Timer is a local timer maintained by the receiver of
   the segments that assists in performing the reassembly function.
   This timer determines how long a receiver waits to receive all
   segments of a segment sequence.

   The Reassembly Timer shall be started on receipt of a segment with
   different sequence identifier (Invoke-Reference-Number).  On receipt
   of all segments composing a sequence, the corresponding reassembly
   timer shall be stopped.






Banan, et. al                Informational                     [Page 42]

RFC 2188                          ESRO                    September 1997


   The value of the Reassembly Timer is defined based on the network
   characteristics and the number of segments.  This requires that the
   transmission of all segments of a single ESRO-SDU must be completed
   within this time limit.

4.4  Structure and Encoding of ESROS PDUs

   Five PDU types are used in the ESRO protocol which are described in
   the following sections.  PDU type coding is presented in Table 15.

   The octets are numbered in increasing order, starting from 1.  The
   bits of an octet are numbered from 1 to 8, where 1 is the low-order
   bit.

4.4.1  ESRO-INVOKE-PDU Format

   Bit string format of the ESRO-INVOKE-PDU is represented in Table 16
   and Table 17.



           _______________________________________
           | PDU Name              |PDU Type Code |
           _______________________________________
           | ESRO-INVOKE           |0             |
           | ESRO-RESULT           |1             |
           | ESRO-ERROR            |2             |
           | ESRO-ACK              |3             |
           | ESRO-FAILURE          |4             |
           | ESRO-SEGMENTED-INVOKE |5             |
           _______________________________________




                   Table 15:  PDU Coding















Banan, et. al                Informational                     [Page 43]

RFC 2188                          ESRO                    September 1997


       __________________________________________________________
       |Bit       |8 |            7           |6 |5 |4 |3 |2 |1 |
       |----------+--+------------------------+--+--+--+--+--+--|
       |Octet 1   | Performer SAP                   |0 |0 |0 |0 |
       |----------+--+------------------------+--+--+--+--+--+--|
       |Octet 2   | Invoke Reference Number                     |
       |----------+--+------------------------+--+--+--+--+--+--|
       |Octet 3   |Parameter Encoding Type    |Operation Value  |
       |----------+--+------------------------+--+--+--+--+--+--|
       |Octet 4   |                                             |
       | ...      | Operation Information                       |
       |Octet N   |                                             |
       |__________|_____________________________________________|




          Table 16:  ESRO-INVOKE-PDU format. ESRO-INVOKE-PDU Type Code =
          0.  Note:  Invoker SAP = Performer SAP - 1.




                         _______________________
                         | Value |   Meaning    |
                         |_______|______________|
                         | 0     |   BER [5]    |
                         |_______|______________|
                         | 1     |   PER [4]    |
                         |_______|______________|
                         | 2     |   XDR [8]    |
                         |_______|______________|
                         | 3     |  Reserved    |
                         |_______|______________|





         Table 17:  Parameter Encoding Type for ESRO-INVOKE-PDU











Banan, et. al                Informational                     [Page 44]

RFC 2188                          ESRO                    September 1997


          ---------------------------------------------------------
         |Bit     |  8  |  7  |  6  |  5  |  4  |  3  |  2  |  1  |
         |--------|-----------|-----|-----|-----|-----|-----|-----|
         |        | Parameter |     |     |     |     |     |     |
         |        | Encoding  |     |     |     |     |     |     |
         |Octet 1 | Type      |  0  |  0  |  0  |  0  | 0   |  1  |
         |        |           |     |     |     |     |     |     |
         |        |           |     |     |     |     |     |     |
         |        |           |     |     |     |     |     |     |
         |--------|-----------------------------------------------|
         |Octet 2 |           Invoke Reference Number             |
         |--------|-----------------------------------------------|
         |Octet 3 |                                               |
         |...     |             Result-parameter                  |
         |Octet N |                                               |
          ---------------------------------------------------------
         ESRO-RESULT-PDU Type Code = 1.


                         Table 18:  ESRO-RESULT-PDU format



                         _______________________
                         | Value |   Meaning    |
                         |_______|______________|
                         | 0     |    Basic     |
                         |_______|______________|
                         | 1     |   Packed     |
                         |_______|______________|
                         | 2     |     XDR      |
                         |_______|______________|
                         | 3     |  Reserved    |
                         |_______|______________|



          Table 19:  Parameter Encoding Type for ESRO-RESULT-PDU




4.4.2  ESRO-RESULT-PDU Format

   Bit string format of the ESRO-RESULT-PDU is represented in Table 18
   and Table 19.





Banan, et. al                Informational                     [Page 45]

RFC 2188                          ESRO                    September 1997


4.4.3  ESRO-ERROR-PDU Format

   Bit string format of the ESRO-ERROR-PDU is represented in Table 20
   and Table 21.

          ---------------------------------------------------------
         |Bit     |  8  |  7  |  6  |  5  |  4  |  3  |  2  |  1  |
         |--------|-----------|-----|-----|-----|-----|-----|-----|
         |        | Parameter |     |     |     |     |     |     |
         |        | Encoding  |     |     |     |     |     |     |
         |Octet 1 | Type      |  0  |  0  |  0  |  0  | 1   |  0  |
         |        |           |     |     |     |     |     |     |
         |        |           |     |     |     |     |     |     |
         |        |           |     |     |     |     |     |     |
         |--------|-----------------------------------------------|
         |Octet 2 |           Invoke Reference Number             |
         |--------|-----------------------------------------------|
         |Octet 3 |               Error Value                     |
         |--------|-----------------------------------------------|
         |Octet 4 |                                               |
         |...     |             Error parameter                   |
         |Octet N |                                               |
          ---------------------------------------------------------
         ESRO-ERROR-PDU Type Code = 2.


                          Table 20:  ESRO-ERROR-PDU format


                         _______________________
                         | Value |   Meaning    |
                         |_______|______________|
                         | 0     |    Basic     |
                         |_______|______________|
                         | 1     |   Packed     |
                         |_______|______________|
                         | 2     |     XDR      |
                         |_______|______________|
                         | 3     |  Reserved    |
                         |_______|______________|


               Table 21:  Parameter Encoding Type for ESRO-ERROR-PDU








Banan, et. al                Informational                     [Page 46]

RFC 2188                          ESRO                    September 1997


             ---------------------------------------------------------
            |Bit     |  8  |  7  |  6  |  5  |  4  |  3  |  2  |  1  |
            |--------|-----------------------|-----|-----|-----|-----|
            |        | ESRO-ACK-PDU Type     |     |     |     |     |
            |Octet 1 |                       |  0  |  0  | 1   |  1  |
            |--------|-----------------------------------------------|
            |Octet 2 |           Invoke Reference Number             |
             ---------------------------------------------------------
             ESRO-ACK-PDU Type Code = 3.


                            Table 22:  Fields of ESRO-ACK-PDU


               _____________________________________________
               | ESRO-ACK-PDU Type |        Meaning         |
               |___________________|________________________|
               | 0                 |Complete 3-way handshake|
               |___________________|________________________|
               | 1                 |        Hold on         |
               |___________________|________________________|



                  Table 23:  Encoding of ESRO-ACK-PDU Type




4.4.4  ESRO-ACK-PDU Format

   Bit string format of the ESRO-ACK-PDU is represented in Table 22 and
   Table 23.

4.4.5  ESRO-FAILURE-PDU Format

   Bit string format of the ESROS-FAILURE-PDU is represented in Table 24
   and Table 25.

   The first nibble of the first octet of ESRO-FAILURE-PDU shall be set
   to zero.










Banan, et. al                Informational                     [Page 47]

RFC 2188                          ESRO                    September 1997


4.4.6  ESRO-INVOKE-SEGMENTED-PDU Format

   Bit string format of the ESRO-INVOKE-SEGMENTED-PDU is represented in
   Table 25 and Table 26.

   Note:  Invoker SAP = Performer SAP - 1.


      ---------------------------------------------------------
     |Bit     |  8  |  7  |  6  |  5  |  4  |  3  |  2  |  1  |
     |--------|-----------------------|-----|-----|-----|-----|
     |Octet 1 |         Not used      |  0  |  1  |  0  |  0  |
     |--------|-----------------------------------------------|
     |Octet 2 |           Invoke Reference Number             |
     |--------|-----------------------------------------------|
     |Octet 3 |                Failure Value                  |
     ----------------------------------------------------------
     ESRO-FAILURE-PDU Type Code = 4.



                     Table 24:  ESRO-FAILURE-PDU format


               ________________________________________
               | Failure Value |       Meaning         |
               |_______________|_______________________|
               | 0             |Transmission failure   |
               |_______________|_______________________|
               | 1             |Out of local resources |
               |_______________|_______________________|
               | 2             | User not responding   |
               |_______________|_______________________|
               | 3             |Out of remote resources|
               |_______________|_______________________|



                 Table 25:  Encoding of failure value












Banan, et. al                Informational                     [Page 48]

RFC 2188                          ESRO                    September 1997


      -----------------------------------------------------------------
     |Bit     |  8   |  7   |  6   |  5   |  4   |  3   |  2   |  1   |
     |--------|---------------------------|------|------|------|------|
     |Octet 1 | Performer Service         |  0   |  1   |  0   |  1   |
     |        | Access Point Selector     |      |      |      |      |
     |--------|-------------------------------------------------------|
     |Octet 2 |            Invoke Reference Number                    |
     |--------|-------------------------------------------------------|
     |        | Parameter   |                                         |
     |        | Encoding    |                                         |
     |Octet 3 | Type        |        Operation Value                  |
     |        |             |                                         |
     |        |             |                                         |
     |        |             |                                         |
     |--------|-------------------------------------------------------|
     |Octet 4 |First/|                                                |
     |        |Other |                  Segment Number                |
     |--------|-------------------------------------------------------|
     |Octet 5 |                                                       |
     |...     |             Operation Information                     |
     |Octet N |                                                       |
      ----------------------------------------------------------------|
      ESRO-INVOKE-PDU Type Code = 5.


                Table 26:  ESRO-INVOKE-SEGMENTED-PDU format


                          _______________________
                          | Value |   Meaning    |
                          |_______|______________|
                          | 0     |    Basic     |
                          |_______|______________|
                          | 1     |   Packed     |
                          |_______|______________|
                          | 2     |     XDR      |
                          |_______|______________|
                          | 3     |  Reserved    |
                          |_______|______________|

     Table 27:  Parameter Encoding Type for ESRO-INVOKE-SEGMENTED-PDU

      o For the first segment, the first/other bit is set to one, and
        the segment number field contains the total number of segments.

      o For segments other than the first one, the first/other bit is
        set to zero, and the segment number field has the sequence
        number of the segment.



Banan, et. al                Informational                     [Page 49]

RFC 2188                          ESRO                    September 1997


   The values of the three fields Performer-SAP, Parameter-Encoding-
   Type, and Operation-Value of the first segment are used by performer
   and these fields are ignored in the segments other than the first
   one.

4.4.7  ESRO-RESULT-SEGMENTED-PDU Format

   Bit string format of the ESRO-RESULT-SEGMENTED-PDU is represented in
   Table 28 and Table 29.

      o For the first segment, the first/other bit is set to one, and
      the
        segment number field contains the total number of segments.

      o For segments other than the first one, the first/other bit is
      set
        to zero, and the segment number field has the sequence number of
        the segment.

   The values of the Parameter-Encoding-Type field of the first segment
   is used by invoker and this field is ignored in the segments other
   than the first one.


      -----------------------------------------------------------------
     |Bit     |  8   |  7   |  6   |  5   |  4   |  3   |  2   |  1   |
     |--------|-------------|------|------|------|------|------|------|
     |        | Parameter   |      |      |      |      |      |      |
     |        | Encoding    |      |      |      |      |      |      |
     |Octet 1 | Type        |  0   |  1   |  0   |  0   |  0   |  1   |
     |        |             |      |      |      |      |      |      |
     |        |             |      |      |      |      |      |      |
     |        |             |      |      |      |      |      |      |
     |--------|-------------------------------------------------------|
     |Octet 2 |            Invoke Reference Number                    |
     |--------|-------------------------------------------------------|
     |Octet 4 |First/|                                                |
     |        |Other |                  Segment Number                |
     |--------|-------------------------------------------------------|
     |Octet 5 |                                                       |
     |...     |                 Result Parameter                      |
     |Octet N |                                                       |
      ----------------------------------------------------------------|
     ESRO-RESULT-SEGMENTED-PDU Type Code = 1.

                Table 28:  ESRO-RESULT-SEGMENTED-PDU format





Banan, et. al                Informational                     [Page 50]

RFC 2188                          ESRO                    September 1997


                          _______________________
                          | Value |   Meaning    |
                          |_______|______________|
                          | 0     |    Basic     |
                          |_______|______________|
                          | 1     |   Packed     |
                          |_______|______________|
                          | 2     |     XDR      |
                          |_______|______________|
                          | 3     |  Reserved    |
                          |_______|______________|

     Table 29:  Parameter Encoding Type for ESRO-RESULT-SEGMENTED-PDU




      -----------------------------------------------------------------
     |Bit     |  8   |  7   |  6   |  5   |  4   |  3   |  2   |  1   |
     |--------|-------------|------|------|------|------|------|------|
     |        | Parameter   |      |      |      |      |      |      |
     |        | Encoding    |      |      |      |      |      |      |
     |Octet 1 | Type        |  0   |  1   |  0   |  0   |  1   |  0   |
     |        |             |      |      |      |      |      |      |
     |        |             |      |      |      |      |      |      |
     |        |             |      |      |      |      |      |      |
     |--------|-------------------------------------------------------|
     |Octet 2 |            Invoke Reference Number                    |
     |--------|-------------------------------------------------------|
     |Octet 3 |First/|                                                |
     |        |Other |                  Segment Number                |
     |--------|-------------------------------------------------------|
     |Octet 4 |                    Error Value                        |
     |--------|-------------------------------------------------------|
     |Octet 5 |                                                       |
     |...     |                 Error Parameter                       |
     |Octet N |                                                       |
      ----------------------------------------------------------------|
     ESRO-ERROR-SEGMENTED-PDU Type Code = 2.


                    Table 30:  ESRO-ERROR-SEGMENTED-PDU

4.4.8  ESRO-ERROR-SEGMENTED-PDU Format

   Bit string format of the ESRO-ERROR-PDU is represented in Table 30
   and Table 31.




Banan, et. al                Informational                     [Page 51]

RFC 2188                          ESRO                    September 1997


      o For the first segment, the first/other bit is set to one, and
        the segment number field contains the total number of segments.

      o For segments other than the first one, the first/other bit is
        set to zero, and the segment number field has the sequence
        number of the segment.

   The values of the Parameter-Encoding-Type field of the first segment
   is used by invoker and this field is ignored in the segments other
   than the first one.

4.5  Concatenation and Separation

   The procedure for concatenation and separation conveys multiple
   ESRO-PDUs in one TSDU. This is accomplished by ESRO-CONCATENATED-PDU.



                          _______________________
                          | Value |   Meaning    |
                          |_______|______________|
                          | 0     |    Basic     |
                          |_______|______________|
                          | 1     |   Packed     |
                          |_______|______________|
                          | 2     |     XDR      |
                          |_______|______________|
                          | 3     |  Reserved    |
                          |_______|______________|



      Table 31:  Parameter Encoding Type for ESRO-SEGMENTED-ERROR-PDU




   An ESRO-CONCATENATED-PDU can contain one or more of the following
   PDUs:  INVOKE, RESULT, ERROR, FAILURE, and ACK.

   The ESRO-PDUs within a concatenated set may be distinguished by means
   of the length indicator.  A one byte length indicator comes before
   each ESRO-PDU.

   The number of ESRO-PDUs in an ESRO-CONCATENATED-PDU is bounded by the
   maximum length of TSDU.





Banan, et. al                Informational                     [Page 52]

RFC 2188                          ESRO                    September 1997


4.5.1  Procedures

   Concatenation


   The ESROS provider concatenates PDUs as follows:

      o PDU type code 8 is used.

      o The length indicator which is the total length of first ESRO-PDU
        (header and data) in octets is placed after PDU type code in
        length indicator field of ESRO- CONCATENATED-PDU (see Section
        4.5.2).

      o The first PDU (header and data) is placed after the length
        indicator field and in the ESRO-PDU field of ESRO-CONCATENATED-
        PDU (see Section 4.5.2).

      o For any additional ESRO-PDU, the length indicator and PDUs are
        concatenated.

   Separation

   When the ESRO service provider receives a PDU with PDU type code 8,
   it separates the concatenated PDUs as described below:

      o Length indicator field coming after type code field (see
        Section 4.5.2) specifies the total length of the first PDU in
        octets.

      o The first PDU is in the ESRO-PDU field after the length
      indicator
        field (see Section 4.5.2).

      o Any additional PDU has its length indicator field specifying the
        total length of PDU, followed by PDU itself (see Section 4.5.2).

      o PDUs are separated until the end of the ESRO-CONCATENATED-PDU is
        reached.

4.5.2  ESRO-CONCATENATED-PDU format

   Bit string format of the ESRO-CONCATENATED-PDU containing multiple
   concatenated ESRO-PDUs is represented in Table  32.







Banan, et. al                Informational                     [Page 53]

RFC 2188                          ESRO                    September 1997


   Length Indicator field

   This field is contained in one octet and comes before each ESROS-PDU
   in the concatenated PDU. The length indicated is total length of the
   ESRO-PDU (including header and data) coming after it in octets.

   ESRO-PDU field

   This field contains an ESRO-INVOKE-PDU, ESRO-RESULT-PDU, ESRO-ERROR-
   PDU, ESRO-FAILURE-PDU, or ESRO-ACK-PDU.

   The length of this field is specified by the length indicator field
   coming before it.

4.6  ES Remote Operations Protocol Parameters

4.6.1  PDU size

      o CLRO_SMALL_PDU_MAX_SIZE:


         ________________________________________________________________
        |Bit     |  8   |  7   |  6   |  5   |  4   |  3   |  2   |  1   |
        |--------|---------------------------|------|------|------|------|
        |Octet 1 |         Not used          |  1   |  0   |  0   |  0   |
        |--------|-------------------------------------------------------|
        |Octet 2 |                  Length Indicator                     |
        |--------|-------------------------------------------------------|
        |Octet 3 |                                                       |
        |...     |                     ESRO-PDU                          |
        |Octet N |                                                       |
        |--------|-------------------------------------------------------|
        |Octet   |                                                       |
        | N+1    |                  Length Indicator                     |
        |--------|-------------------------------------------------------|
        |Octet   |                                                       |
        | N+2    |                     ESRO-PDU                          |
        | ...    |                                                       |
        |--------|-------------------------------------------------------|
        | ...    |                        ...                            |
        |________|_______________________________________________________|
        ESRO-CONCATENATED-PDU Type Code = 8.

                     Table 32:  ESRO-CONCATENATED-PDU format







Banan, et. al                Informational                     [Page 54]

RFC 2188                          ESRO                    September 1997


        The value of this parameter should be chosen based on the
        specifics of the subnetwork in use.  For example, in CDPD the
        maximum size of SN-Userdata size can be up to 2048 bytes (see
        part 404-2.b of CDPD Specification V1.1).  Based on this value
        and IP and UDP protocol information fields, the value of
        CLRO_SMALL_PDU_MAX_SIZE may be determined for CDPD. Again based
        on the specifics of the subnetwork, the optimum value of
        CLRO_SMALL_PDU_MAX_SIZE may best be determined based on field
        experience and may be smaller than the maximum size that the
        subnetwork supports.

      o CLRO_SEGMENTED_PDU_MAX_SIZE

        The value of this parameter should be chosen based on the
        specifics of the subnetwork in use.  The optimum value of
        CLRO_SEGMENTED_PDU_MAX_SIZE may best be determined based on
        field experience.

      o CLRO_MAX_PDU_SEGMENTS

        The value of this parameter should be chosen based on the
        specifics of the subnetwork in use.  The optimum value of
        CLRO_MAX_PDU_SEGMENTS may best be determined based on field
        experience.  In any case, this value should be smaller than 127.

4.6.2  Timers

      o INVOKE_PDU_RETRANSMISSION_INTERVAL:

        The INVOKE_PDU retransmission interval should be specified and
        optimized based on the characteristics of the network in use.

      o RESULT_ERROR_PDU_RETRANSMISSION_INTERVAL:

        The RESULT and ERROR-PDU retransmission interval should be
        specified and optimized based on the characteristics of the
        network in use.

      o MAX_RETRANSMISSIONS:

        The maximum number of retransmissions should be specified and
        optimized based on the characteristics of the network in use.

      o INACTIVITY_TIME:

        The minimum waiting time during which no duplicate PDU is
        received should be specified and optimized based on the
        characteristics of the network in use.



Banan, et. al                Informational                     [Page 55]

RFC 2188                          ESRO                    September 1997


      o REFERENCE_NUMBER_TIME: The reference number lifetime timer
        should be specified and optimized based on the characteristics
        of the network in use.

4.6.3  Use of lower layers

   ESRO protocol uses UDP port number 259.

5  ACKNOWLEDGMENTS

   Development of this specification was funded by AT&T Wireless
   Services (AWS). This protocol specification has been derived from
   AT&T Wireless Services' document titled:  "Limited Size Remote
   Operation Services (LSROS)", Revision 0.8, dated April 20, 1995.

   This specification is technically consistent with CDPD Forum's
   Implementor's Guidelines Part 1028, Release 1.03, June 21, 1996.

6  SECURITY CONSIDERATIONS

   ESROS has no authentication mechanism.  Authentication is the
   responsibility of the performer (which is outside of the scope of

   ESROS) and the performer is not expected to honor the invoker when it
   is not authenticated.

7  AUTHORS' ADDRESSES

   Mohsen Banan
   Neda Communications, Inc.
   17005 SE 31st Place
   Bellevue, WA 98008

   EMail: mohsen@neda.com


   Mark S. Taylor
   Director of Strategic Engineering
   AT&T Wireless Services
   Wireless Data Division
   10230 NE Points Drive
   Kirkland, WA 98033-7869 USA

   EMail: mark.taylor@airdata.com







Banan, et. al                Informational                     [Page 56]

RFC 2188                          ESRO                    September 1997


   Jia-bing Cheng
   AT&T Wireless Services
   Wireless Data Division
   10230 NE Points Drive
   Kirkland, WA 98033-7869 USA

   EMail: jcheng@airdata.com

References

   [1] Remote Operations:  Model, Notation and Service Definition, March
        1988. Recommendation X.219.

   [2] Remote Operations:  Protocol Specification, March 1988.
        Recommendation X.229.

   [3] Specification of Abstract Syntax Notation One, 1988.
        Recommendation X.208.

   [4] Information Processing --- Open Systems
        Interconnection --- Specification of Packed Encoding Rules for
        Abstract Syntax Notation One (ASN.1). International Standard
        8825-2.

   [5] Information Processing --- Open Systems
        Interconnection --- Specification of Basic Encoding Rules for
        Abstract Syntax Notation One (ASN.1), 1987. International
        Standard 8825.

   [6] Srinivasan, R., "Binding protocols for onc rpc version 2".
        RFC 1833, Sun Microsystems Inc, August 1995.

   [7] Srinivasan, R., "Rpc:  Remote procedure call protocol
        specification version 2". RFC 1831, Sun Microsystems Inc, August
        1995.

   [8] Srinivasan, R., "Xdr:  External data representation standard".
        RFC 1832, Sun Microsystems Inc, August 1995.













Banan, et. al                Informational                     [Page 57]

