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Versions: 00 RFC 2188

Internet-Draft              Expires: June 1997          Internet-Draft


Network Working Group                                   M. Banan, Neda
                                                        M. Taylor, AWS
Category:  Informational                                 J. Cheng, AWS
                                                          January 1997


         AT&T/Neda's Efficient Short Remote Operations (ESRO)
                  Protocol Specification Version 1.2
                     <draft-rfced-info-banan-esro-00.txt>





Status of this Memo:

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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,
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.

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Contents

1  INTRODUCTION                                                      4
   1.1 Relationship To Existing Remote Operation Services    .       4
       1.1.1 ESRO and RPC    .   .   .   .   .   .   .   .   .       4
       1.1.2 ESRO and ROSE   .   .   .   .   .   .   .   .   .       4
   1.2 Overview of ESROS     .   .   .   .   .   .   .   .   .       5
   1.3 The Remote Operation Model    .   .   .   .   .   .   .       5

2  ESRO SERVICE DEFINITIONS                                          7
   2.1 Acknowledged Result Service Mode  .   .   .   .   .   .       8
       2.1.1 Performer side .   .   .   .   .   .   .   .   .        8
       2.1.2 Invoker side    .   .   .   .   .   .   .   .   .      10
   2.2 Non-acknowledged Result   .   .   .   .   .   .   .   .      10
       2.2.1 Performer side .   .   .   .   .   .   .   .   .       11
       2.2.2 Invoker side    .   .   .   .   .   .   .   .   .      11
   2.3 Serialized Use of ESRO Services   .   .   .   .   .   .      11
       2.3.1 Invoker     .   .   .   .   .   .   .   .   .   .      11
       2.3.2 Performer   .   .   .   .   .   .   .   .   .   .      11
   2.4 ESROS-INVOKE Service  .   .   .   .   .   .   .   .   .      12
       2.4.1 Operation-value     .   .   .   .   .   .   .   .      12
       2.4.2 Performer-address   .   .   .   .   .   .   .   .      13
       2.4.3 Invoker-address     .   .   .   .   .   .   .   .      13
       2.4.4 Invoke-argument-encoding-type   .   .   .   .   .      14
       2.4.5 Invoke-argument     .   .   .   .   .   .   .   .      14
       2.4.6 Invoke-ID   .   .   .   .   .   .   .   .   .   .      14
       2.4.7 Failure-value   .   .   .   .   .   .   .   .   .      14
   2.5 ESROS-RESULT Service  .   .   .   .   .   .   .   .   .      15
       2.5.1 Result-argument-encoding-type   .   .   .   .   .      15
       2.5.2 Result-argument     .   .   .   .   .   .   .   .      16
       2.5.3 Invoke-ID   .   .   .   .   .   .   .   .   .   .      16
       2.5.4 Failure-value   .   .   .   .   .   .   .   .   .      17
   2.6 ESROS-ERROR Service   .   .   .   .   .   .   .   .   .      17
       2.6.1 Error-value     .   .   .   .   .   .   .   .   .      17
       2.6.2 Error-argument-encoding-type    .   .   .   .   .      18
       2.6.3 Error-argument .   .   .   .   .   .   .   .   .       18
       2.6.4 Invoke-ID   .   .   .   .   .   .   .   .   .   .      19
       2.6.5 Failure-value   .   .   .   .   .   .   .   .   .      19
   2.7 ESROS-FAILURE Service     .   .   .   .   .   .   .   .      19
       2.7.1 Failure-value   .   .   .   .   .   .   .   .   .      20
       2.7.2 Invoke-ID   .   .   .   .   .   .   .   .   .   .      20

3  ESRO SERVICE NOTATION                                            20
   3.1 ES-OPERATION Notation     .   .   .   .   .   .   .   .      21
   3.2 Mapping of ESROS Notation     .   .   .   .   .   .   .      21
       3.2.1 Invocation of an Operation .   .   .   .   .   .       21
       3.2.2 Reply of an Operation   .   .   .   .   .   .   .      21

4  REMOTE OPERATIONS PROTOCOL                                       23
   4.1 Overview of the Protocol  .   .   .   .   .   .   .   .      23
       4.1.1 Service Provision (Invoker User)    .   .   .   .      23

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       4.1.2 Service Provision (Performer User) .   .   .   .       24
   4.2 Protocol Procedures   .   .   .   .   .   .   .   .   .      24
       4.2.1 Service Access Point (SAP) Bind Procedure   .   .      24
       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     .   .   .   .      27
       4.3.3 2-Way Handshake Functional Unit     .   .   .   .      34
       4.3.4 Segmentation and Reassembly     .   .   .   .   .      39
   4.4 Structure and Encoding of ESROS PDUs  .   .   .   .   .      41
       4.4.1 ESRO-INVOKE-PDU Format .   .   .   .   .   .   .       41
       4.4.2 ESRO-RESULT-PDU Format .   .   .   .   .   .   .       43
       4.4.3 ESRO-ERROR-PDU Format   .   .   .   .   .   .   .      43
       4.4.4 ESRO-ACK-PDU Format     .   .   .   .   .   .   .      45
       4.4.5 ESRO-FAILURE-PDU Format     .   .   .   .   .   .      45
       4.4.6 ESRO-INVOKE-SEGMENTED-PDU Format    .   .   .   .      45
       4.4.7 ESRO-RESULT-SEGMENTED-PDU Format    .   .   .   .      48
       4.4.8 ESRO-ERROR-SEGMENTED-PDU Format     .   .   .   .      50
   4.5 Concatenation and Separation  .   .   .   .   .   .   .      50
       4.5.1 Procedures .   .   .   .   .   .   .   .   .   .       51
       4.5.2 ESRO-CONCATENATED-PDU format    .   .   .   .   .      52
   4.6 ES Remote Operations Protocol Parameters  .   .   .   .      52
       4.6.1 PDU size    .   .   .   .   .   .   .   .   .   .      52
       4.6.2 Timers .   .   .   .   .   .   .   .   .   .   .       54
       4.6.3 Use of lower layers     .   .   .   .   .   .   .      54

5  ACKNOWLEDGMENTS                                                  54

6  SECURITY CONSIDERATIONS                                          54

7  AUTHORS' ADDRESSES                                               55



















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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 Primitives

  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)


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].


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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.


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.





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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














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               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:

 1. Acknowledged Result, and

 2. Non-Acknowledged Result


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             _____________________________________________
             | 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.





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       ____________________________________________________________
       | 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



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______________________________________________________________
|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.




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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.






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               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.





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       _____________________________________________________________
       | 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.






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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.



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.


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               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.








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______________________________________________________________
| 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.







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               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.





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       ________________________________________________________
       | 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).



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               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.






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       _____________________________________________
       | 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.



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 1. The operation's Argument data type.

 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


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












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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.

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,




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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 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


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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:


  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

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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.

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.



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______________________________________________________
|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.

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.



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_______________________________________________________________
|      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.

     o Issue ESROS-INVOKE-P.confirm primitive.

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     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.

 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).


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     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).

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

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____________________________________________________________________
|      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.


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:


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 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.

 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:

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     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).

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

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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.

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.


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______________________________________________________________
|      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.


 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.



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 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
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



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___________________________________________________________________
|      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.


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. 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 (as a single PDU or
       as a sequence of segment PDUs.)

     o Set Inactivity timer.

 4. Internal failure:


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     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. Duplicate ESRO-INVOKE-PDU:

     o Reset invoke reference number timer.

 8. Invoke reference number timer:


     o Release the invoke reference number.


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.

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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.

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.







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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.

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


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    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.

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.







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 _______________________________________
 | 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


__________________________________________________________
|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


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   ---------------------------------------------------------
  |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.


4.4.3  ESRO-ERROR-PDU Format


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






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   ---------------------------------------------------------
  |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





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   ---------------------------------------------------------
  |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.


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.



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   ---------------------------------------------------------
  |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







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   -----------------------------------------------------------------
  |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














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                       _______________________
                       | 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.


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.








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   -----------------------------------------------------------------
  |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








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

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





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   -----------------------------------------------------------------
  |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.


  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.


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                       _______________________
                       | 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.


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.






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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.


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:

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   ________________________________________________________________
  |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

    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.




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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.

  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


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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


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] R. Srinivasan. Binding protocols for onc rpc version 2. Request
    for Comments 1833, Sun Microsystems Inc, 08 1995.

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[7] R. Srinivasan. Rpc:  Remote procedure call protocol specification
    version 2. Request for Comments 1831, Sun Microsystems Inc, 08
    1995.
[8] R. Srinivasan. Xdr:  External data representation standard.
    Request for Comments 1832, Sun Microsystems Inc, 08 1995.


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