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rfc:rfc2188

Network Working Group M. Banan Request for Comments: 2188 Neda Category: Informational M. Taylor

                                                                 AWS
                                                            J. Cheng
                                                                 AWS
                                                      September 1997
        AT&T/Neda's Efficient Short Remote Operations (ESRO)
                 Protocol Specification Version 1.2

Status of this Memo

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

IESG Note

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

Abstract

This document specifies the service model, the notation and protocol
for Efficient Short Remote Operations (ESRO). The ESRO service is
similar to and is consistent with other Remote Procedure Call
services.  The emphasis of ESRO service definition and the ESRO
protocol is on efficiency.  ESRO is designed specifically with
wireless network (e.g., CDPD) usage in mind.
ESRO protocol provides reliable connectionless remote operation
services on top of UDP (or any other non-reliable connectionless
transport service) with minimum overhead.  ESRO protocol supports
segmentation and reassembly, concatenation and separation as well as
multiplexing for service users (applications).
ESRO allows for trade-offs between efficiency and reliability by
specifying both 2-way hand-shake and 3-way hand-shake based protocols.
Encoding mechanisms for presentation of the parameters of remote
operations are outside the scope of this document.  But,
identification (tagging) of the encoding mechanism in use (e.g., XDR,

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

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

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

Contents

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

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

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

1 INTRODUCTION

 Efficient Short Remote Operations (ESRO) provide an efficent
 mechanism for realization of Remote Procedure Call.  This document
 specifies many aspects of ESRO including:
   o Service Model
   o Service Primitves
   o A Notation for user of the Service
   o Confirmed Connectionless Protocol (based on a 3-way hand-shake)
   o Unconfirmed Connectionless Protocol (based on a 2-way hand-shake)

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

1.1 Relationship To Existing Remote Operation Services

 The overall model of ESRO is similar to and consistent with many
 existing protocols.  ESRO's distinguishing characteristic is
 efficiency.
 A brief comparison of ESRO and Remote Procedure Calls [7] and Remote
 Operation Service Elements [1] follows.

1.1.1 ESRO and RPC

 Remote Procedure Call (RPC) is specified in [7] (RFC-1831) and [6]
 (RFC-1833).
 RPC specifications define a remote procedure model that is
 essentially same as ESRO. RPC's notation uses a syntax quite
 different from that of ESRO. RPC can rely on a connection oriented or
 connectionless transport mechanism.  When using the connectionless
 mechanism, the retransmission and reliability issues are considered
 beyond the scope of the RPC specification.  RPC is usually used in
 combination with External Data Representation, XDR [8] (RFC-1832).

1.1.2 ESRO and ROSE

 ROSE is specified in [1] and [2].  The service definition for ESRO
 Service (ESROS) specified in this document is similar ROSE's
 Notation.  The Notation specified in this document for ESROS is
 similar ROSE's Notation.  The ESRO protocol specified in this
 document is very different from the ROSE protocol [2].
 The operation model for ESRO Service (ESROS) is based on Remote
 Operations Services Element (ROSE) in [1].  In ESROS model both
 entities can invoke operations.
 ESRO protocols can accomplish short operations with much less
 overhead than ROSE.

1.2 Overview of ESROS

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

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

1.3 The Remote Operation Model

 ESROS provides for performance of operations between two peer
 sublayers.  Users of the ESROS assume the roles of invoker and
 performer which invoke and perform the operations respectively.  An
 ESROS-User can assume both roles and be an invoker for some
 operations and be a performer for other operations.  The performer is
 expected to report either the result of the operation or an error.  A
 result reply is sent to the invoker if the operation is successful,
 and an error reply is sent if the operation is unsuccessful.  If the
 performer is unreachable, the ESROS sends a failure indication
 primitive to the invoker.
 Operations are asynchronous and the invoker may continue to invoke
 further operations without waiting for a reply.  Synchronous or
 serialized operations are also supported as a subset and a special
 case of asynchronous service.  By default the ESRO service provider
 on both invoker and performer sides supports the asynchronous
 operation invocation.  However, if one side is to support only
 serialized (synchronous) mode, it should be in agreement with the
 peer side.
 ESROS has no authentication mechanism.  Authentication is the
 responsibility of the performer (which is outside of the scope of
 ESROS) and the performer is not expected to honor the invoker when it
 is not authenticated.
 The ESROS operation model is represented in Figure 1.  In this
 example, the ESROS User on the left is the Invoker and the ESROS User
 on the right is the Performer.  The Provider is the entity providing
 a service to the layer above it.

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

 ESROS      -------------------    -------------------     ESROS
 User      | Layer above ESROS |  | Layer above ESROS |    User
 (Invoker) |                   |  |                   |  (Performer)
            -------------------    -------------------
    ^                |                      |                ^
    |                |                      |                |
    v                |                      |                v
 ESROS     -------------------    -------------------      ESROS
 Provider |       ESROS       |  |       ESROS       |    Provider
           -------------------    -------------------
                     |                      |
                     |                      |
                     |                      |
           -------------------    -------------------
          |    UDP           |   |   UDP             |
           -------------------    -------------------
                   _                    _/
                     _                _/
                       _    .       _/
                         _ . .* . _/
                           . * .* .
                             * . *
                  Figure 1:  ES Remote Operation Model

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

                Invoker        Performer
               ESRO SAP         ESRO SAP
                     |               |
                     |               |
 ESROS-INVOKE.req.   |               | ESROS-INVOKE.ind.
 -------->-----------|               |-------->---------
                     |               |
 ESROS-INVOKE-P.conf.|               |
 --------<-----------|               |
                     |               |
                     |               |
                     |               |
 ESROS-RESULT.ind.   |               | ESROS-RESULT.req.
 --------<-----------|               |--------<---------
                     |               |
                     |               | ESROS-RESULT.conf.
                     |               |-------->---------
                     |               |
                     |               |
 ESROS-ERROR.ind.    |               | ESROS-ERROR.req.
 --------<-----------|               |--------<---------
                     |               |
                     |               | ESROS-ERROR.conf.
                     |               |-------->---------
                     |               |
                     |               |
                     |               |
                     |               |
 ESROS-FAILURE.ind.  |               | ESROS-FAILURE.ind.
 --------<-----------|               |-------->---------
                     |               |
           Figure 2:  Time sequence diagram for ESRO services

2 ESRO SERVICE DEFINITIONS

 ESRO service primitives are illustrated in Figure 2, Table 1 and
 Table 2.  The description of services and primitives comes in the
 following sections.
 ESROS-User accesses ESRO services through Efficient Short Remote
 Operations Service Access Point (ESRO-SAP) as shown in Figure 2.
 The RESULT.request, ERROR.request and FAILURE.indication service
 primitives can be implemented in two different modes:

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

  1. Acknowledged Result, and
  2. Non-Acknowledged Result
               _____________________________________________
               | ESRO Service   |Type                      |
               |________________|__________________________|
               | ESROS-INVOKE   |Non-confirmed             |
               | ESROS-INVOKE-P |Provider-initiated        |
               | ESROS-RESULT   |Confirmed / Non-confirmed |
               | ESROS-ERROR    |Confirmed / Non-confirmed |
               | ESROS-FAILURE  |Provider initiated        |
               |________________|__________________________|
                         Table 1:  ESRO Services
 as described below.  The difference between different modes is in
 their reliability of service and efficiency.  Reliability of service
 is defined based on the understanding of invoker and performer about
 the success or failure of the operation on the peer side.  Table 3
 and Table 4 summarize understanding of performer about success or
 failure on invoker side in different situations.  In these tables the
 FAILURE.indication refers to the primitive generated by protocol and
 not the failure of local provider.

2.1 Acknowledged Result Service Mode

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

2.1.1 Performer side

 In this type of service, the RESULT.confirm and ERROR.confirm
 primitives on performer side are generated if the result/error is
 acknowledged by invoker.
 The FAILURE.indication on performer side is generated if result/error
 is not acknowledged by invoker or if there is a local failure on
 performer side.
 >From the protocol point of view, the FAILURE.indication might be
 because either the result/error PDU or the ack PDU is lost.  The
 outcome of this is that a FAILURE.indication is not robust as the
 operation may have been successful from the invoker's perspective.
 One method of compensating for this shortcoming is having the
 performer verify the FAILURE.indication in a separate operation.

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

 ____________________________________________________________
 | Primitive                |Parameters                     |
 |--------------------------+-------------------------------|
 |                          |Operation-value                |
 |                          |Performer-address              |
 | ESROS-INVOKE.request     |Invoke-argument-encoding-type  |
 |                          |Invoke-argument                |
 |--------------------------+-------------------------------|
 |                          |Operation-value                |
 |                          |Invoker-address                |
 | ESROS-INVOKE.indication  |Invoke-argument-encoding-type  |
 |                          |Invoke-argument                |
 |                          |Invoke-ID                      |
 |--------------------------+-------------------------------|
 | ESROS-INVOKE-P.confirm   |Invoke-ID                      |
 |==========================================================|
 |                          |                               |
 |                          |Result-argument-encoding-type  |
 | ESROS-RESULT.request     |Result-argument                |
 |                          |Invoke-ID                      |
 |--------------------------+-------------------------------|
 |                          |Result-argument-encoding-type  |
 | ESROS-RESULT.indication  |Result-argument                |
 |                          |Invoke-ID                      |
 |--------------------------+-------------------------------|
 | ESROS-RESULT.confirm     |Invoke-ID                      |
 |==========================================================|
 |                          |                               |
 |                          |Error-value                    |
 |                          |Error-argument-encoding-type   |
 | ESROS-ERROR.request      |Error-argument                 |
 |--------------------------+-------------------------------|
 |                          |Error-value                    |
 |                          |Error-argument-encoding-type   |
 | ESROS-ERROR.indication   |Error-argument                 |
 |                          |Invoke-ID                      |
 |--------------------------+-------------------------------|
 | ESROS-ERROR.confirm      |Invoke-ID                      |
 |==========================================================|
 |                          |                               |
 |                          |Failure-value                  |
 | ESROS-FAILURE.indication |Invoke-ID                      |
 |__________________________|_______________________________|
  Table 2:  ESRO service primitives and associated parameters

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

 ______________________________________________________________
 |Service Mode        |Performer          |Invoker            |
 |--------------------+-------------------+-------------------|
 |Acknowledged Result |RESULT.confirm     |RESULT.indication  |
 |                    |-------------------+-------------------|
 |                    |FAILURE.indication |RESULT.indication  |
 |                    | (protocol)        |                   |
 |                    |-------------------+-------------------|
 |                    |FAILURE.indication |FAILURE.indication |
 |                    | (protocol)        | (protocol)        |
 |____________________|___________________|___________________|
  Table 3:  Success and Failure in Acknowledged Result Mode
 __________________________________________________________________
 |Service Mode            |Performer           |Invoker            |
 |------------------------+--------------------+-------------------|
 |Non-acknowledged Result |RESULT.confirm      |RESULT.indication  |
 |                        +--------------------+-------------------|
 |                        |RESULT.confirm      |FAILURE.indication |
 |                        |                    | (protocol)        |
 |                        +--------------------+-------------------|
 |                        |FAILURE.indication  |                   |
 |                        |(protocol)          |                   |
 |                        |does not            |---                |
 |                        |exist               |                   |
 |________________________|____________________|___________________|
  Table 4:  Success and Failure in Non-acknowledged Result Mode

2.1.2 Invoker side

 When invoker receives failure indication, the performer has the
 failure indication too.
 This type of service can be implemented by protocols based on 3-Way
 handshaking.

2.2 Non-acknowledged Result

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

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

2.2.1 Performer side

 In this type of service, the RESULT.confirm and ERROR.confirm
 primitives on performer side are generated without receiving
 additional information from the invoker peer.  In other words, these
 Primitives have no protocol-related meaning and convey no
 information, other than end-of-operation.
 The FAILURE.indication on performer side is not generated by
 protocol.  The only case that can generate FAILURE.indication on
 performer side is local failure in service provider on performer
 side.

2.2.2 Invoker side

 The FAILURE.indication on invoker side can be the resultof not
 receiving result/error/failure from peer performer or it can result
 from failure in local service provider.
 This type of service can be implemented by protocols based on 2-Way
 handshaking.

2.3 Serialized Use of ESRO Services

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

2.3.1 Invoker

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

2.3.2 Performer

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

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

                Invoker        Performer
               ESROS AP         ESROS AP
                     |               |
                     |               |
 ESROS-INVOKE.req.   |               | ESROS-INVOKE.ind.
 -------->-----------|               |-------->---------
                     |               |
 ESROS-INVOKE-P.conf.|               |
 --------<-----------|               |
                     |               |
 ESROS-FAILURE.ind.  |               |
 --------<-----------|               |
                     |               |
 Figure 3:  Time sequence diagram for ESROS-INVOKE service

2.4 ESROS-INVOKE Service

 The ESROS-INVOKE service is used by an ESROS-User (the invoker) to
 cause the invocation of an OPERATION to be performed by the other
 ESROS-User (the performer).
 ESROS Invoker User issues ESROS-INVOKE.request primitive to invoke an
 operation.
 ESROS-INVOKE.indication primitive provides the ESROS Performer User
 with the parameters of the invoked operation.
 ESRO Service Provider issues the ESROS-INVOKE-P.confirm primitive to
 provide the ESROS Invoker User with Invoke-ID of the invoked
 operation.
 The related service structure consists of three service primitives as
 illustrated in Figure 3 and Table 5.

2.4.1 Operation-value

 This value is the identifier of the operation to be invoked.  The
 value is agreed upon between the ESROS Users.  This parameter has to
 be supplied by the invoker of the service.
 ESROS Invoker User provides the Operation-value parameter for the
 ESROS-INVOKE.request primitive.  The Operation-value parameter of
 ESROS-INVOKE.indication is provided to the ESROS Performer User.

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

        _____________________________________________________________
        | Primitive                |Parameters                       |
        |__________________________|_________________________________|
        |                          |Operation-value                  |
        |                          |Performer-address                |
        | ESROS-INVOKE.request     |Invoke-argument-encoding-type    |
        |                          |Invoke-argument                  |
        |__________________________|_________________________________|
        |                          |Operation-value                  |
        |                          |Invoker-address                  |
        |                          |Invoke-argument-encoding-type    |
        | ESROS-INVOKE.indication  |Invoke-argument                  |
        |                          |Invoke-ID                        |
        |__________________________|_________________________________|
        | ESROS-INVOKE-P.confirm   |Invoke-ID                        |
        |                          |Failure-value                    |
        |__________________________|_________________________________|
        | ESROS-FAILURE.indication |Invoke-ID                        |
        |__________________________|_________________________________|
   Table 5:  ESROS-INVOKE service primitives and associated parameters

2.4.2 Performer-address

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

2.4.3 Invoker-address

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

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

2.4.4 Invoke-argument-encoding-type

 This parameter identifies the encoding type of the Invoke-argument
 (see next subsection).  The encoding type has to be agreed upon
 between ESROS Users.  This parameter has to be supplied by the
 invoker of the service.
 ESROS Invoker User provides the Invoke-argument-encoding-type
 parameter for the ESROS-INVOKE.request primitive.  The Invoke-
 argument-encoding-type parameter of ESROS-INVOKE.indication is
 provided to the ESROS Performer User.

2.4.5 Invoke-argument

 This parameter is the argument of the invoked operation.  The type
 has to be agreed between the ESROS Users.  This parameter has to be
 supplied by the invoker of the service.  Encoding type of the
 Invoke-argument is specified through the Invoke-argument-encoding-
 type parameter (see previous subsection).
 ESROS Invoker User provides the Invoke-argument parameter for the
 ESROS-INVOKE.request primitive.  The Invoke-argument parameter of
 ESROS-INVOKE.indication is provided to the ESROS Performer User.

2.4.6 Invoke-ID

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

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

2.4.7 Failure-value

 This parameter identifies the failure that occurred during the
 processing or transmission of any of the service primitives of ESROS.
                Invoker        Performer
               ESROS AP         ESROS AP
                     |               |
                     |               |
 ESROS-RESULT.ind.   |               | ESROS-RESULT.req.
 --------<-----------|               |--------<---------
                     |               |
                     |               | ESROS-RESULT.conf.
                     |               |-------->---------
                     |               |
                     |               | ESROS-FAILURE.ind.
                     |               |-------->---------
                     |               |
 Figure 4:  Time sequence diagram for ESROS-RESULT service
 This parameter has to be supplied by the ESROS provider (see also
 Section 2.7).

2.5 ESROS-RESULT Service

 The ESROS-RESULT service is used by an ESROS User to reply to a
 previous ESROS-INVOKE.indication in the case of a successfully
 performed operation.  This service is either confirmed or non-
 confirmed based on the service mode (see Section 2).
 The related service structure consists of three service primitives as
 illustrated in Figure 4 and Table 6.

2.5.1 Result-argument-encoding-type

 This parameter identifies the encoding type of the Result-argument
 (see next subsection).  The encoding type has to be agreed upon
 between the ESROS Users.  This parameter has to be supplied by the
 ESROS Performer User.
 ESROS Performer User provides the Result-argument-encoding-type
 parameter for the ESROS-RESULT.request primitive.  The Result-
 argument-encoding-type parameter of ESROS-RESULT.indication is
 provided to the ESROS Invoker User.

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

 ______________________________________________________________
 | Primitive                |Parameters                       |
 |__________________________|_________________________________|
 |                          |Result-argument-encoding-type    |
 |                          |Result-argument                  |
 | ESROS-RESULT.request     |Invoke-ID                        |
 |__________________________|_________________________________|
 |                          |                                 |
 |                          |Result-argument-encoding-type    |
 |                          |Result-argument                  |
 | ESROS-RESULT.indication  |Invoke-ID                        |
 |__________________________|_________________________________|
 |                          |                                 |
 | ESROS-RESULT.confirm     |Invoke-ID                        |
 |                          |Failure-value                    |
 |                          |                                 |
 |__________________________|_________________________________|
 | ESROS-FAILURE.indication |Invoke-ID                        |
 |__________________________|_________________________________|
 Table 6:  ESROS-RESULT service primitives and associated parameters

2.5.2 Result-argument

 This parameter is the result of an invoked and successfully performed
 operation.  The type has to be agreed between the ESROS Users.  This
 parameter has to be supplied by the invoker of the service.  Encoding
 type of the Result-argument is specified through the Result-
 argument-encoding-type parameter (see previous subsection).
 ESROS Performer User provides the Result-argument parameter for the
 ESROS-RESULT.request primitive.  The Result-argument parameter of
 ESROS-RESULT.indication is provided to the ESROS Invoker User.

2.5.3 Invoke-ID

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

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

                Invoker        Performer
               ESROS AP         ESROS AP
                     |               |
                     |               |
 ESROS-ERROR.ind.    |               | ESROS-ERROR.req.
 --------<-----------|               |--------<---------
                     |               |
                     |               | ESROS-ERROR.conf.
                     |               |-------->---------
                     |               |
                     |               | ESROS-FAILURE.ind.
                     |               |-------->---------
 Figure 5:  Time sequence diagram for ESROS-ERROR service

2.5.4 Failure-value

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

2.6 ESROS-ERROR Service

 The ESROS-ERROR service is used by an ESROS User to reply to a
 previous ESROS-INVOKE.indication in the case of an unsuccessfully
 performed operation.  This service is either confirmed or non-
 confirmed based on the service mode (see Section 2).
 The related service structure consists of three service primitives as
 illustrated in Figure 5 and Table 7.

2.6.1 Error-value

 This parameter identifies the error in reply to a previous ESROS-
 INVOKE.indication in the case of an unsuccessfully performed
 operation.  The value has to be agreed between the ESROS-Users.  This
 parameter has to be supplied by the ESROS Performer User.
 ESROS Performer User provides the Error-argument parameter for the
 ESROS-ERROR.request primitive.  The Error-argument parameter of
 ESROS-ERROR.indication is provided to the ESROS Invoker User.

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

        ________________________________________________________
        | Primitive                |Parameters                  |
        |__________________________|____________________________|
        |                          |Error-value                 |
        |                          |Error-argument-encoding-type|
        | ESROS-ERROR.request      |Error-argument              |
        |__________________________|____________________________|
        |                          |                            |
        |                          |Error-value                 |
        |                          |Error-argument-encoding-type|
        | ESROS-ERROR.indication   |Error-argument              |
        |                          |Invoke-ID                   |
        |                          |                            |
        |__________________________|____________________________|
        | ESROS-ERROR.confirm      |Invoke-ID                   |
        |                          |Failure-value               |
        |                          |                            |
        |__________________________|____________________________|
        | ESROS-FAILURE.indication |Invoke-ID                   |
        |__________________________|____________________________|
  Table 7:  ESROS-ERROR service primitives and associated parameters

2.6.2 Error-argument-encoding-type

 This parameter identifies the encoding type of the Error-argument
 (see next subsection).  The encoding type has to be agreed upon
 between the ESROS Users.  This parameter has to be supplied by the
 ESROS Performer User.
 ESROS Performer User provides the Error-argument-encoding-type
 parameter for the ESROS-ERROR.request primitive.  The Error-
 argument-encoding-type parameter of ESROS-ERROR.indication is
 provided to the ESROS Invoker User.

2.6.3 Error-argument

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

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

                Invoker        Performer
               ESROS AP         ESROS AP
                     |               |
                     |               |
 ESROS-FAILURE.ind.  |               |
 --------<-----------|               |
                     |               |
                     |               | ESROS-FAILURE.ind.
                     |               |--------->---------
                     |               |
 Figure 6:  Time sequence diagram for ESROS-FAILURE service
 ESROS Performer User provides the Error-argument parameter for the
 ESROS-ERROR.request primitive.  The Error-argument parameter of
 ESROS-ERROR.indication is provided to the ESROS Invoker User.

2.6.4 Invoke-ID

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

2.6.5 Failure-value

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

2.7 ESROS-FAILURE Service

 The ESROS-FAILURE service is used by ESROS provider to indicate the
 failure in providing an ESROS-INVOKE, ESROS-RESULT, or ESROS-ERROR
 service.
 The related service structure consists of one service primitive as
 illustrated in Figure 6 and Table 8.

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

        _____________________________________________
        | Primitive                |Parameters       |
        |__________________________|_________________|
        |                          |Failure-value    |
        | ESROS-FAILURE.indication |Invoke-ID        |
        |__________________________|_________________|
 Table 8:  ESROS-FAILURE service primitives and associated parameters
        _________________________________________
        | Failure Value |Meaning                 |
        |_______________|________________________|
        | 0             |Transmission failure    |
        | 1             |Out of local resources  |
        | 2             |User not responding     |
        | 3             |Out of remote resources |
        | 4             |Reassembly failure      |
        |_______________|________________________|
           Table 9:  Encoding of Failure-value

2.7.1 Failure-value

 This parameter identifies the failure that occurred during the
 processing or transmission of any of the service primitives of ESROS.
 This parameter has to be supplied by the ESROS provider.
 The values for encoding of Failure-value are presented in Table 9.

2.7.2 Invoke-ID

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

3 ESRO SERVICE NOTATION

 Users of ESRO services (invoker and performer) need to agree on a
 well defined set of parameters which are enumerated below.
  1. The operation's Argument data type.

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

  2. The operation's Result data type.
  3. The operation's Error data type.
  4. The operation's value.  A specific tag which uniquely identifies
      the operation.
 The invoker and the performer can specify these parameters using a
 variety of mechanisms.  The notation specified in this section is one
 such mechanism.  It is not the only machanism and ESRO protocol can
 be used independent of this notation.

3.1 ES-OPERATION Notation

 The Remote Operations and Operation Errors are specified in this
 section.  The notation is defined by means of the macro facility
 defined in [3].
 The macros enabling the specification of operations and errors are
 listed in Figure 7.
 Note that this notation is very similar to the abstract operation
 defined in [1].  The value form of ES-OPERATION is always an integer.

3.2 Mapping of ESROS Notation

3.2.1 Invocation of an Operation

 An operation is mapped onto the ESRO Services.
 The invocation of an operation is mapped on the ESRO-INVOKE service.
 The value assigned to the operation is mapped on the Operation-value
 parameter of that service.  The value of the Named-Type in the
 ARGUMENT clause of the OPERATION Macro is mapped on the Argument
 parameter of that service.

3.2.2 Reply of an Operation

 If an operation was successfully performed, the reply is mapped on
 the ESRO-RESULT service.
 The value of the Named-Type in the RESULT clause of the OPERATION
 DEFINITIONS ::=
 BEGIN
   ES-OPERATION, ERROR;
   -- macro definition for operations

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

   ES-OPERATION MACRO ::=
   BEGIN
         TYPE NOTATION   ::=     Argument Result Errors
         VALUE NOTATION  ::=     value (localValue INTEGER)
         Argument        ::=     "ARGUMENT" NamedType | empty
         Result          ::=     "RESULT" ResultType | empty
         ResultType      ::=     NamedType | empty
         Errors          ::=     "ERRORS" "{"ErrorNames"}" | empty
         ErrorNames      ::=     ErrorList | empty
         ErrorList       ::=     Error | ErrorList "," Error
         Error           ::=     value (ERROR) | type
         NamedType       ::=     identifier type | type
   END
   -- macro definition for operations errors
   ERROR MACRO   ::=
   BEGIN
         TYPE NOTATION   ::=     Parameter
         VALUE NOTATION  ::=     value (localValue INTEGER)
         Parameter       ::=     "PARAMETER" NamedType | empty
         NamedType       ::=     identifier type | type
   END
 END
          Figure 7:  ES Remote Operation Notation
 macro is mapped on the Result parameter of that service.
 If an operation was not successfully performed, the reply is mapped
 on the ESRO-ERROR service.
 In this case one of the errors in the Identifier List of Error Names
 in the ERROR clause of the OPERATION macro may be applied.  The value
 assigned to the applied error is mapped onto the Error parameter of
 that service.  The value of the Named-Type in the PARAMETER clause of
 the ERROR macro of the applied error is mapped on the Error-
 parameter of that service.

4 REMOTE OPERATIONS PROTOCOL

4.1 Overview of the Protocol

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

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

 Two Functional Units are defined which realize the services with 2-
 Way handshake and 3-Way handshake, called 2-Way Handshake Functional
 Unit and 3-Way Handshake Functional Unit respectively.
 The procedures specified in this section refer to Protocol Data Units
 (PDUs) which are defined in Section 4.4.

4.1.1 Service Provision (Invoker User)

    o An ESROS user binds to an ESRO Service Access Point (SAP) and
      specifies whether 3-Way or 2-Way handshake Functional Unit is to
      be associated with the SAP.
    o An ESROS user initiates the transfer of a PDU using the INVOKE
      service.
    o On receipt of an ESROS-INVOKE.request service primitive from the
      ESROS user:
  1. - The ESROS provider generates an Invoke ID,
  1. - Communicates the Invoke-ID to the invoker of the service

through the ESROS-INVOKE-P.confirm primitive,

4.1.2 Service Provision (Performer User)

    o An ESROS user binds to an ESRO Service Access Point (SAP) and
      specifies whether 3-Way or 2-Way handshake Functional Unit is to
      be associated with the SAP.
    o On receipt of an ESRO-INVOKE-PDU, the ESROS provider issues an
      ESROS-INVOKE.indication to the ESROS performer user.
    o On receipt of ESROS-RESULT.request or ESROS-ERROR.request from
      the performer, the provider creates the ESRO-RESULT-PDU or
      ESRO-ERROR-PDU.
    o In the case that the provider receives an ESRO-ACK-PDU for the
      transmitted ESRO-RESULT-PDU or ESRO-ERROR-PDU, if the
      corresponding SAP is associated with the 3-Way Handshake
      Functional Unit, it passes an ESROS-RESULT.confirm or ESROS-
      ERROR.confirm to the performer user.  If the corresponding SAP
      is associated with the 2-Way handshake Functional Unit, the
      ESRO-ACK-PDU is dropped as an invalid PDU.
    o In the case that the provider is not able to deliver the
      ESRO-RESULT-PDU or ESRO-ERROR-PDU, it issues an ESROS-
      FAILURE.indication to the performer user.  In the case that the

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

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

4.2 Protocol Procedures

4.2.1 Service Access Point (SAP) Bind Procedure

 To access the ESRO Services, an ESROS user binds to an ESRO Service
 Access Point and specifies the SAP to be associated with 3-Way
 handshake Functional Unit or 2-Way handshake Functional Unit.  ESROS
 provider generates a SAP descriptor which is passed to the user.  The
 handshaking for all Invoke.requests addressed to that SAP and all
 PDUs addressed to that SAP will be either 3-Way or 2-Way based on the
 Functional Unit associated with SAP and specified by user at SAP bind
 time.
 It is the responsibility of the ESROS peer users (invoker and
 performer) to address their operations to the appropriate SAP (3-Way
 or 2-Way) based on the agreement between users.

4.2.2 Invoke Service Procedure

 An ESROS user initiates the transfer of a PDU using the INVOKE
 service.
 On receipt of an ESRO-INVOKE-PDU, the ESROS provider sends an ESROS-
 INVOKE.indication primitive to the ESROS performer user.

4.2.3 Invoke ID Assignment Procedure

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

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

    o Invoke-Reference-Number:  Uniquely identifies the invocation
      between the two peers.  This is a PDU field with a length of 8
      bits (see section 4.4).
    o Invoke-ID-Parameter:  Uniquely identifies the invocation to the
      service user.  This Invoke-ID-Parameter is a combination of the
      Invoke-Reference-Number described above and the invoker address,
      performer address, and the SAP Selector.
 The provider communicates the Invoke-ID-Parameter to the invoker of
 the INVOKE service through the ESROS-INVOKE-P.confirm primitive.
 The Invoke-Reference-Number distinguishes several invocations of the
 service in progress (asynchronous operations).  It is also used as
 segment identifier when a Service Data Unit (SDU) is transferred
 using segmentation and reassembly.  The ESROS provider may begin to
 reuse the Invoke-Reference-Number values whenever it chooses, subject
 to the constraint that it may not reuse an Invoke-Reference-Number
 value that was previously assigned to an invocation of the service
 for which it expects, but has not yet received, a reply.  In other
 words the provider does not reuse a previously used Invoke-
 Reference-Number unless the corresponding service is fully completed.
 The same value of the Invoke-Reference-Number can be reused to
 identify the invocation between different peer entities.  In that
 case, the combination of the peer entity's address and the Invoke-
 Reference-Number guarantees unique identification of each invocation.

4.2.4 Functional Unit Selection Procedure

 When an ESRO Services user binds to an ESRO SAP, it associates its
 SAP descriptor to 3-Way Handshake Functional Unit or 2-Way Handshake
 Functional Unit.
 Based on the Functional Unit associated with SAP, provider selects
 the corresponding Functional Unit for all Invoke Requests or PDUs
 addressed to that SAP.

4.3 Connectionless PDU Transfer For Small PDUs

4.3.1 Overview

 PDUs sent by UDP use port ESRO_CL_PORT. PDUs carried by UDP are
 restricted to CLRO_SMALL_PDU_MAX_SIZE bytes (see 4.6.1)
 Each PDU is encapsulated in a single UDP datagram.

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

 For PDUs larger than CLRO_SMALL_PDU_MAX_SIZE but smaller than
 CLRO_SEGMENTED_PDU_MAX_SIZE bytes (see 4.6.1), segmentation and
 reassembly is used and each segment is transmitted in a UDP datagram.
 PDUs sent using UDP may be lost, and hence a retransmission strategy
 is defined.  When a PDU is segmented, the retransmission strategy is
 not applied to individual segments (i.e., loss of one segment results
 in retransmission of the whole SDU).
 The optimal UDP retransmission policy will vary with the performance
 of the network and the needs of the transmitter, but the following
 are considered:
 The retransmission interval should be based on prior statistics if
 possible.  Too aggressive retransmission can easily slow response
 time of the network at large.  Depending on how well connected the
 invoker is to its performer, the minimum retransmission interval
 should be RETRANSMISSION_INTERVAL (see 4.6.2) seconds.
 Delivery of PDUs is asynchronous which means the ESROS does not wait
 for the result of a transmitted PDU and continues delivering the next
 PDUs.
     ______________________________________________________
     |From Idle to:                      |Event            |
     |___________________________________|_________________|
     |CL-Invoker Transition Diagram      |ESRO-INVOKE.req  |
     | 2-way Handshake (Connectionless)  |                 |
     |___________________________________|_________________|
     |CL-Invoker Transition Diagram      |ESRO-INVOKE.req  |
     | 3-way Handshake (Connectionless)  |                 |
     |___________________________________|_________________|
     |CL-Performer Transition Diagram    |INVOKE-PDU       |
     | 3-way Handshake (Connectionless)  |                 |
     |___________________________________|_________________|
     |CL-Performer Transition Diagram    |INVOKE-PDU       |
     | 2-way Handshake  (Connectionless) |                 |
     |___________________________________|_________________|
              Table 10:  ESROS Finite State Machine
 This section describes the ESROS protocols in terms of state
 diagrams.  The ESROS Finite State Machine is expressed as four
 separate transition diagrams.  This is illustrated in Table 10.

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

 Details of each of the two transition diagrams for connectionless
 transmission and different handshakings are described in the
 following sections.  The state diagrams show the state, the events,
 the actions taken and the resultant state.The ESROS state transition
 diagrams for connectionless data transmission are presented in Table
 11, Table 12, Table 13, and Table 14.
 Transitions are identified by numbers on the state diagrams.  The
 corresponding actions are listed next to each table.

4.3.2 3-Way Handshake Functional Unit

 This unit implements the Acknowledged Result model of ESRO Services.
 3-Way handshaking is used in this unit.
 The RESULT.confirm and ERROR.confirm primitives on performer are
 generated when ESRO-ACK-PDU is received.
 The FAILURE.indication on performer side is resulted from remote or
 local failures.  Not receiving ESRO-ACK-PDU or local failure can
 generate FAILURE.indication primitive.
 The FAILURE.indication on invoker side is generated if a local
 failure happens or a ESRO-FAILURE-PDU is received.

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

 _______________________________________________________________
 |      State      |STA01     |STA02      |STA03    |STA04     |
 |                 |CL Invoker|Invoke PDU |ACK-PDU  |Invoker   |
 |Event            |Start     |Send       |Send     |RefNu Wait|
 |-----------------+----------+-----------+---------+----------+
 |U: INVOKE.request|(1) STA02 |           |         |          |
 |-----------------+----------+-----------+---------+----------+
 |T: INVOKE PDU    |          |(2) STA02  |         |          |
 | Retransmit      |          |           |         |          |
 |-----------------+----------+-----------+---------+----------+
 |T: Last Timer    |          |(3) STA04  |         |          |
 |-----------------+----------+-----------+---------+----------+
 |P: Result-PDU    |          |           |         |(9) STA04 |
 |-----------------+----------+-----------+---------+----------+
 |P: Failure-PDU   |          |(5) STA04  |         |          |
 |-----------------+----------+-----------+---------+----------+
 |P: ACK-PDU       |          |(6) STA02  |         |          |
 | (Hold On)       |          |           |         |          |
 |-----------------+----------+-----------+---------+----------+
 |P: Duplicate     |          |           |(7) STA03|          |
 |  Result-PDU     |          |           |         |          |
 |-----------------+----------+-----------+---------+----------+
 |T: RefNu Timer   |          |           |         |(8) STA01 |
 |-----------------+----------+-----------+---------+----------+
 |P: Result-PDU    |          |(4) STA03  |         |          |
 |-----------------+----------+-----------+---------+----------+
 |T: Inactivity    |          |           |(10)     |          |
 | Timer           |          |           |STA04    |          |
 |_________________|__________|___________|_________|__________|
 Table 11:  ESROS State Transition Diagram-Connectionless Transmission,
 3-Way HS. P = Protocol, T = Timer, U = User, I = Internal.
 The transmission of INVOKE, RESULT, and ERROR SDUs can be in a single
 PDU (when it fits in one UDP) or a sequence of segment PDUs.
 3-Way Handshake Connectionless Transmission:  Invoker
 For each transition number in the state diagram Table 11, the
 corresponding actions are listed below:
  1. INVOKE.request:
    o Assign Invoke-ID.

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

    o Issue ESROS-INVOKE-P.confirm primitive.
    o Assign invoke reference number.
    o Send operation in one ESRO-INVOKE-PDU or in segmented INVOKE-
      PDUs depending on the size of the operation.
    o Initialize retransmission counter.
    o Initialize retransmission timer.
  2. Invoke PDU Retransmit:
    o Retransmit operation in one ESRO-INVOKE-PDU or segmented PDUs
      while number of retransmissions is less than
      MAX_RETRANSMISSIONS.
    o Increment the retransmission counter.  When MAX_RETRANSMISSIONS
      reached, start LAST_TIMER, otherwise initialize retransmission
      timer.
  3. Last Timer:
    o Issue ESROS-FAILURE.indication primitive.
    o Initialize reference number timer.
  4. ESRO-RESULT-PDU or ESRO-ERROR-PDU (or reassembled ESRO-
     RESULT-SEGMENTED-PDU or ESRO-ERROR-SEGMENTED-PDU when the PDU is
     received in segmented format):
    o Send ESRO-ACK-PDU.
    o Issue ESROS-RESULT.indication or ESROS-ERROR.indication
      primitive.
    o Initialize inactivity timer.
  5. ESRO-FAILURE-PDU:
    o Issue ESROS-FAILURE.indication primitive with User not
      Responding failure cause.
    o Initialize reference number timer.

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

  6. ESRO-ACK-PDU (Hold on):
    o For future use (no action).
  7. Duplicate ESRO-RESULT-PDU or ESRO-ERROR-PDU:
    o Initialize inactivity timer (Ignore PDU).
    o Send ESRO-ACK-PDU.
  8. Invoke reference number timer:
    o Release the invoke reference number.
  9. ESRO-RESULT-PDU or ESRO-ERROR-PDU:
    o Reset Invoke reference number timer.
  10. Inactivity timer:
    o Initialize reference number timer.
 On receipt of an ESROS-INVOKE.request, ESROS provider generates an
 Invoke- Reference-Number and an Invoke-ID (see Section 4.2.3).  The
 provider issues an ESROS-INVOKE-P.confirm primitive and passes the
 Invoke-ID to the invoker.
 The ESROS provider initiates the timer for the Invoke-ID and
 transmits the PDU. Based on the size of SDU, if segmentation is
 required, the SDU is segmented and transmitted in a sequence of
 segmented PDUs.  If the ESRO-RESULT-PDU or ESRO-ERROR-PDU associated
 with the invoke ID is not received within the
 INVOKE_PDU_RETRANSMISSION_INTERVAL (see 4.6.2) period, the SDU is
 retransmitted (in one PDU or segmented and transmitted in a sequence
 of segment PDUs).  The retransmission is repeated for a maximum of
 MAX_RETRANSMISSIONS unless an ESRO-RESULT-PDU or ESRO-ERROR-PDU is
 received.
 If the ESRO-RESULT-PDU or ESRO-ERROR-PDU is received in a segmented
 format, the reassembly process reassembles the sequence of segment
 PDUs.
 In the case that the Hold-on ESRO-ACK-PDU is received from the
 performer, the provider stops retransmitting the ESRO-INVOKE-PDU and
 waits for the ESRO- RESULT-PDU or ESRO-ERROR-PDU for a period equal
 to the multiplication of INVOKE_PDU_RETRANSMISSION_INTERVAL (see
 4.6.2) and MAX_RETRANSMISSIONS (see 4.6.2, for future use).

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

 In the case that the ESRO-INVOKE-PDU is sent MAX_RETRANSMISSIONS (see
 4.6.2) times and no ESRO-RESULT-PDU or ESRO-ERROR-PDU is received,
 the ESROS provider sends an ESROS-FAILURE.indication primitive, with
 the Invoke-ID of the failed PDU and the Failure-value as parameters,
 to the invoker.
 When an ESRO-RESULT-PDU or ESRO-ERROR-PDU is received (whether in one
 PDU or reassembled from a sequence of segmented PDUs), the provider
 issues an ESROS-RESULT.indication or ESROS-ERROR.indication to the
 invoker user, sends an ESRO-ACK-PDU and initializes the inactivity
 timer.  In the case that duplicate ESRO- RESULT-PDU or ESRO-ERROR-PDU
 ____________________________________________________________________
 |      State      |STA01        |STA02      |STA03     |STA04      |
 |                 |CL Performer |Invoke PDU |ACK-PDU   |Performer  |
 |Event            |Start        |Received   |Wait      |RefNu Wait |
 |-----------------+-------------+-----------+----------+-----------|
 |P: Invoke-PDU    |(1) STA02    |           |          |           |
 |-----------------+-------------+-----------+----------+-----------|
 |U: RESULT.req.   |             |(2) STA03  |          |           |
 |-----------------+-------------+-----------+----------+-----------|
 |P: ACK-PDU       |             |           |(3) STA04 |           |
 |-----------------+-------------+-----------+----------+-----------|
 |P: Invoke-PDU    |             |(4) STA02  |(6) STA03 |(7) STA04  |
 | Duplicate       |             |           |          |           |
 |-----------------+-------------+-----------+----------+-----------|
 |T: Result-PDU    |             |           |(5) STA03 |           |
 | Retransmission  |             |           |          |           |
 | Timer           |             |           |          |           |
 |-----------------+-------------+-----------+----------+-----------|
 |I: Failure       |             |(8) STA01  |          |           |
 |-----------------+-------------+-----------+----------+-----------|
 |T: Last Time     |             |           |(9) STA04 |           |
 |-----------------+-------------+-----------+----------+-----------|
 |T: RefNu Timer   |             |           |          |(10) STA01 |
 |-----------------+-------------+-----------+----------+-----------|
 |P: ACK-PDU       |             |           |          |(11) STA04 |
 | Duplicate       |             |           |          |           |
 |-----------------+-------------+-----------+----------+-----------|
 |U/P: Hold On ACK |             |(12) STA02 |          |           |
 ____________________________________________________________________
 Table 12:  ESROS State Transition Diagram-Connectionless Transmission,
 3-Way HS: Performer.  P = Protocol, T = Timer, U = User, I = Internal.

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

 are received, they are ignored, the inactivity timer is reset, and an
 ESRO-ACK-PDU is retransmitted.
 When no duplicate ESRO-RESULT-PDU or ESRO-ERROR-PDU is received for a
 period equal to INACTIVITY_TIME (see 4.6.2), or in the case of ESRO-
 INVOKE- PDU retransmission time-out, or in the case of internal
 failure, the provider initializes the reference number timer.  After
 REFERENCE_NUMBER_TIME (see 4.6.2), the reference number is released.
 3-Way Handshake Connectionless Transmission:  Performer
 For each transition number in the state diagram above, the
 corresponding actions are listed below:
   1. ESRO-INVOKE-PDU (as a single PDU or a sequence of segment PDUs):
    o Issue ESROS-INVOKE.indication primitive.
  2. ESROS-RESULT.request or ESROS-ERROR.request:
    o Add invoke reference number to the active list.
    o Transmit ESRO-RESULT-PDU or ESRO-ERROR-PDU (in a single PDU or
      a sequence of segment PDUs).
    o Set ESRO-RESULT-PDU or ESRO-ERROR-PDU retransmission timer.
  3. ESRO-ACK-PDU:
    o Initialize invoke reference number timer.
    o Issue ESROS-RESULT.confirm or ESROS-ERROR.confirm.
  4. Duplicate ESRO-INVOKE-PDU:
    o No action (ignore the duplicate ESRO-INVOKE-PDU).
  5. ESRO-RESULT-PDU or ESRO-ERROR-PDU retransmission timer:
    o Retransmit ESRO-RESULT-PDU or ESRO-ERROR-PDU (in a single PDU
      or in a segmented format) while number of retransmissions is
      less than MAX_RETRANSMISSIONS.
    o Increment the transmission counter.

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

  6. Duplicate ESRO-INVOKE-PDU:
    o Retransmit ESRO-RESULT-PDU or ESRO-ERROR-PDU.
    o Reset ESRO-RESULT-PDU or ESRO-ERROR-PDU retransmission timer.
    o Re-initialize the number of retransmissions counter to 1.
  7. Duplicate ESRO-INVOKE-PDU:
    o Reset invoke reference number timer.
  8. Internal failure:
    o Send ESRO-FAILURE-PDU.
    o Release the invoke reference number.
  9. Last time:
    o Issue ESROS-FAILURE.indication.
    o Initialize invoke reference number timer.
  10. Invoke reference number timer:
    o Release the invoke reference number.
  11. Duplicate ESRO-ACK-PDU:
    o Reset invoke reference number timer.
  12. Hold-on ACK request:
    o Send hold-on ESRO-ACK-PDU (for future use).
 On receipt of an ESRO-INVOKE-PDU, the ESROS provider issues an
 ESROS-INVOKE.indication to the ESROS performer user.  The provider
 ignores the duplicate ESRO-INVOKE-PDUs.
 In the case of internal failure or no response from performer user,
 the provider sends an ESRO-FAILURE-PDU and releases the invoke
 reference number.
 On receipt of a Hold-on request from the performer user, or based on
 other information, provider sends a Hold-on ESRO-ACK-PDU (future
 use).

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

 On receipt of either ESROS-RESULT.request or ESROS-ERROR.request from
 the ESROS performer user, the ESROS provider initiates the
 retransmission timer for the ESRO-RESULT-PDU or ESRO-ERROR-PDU and
 transmits the ESRO-RESULT-PDU or ESRO-ERROR-PDU in a single PDU or in
 a sequence of segment PDUs.  If the ESRO-ACK-PDU associated with the
 Invoke-ID is not received within
 RESULT_ERROR_PDU_RETRANSMISSION_INTERVAL (see 4.6.2), the PDU is
 retransmitted.
 When provider is waiting for ESRO-ACK-PDU and a duplicate ESRO-
 INVOKE-PDU arrives, ESRO-RESULT-PDU or ESRO-ERROR-PDU is
 retransmitted (in a single PDU or in a sequence of segment PDUs), the
 retransmission timer is reset and counter for number of
 retransmissions is re-initialized to 1.
 If after MAX_TRANSMISSIONS (see 4.6.2) no ESRO-ACK-PDU is received,
 the provider issues an ESROS-FAILURE.indication primitive, with the
 Invoke-ID of the failed PDU and the Failure-value as parameters, to
 the performer user.  Then the provider sets the reference number
 timer and releases the reference number after REFERENCE_NUMBER_TIME
 (see 4.6.2).
 On receipt of ESRO-ACK-PDU associated with the Invoke-ID before
 MAX_TRANSMISSIONS (see 4.6.2), the provider issues a ESROS-
 RESULT.confirm or ESROS-ERROR.confirm primitive and sets the
 reference number timer and releases the reference number after
 REFERENCE_NUMBER_TIME (see 4.6.2).
 The duplicate ESRO-ACK-PDU and duplicate ESRO-INVOKE-PDUs are ignored
 while provider waits for the reference number timer to expire.

4.3.3 2-Way Handshake Functional Unit

 This Functional Unit implements the Not-Acknowledged Result model of
 ESRO Services.  2-Way handshaking is used in this unit.
 The RESULT.confirm and ERROR.confirm primitives on performer side are
 generated based on time-out, i.e.  when no duplicate ESRO-INVOKE-PDU
 is received in a specified period of time, provider issues
 RESULT.confirm or ERROR.confirm primitive.
 The FAILURE.indication on performer side is generated as a result of
 local failure or after time-out of retransmission of ESRO-RESULT-PDU
 or ESRO-ERROR-PDU.
 The FAILURE.indication on invoker side is generated if a local
 failure happens or a ESRO-FAILURE-PDU is received.

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

 The transmission of INVOKE, RESULT, and ERROR PDUs can be in a single
 PDU (when it fits in one PDU) or a sequence of segmented PDUs.
 2-Way Handshake Connectionless Transmission:  Invoker
 For each transition number in the state diagram above, the
 corresponding actions are listed below:
  1. INVOKE.request:
    o Assign Invoke-ID.
    o Issue ESROS-INVOKE-P.confirm primitive.
    o Assign invoke reference number.
    o Send ESRO-INVOKE-PDU in a single PDU or as a sequence of
      segment PDUs.
    o Initialize retransmission counter.
    ______________________________________________________________
    |      State              |STA01      |STA02      |STA03     |
    |                         |2-Way HS   |Invoke PDU |Invoker   |
    |                         |CL Invoker |Send       |RefNu Wait|
    |Event                    |Start      |           |          |
    |-------------------------+-----------+-----------+----------|
    |U: INVOKE.req.           |(1) STA02  |           |          |
    |-------------------------+-----------+-----------+----------|
    |T: Invoke PDU Retransmit |           |(2) STA02  |          |
    |-------------------------+-----------+-----------+----------|
    |T: Last Timer            |           |(3) STA03  |          |
    |-------------------------+-----------+-----------+----------|
    |P: Result/Error PDU      |           |(4) STA03  |          |
    |-------------------------+-----------+-----------+----------|
    |P: Failure-PDU           |           |(5) STA03  |          |
    |-------------------------+-----------+-----------+----------|
    |P: Duplicate Result PDU  |           |           |(6) STA03 |
    |-------------------------+-----------+-----------+----------|
    |T: RefNu Timer           |           |           |(7) STA01 |
    |_________________________|___________|___________|__________|
    Table 13:  ESROS State Transition Diagram-Connectionless Transmission,
    2-Way HS: Invoker p = Protocol, T = Timer, U = User, I = Internal.

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

  2. Invoke PDU Retransmit:
    o Retransmit ESRO-INVOKE-PDU (in a single PDU or in a sequence of
      segment PDUs) while number of retransmissions is less than
      MAX_RETRANSMISSIONS.
    o Increment the transmission counter.  When MAX_RETRANSMISSIONS
      reached, start LAST_TIMER.
  3. Last Timer:
    o Issue ESROS-FAILURE.indication primitive.
    o Initialize reference number timer.
  4. ESRO-RESULT-PDU or ESRO-ERROR-PDU:
    o Issue ESROS-RESULT.indication or ESROS-ERROR.indication
      primitive.
    o Initialize reference number timer.
  5. ESRO-FAILURE-PDU:
    o Issue ESROS-FAILURE.indication primitive with User not
      Responding failure cause.
    o Initialize reference number timer.
  6. Duplicate ESRO-RESULT-PDU or ESRO-ERROR-PDU:
    o Reset Invoke reference number timer.
  7. Invoke reference number timer:
    o Release the invoke reference number.
 On receipt of an ESROS-INVOKE.request, ESROS provider generates an
 Invoke- Reference-Number and an Invoke-ID (see 4.2.3).  The provider
 issues an ESROS-INVOKE-P.confirm primitive and passes the Invoke-ID
 to the invoker.
 The ESROS provider initiates the timer for the Invoke-ID and
 transmits the PDU. The PDU is transmitted as a single PDU or a
 sequence of segment PDUs.  If the ESRO- RESULT-PDU or ESRO-ERROR-PDU
 associated with the invoke ID is not received within the

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

 INVOKE_PDU_RETRANSMISSION_INTERVAL (see 4.6.2) period, the PDU is
 retransmitted.  The retransmission is repeated for a maximum of
 MAX_RETRANSMISSIONS unless an ESRO-RESULT-PDU or ESRO-ERROR-PDU is
 received.
 In the case that the ESRO-INVOKE-PDU is sent MAX_RETRANSMISSIONS (see
 4.6.2) times and no ESRO-RESULT-PDU or ESRO-ERROR-PDU is received,
 the ESROS provider sends an ESROS-FAILURE.indication primitive, with
 the Invoke-ID of the failed PDU and the Failure-value as parameters,
 to the invoker.  If ESRO- FAILURE-PDU is received, the ESROS provider
 sends and ESROS-FAILURE.indication primitive, with the Invoke-Id of
 the failed PDU and the Failure- value as parameters to the invoker.
 When an ESRO-RESULT-PDU or ESRO-ERROR-PDU is received, the provider
 issues an ESROS-RESULT.indication or ESROS-ERROR.indication to the
 invoker user, and initializes the Reference-Number timer.  In the
 case that duplicate ESRO-RESULT-PDU or ESRO-ERROR-PDU are received,
 they are ignored.  In the case of internal failure, the provider
 initializes the reference number timer.  After REFERENCE_NUMBER_TIME
 (see 4.6.2), the reference number is released.
 2-Way Handshake Connectionless Transmission:  Performer
 ___________________________________________________________________
 |      State        |STA01      |STA02     |STA03     |STA04      |
 |                   |2-Way HS CL|Invoke PDU|Result PDU|Performer  |
 |Event              |Performer  |Received  |Retransmit|RefNu Wait |
 |                   |Start      |          |          |           |
 |-------------------+-----------+----------+----------+-----------|
 |P: Invoke-PDU      |(1) STA02  |          |          |           |
 |-------------------+-----------+----------+----------+-----------|
 |P: Invoke-PDU      |           |(2) STA02 |(5) STA03 |(7) STA04  |
 | Duplicate         |           |          |          |           |
 |-------------------+-----------+----------+----------+-----------|
 |U: RESULT.req.     |           |(3) STA03 |          |           |
 |-------------------+-----------+----------+----------+-----------|
 |I: Failure         |           |(4) STA01 |          |           |
 |-------------------+-----------+----------+----------+-----------|
 |T: Inactivity Timer|           |          |(6) STA04 |           |
 |-------------------+-----------+----------+----------+-----------|
 |T: RefNu Timer     |           |          |          |(8) STA01  |
 ___________________________________________________________________
 Table 14:  ESROS State Transition Diagram-Connectionless Transmission,
 2-Way HS: Performer.  P = Protocol, T = Timer, U = User, I = Internal.

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

 For each transition number in the state diagram above, the
 corresponding actions are listed below:
  1. ESRO-INVOKE-PDU (received in a single PDU or reassembled from a
     sequence of segment PDUs):
    o Issue ESROS-INVOKE.indication primitive.
  2. Duplicate ESRO-INVOKE-PDU:
    o No action (ignore the duplicate ESRO-INVOKE-PDU).
  3. EROS-RESULT.request or ESROS-ERROR.request:
    o Add invoke reference number to the active list.
    o Transmit ESRO-RESULT-PDU or ESRO-ERROR-PDU (as a single PDU or
      as a sequence of segment PDUs.)
    o Set Inactivity timer.
  4. Internal failure:
    o Send ESRO-FAILURE-PDU.
    o Release the invoke reference number.
  5. Duplicate ESRO-INVOKE-PDU:
    o Retransmit ESRO-RESULT-PDU or ESRO-ERROR-PDU (as a single PDU
      or as a sequence of segment PDUs.)
    o Set Inactivity timer.
  6. Inactivity Timer:
    o Issue ESROS-RESULT.confirm.
    o Initialize invoke reference number timer.
  7. Dplicate ESRO-INVOKE-PDU:
    o Reset invoke reference number timer.
  8. Invoke reference number timer:
    o Release the invoke reference number.

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

 On receipt of an ESRO-INVOKE-PDU (as a single PDU or reassembled from
 a sequence of segment PDUs), the ESROS provider issues an ESROS-
 INVOKE.indication to the ESROS performer user.  The provider ignores
 the duplicate ESRO-INVOKE-PDUs.
 In the case of internal failure or no response from performer user,
 the provider sends an ESRO-FAILURE-PDU and releases the invoke
 reference number.
 On receipt of either ESROS-RESULT.request or ESROS-ERROR.request from
 the ESROS performer user, the ESROS provider initiates the inactivity
 timer for the ESRO- RESULT-PDU or ESRO-ERROR-PDU and transmits the
 ESRO-RESULT-PDU or ESRO-ERROR-PDU (in a single PDU or as a sequence
 of segment PDUs.)  If a duplicate ESRO-INVOKE-PDU associated with the
 Invoke-ID is received within INACTIVITY_TIME interval (see 4.6.2),
 the PDU is retransmitted.
 If no duplicate ESRO-INVOKE-PDU is received within the
 INACTIVITY_TIME interval (see 4.6.2), provider issues a ESROS-
 RESULT.confirm or ESROS-ERROR.confirm primitive and sets the
 reference number timer and releases the reference number after
 REFERENCE_NUMBER_TIME (see 4.6.2).
 The duplicate ESRO-INVOKE-PDUs are ignored while provider waits for
 the reference number timer to expire.

4.3.4 Segmentation and Reassembly

 Small ESRO Service Data Units (ESRO-SDUs) can benefit from the
 efficiencies of connectionless feature of ESROS (See Section 4.3.1).
 When an ESRO-SDU is too large to fit in a single connectionless PDU
 it is segmented and reassembled.  There might be similar mechanisms
 in the upper layers with different levels of efficiency.  When in
 addition to the ESROS segmentation/reassembly, the upper layers are
 capable of segmentation/reassembly services, then the ESROS user can
 decide whether to use ESROS segmenting/reassembly mechanism depending
 on the factors such as reliability of the underlying network.
 In the case of segmentation/reassembly in ESROS layer, transmission
 of operation segments is not acknowledged.  This results in an
 efficient transmission over a reliable underlying network.  However
 failure of one segment results in retransmission of all segments.
 When acknowledged segments are desired, the ESROS user should
 implement it using the acknowledged result service of ESROS.

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

 The ESROS segmentation/reassembly is accommodated by:
    o Use of two additional PDU codes for segmented INVOKE PDU.
    o Use of one byte segmentation information, which contains
      First/Other flag and segment number.
    o Use of unused bits of RESULT and ERROR PDUs to identify a
      segmented RESULT or ERROR PDU.
 Segmentation and Assembly applies to INVOKE, RESULT, and ERROR SDUs.
 The sender of the message is responsible for segmenting the ESRO-SDU
 into segments that fit in CL PDUs.  The segmented ESRO-SDU is sent in
 a sequence of segments each carrying a segment of the SDU. The
 Invoke-Reference-Number is a unique identifier that is used as the
 segment identifier which relates all segments of an ESRO-SDU. In
 addition to this identifier, the first segment specifies the total
 number of segments (number-of-segments).  Other segments have a
 segment sequence number (segment- number).  The receiver is
 responsible for sequencing (based on segment-number) and reassembling
 the entire ESRO-SDU.
 Segmenting/Reassembling over the Connectionless ESRO Service
 The sender maps the original ESRO-SDU into an ordered sequence of
 segments.  Several ESRO-SDU segment sequences can exist over the same
 ESROS association, distinguished by their Invoke-Reference-Number
 (used as segment identifier.)
 All segments in the sequence have the same Invoke-Reference-Number
 assigned by sender.
 The first segment specifies the total number of segments.  All
 segments in the sequence except the first one shall be sequentially
 numbered, starting at 1 (first segment has an implicit segment number
 of 0).
 Each segment is transmitted in one UDP PDU and is sent by sender.
 All segments of a segmented ESRO-SDU are identified by the same
 Invoke-Reference-Number.  For a given operation, the receiver should
 not impose any restrictions on the order of arrival of segments.
 There is no requirement that any segment content be of
 CLRO_SMALL_PDU_MAX_SIZE for connectionless transmission; however, no
 more than CLRO_MAX_PDU_SEGMENTS segments can be derived from a single
 ESRO-SDU.

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

 The receiver reassembles a sequence of segments into a single ESRO-
 SDU. An ESRO-SDU shall not be further processed unless all segments
 of the ESRO-SDU are received.  Failure to receive the SDU shall be
 determined by the following event:
    o Expiration of Reassembly Timer (see Section 4.3.4).
 In the event of the above mentioned failure, the receiver shall
 discard a partially assembled sequence.
 The reassembly is done as described below:
    o In the case of segmented Invoke ESRO-SDU, the encoding type and
      operation-value fields are carried in the first segment used for
      the whole operation.  These three fields are ignored in the
      segments other than the first one.
    o In the case of segmented Result ESRO-SDU, the encoding type of
      the first segment is used for all segments.  The encoding type
      field of segments other than the first one are ignored.
    o In the case of segmented Error ESRO-SDU, the encoding type and
      Error-value field of the first segment are used for all
      segments.  These two fields are ignored in segments other than
      the first one.
 Sender sends all segments of a segmented ESRO-SDU one after the
 other.  There is no mechanism for retransmission of a single segment.
 In the case that the sender receives a failure indication for a
 segment, it means that receiver has failed in reassembly process, and
 the sender retransmits the whole ESRO-SDU (all segments).
 Reassembly Timer
 The Reassembly Timer is a local timer maintained by the receiver of
 the segments that assists in performing the reassembly function.
 This timer determines how long a receiver waits to receive all
 segments of a segment sequence.
 The Reassembly Timer shall be started on receipt of a segment with
 different sequence identifier (Invoke-Reference-Number).  On receipt
 of all segments composing a sequence, the corresponding reassembly
 timer shall be stopped.

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

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

4.4 Structure and Encoding of ESROS PDUs

 Five PDU types are used in the ESRO protocol which are described in
 the following sections.  PDU type coding is presented in Table 15.
 The octets are numbered in increasing order, starting from 1.  The
 bits of an octet are numbered from 1 to 8, where 1 is the low-order
 bit.

4.4.1 ESRO-INVOKE-PDU Format

 Bit string format of the ESRO-INVOKE-PDU is represented in Table 16
 and Table 17.
         _______________________________________
         | PDU Name              |PDU Type Code |
         _______________________________________
         | ESRO-INVOKE           |0             |
         | ESRO-RESULT           |1             |
         | ESRO-ERROR            |2             |
         | ESRO-ACK              |3             |
         | ESRO-FAILURE          |4             |
         | ESRO-SEGMENTED-INVOKE |5             |
         _______________________________________
                 Table 15:  PDU Coding

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

     __________________________________________________________
     |Bit       |8 |            7           |6 |5 |4 |3 |2 |1 |
     |----------+--+------------------------+--+--+--+--+--+--|
     |Octet 1   | Performer SAP                   |0 |0 |0 |0 |
     |----------+--+------------------------+--+--+--+--+--+--|
     |Octet 2   | Invoke Reference Number                     |
     |----------+--+------------------------+--+--+--+--+--+--|
     |Octet 3   |Parameter Encoding Type    |Operation Value  |
     |----------+--+------------------------+--+--+--+--+--+--|
     |Octet 4   |                                             |
     | ...      | Operation Information                       |
     |Octet N   |                                             |
     |__________|_____________________________________________|
        Table 16:  ESRO-INVOKE-PDU format. ESRO-INVOKE-PDU Type Code =
        0.  Note:  Invoker SAP = Performer SAP - 1.
                       _______________________
                       | Value |   Meaning    |
                       |_______|______________|
                       | 0     |   BER [5]    |
                       |_______|______________|
                       | 1     |   PER [4]    |
                       |_______|______________|
                       | 2     |   XDR [8]    |
                       |_______|______________|
                       | 3     |  Reserved    |
                       |_______|______________|
       Table 17:  Parameter Encoding Type for ESRO-INVOKE-PDU

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

  1. ——————————————————–

|Bit | 8 | 7 | 6 | 5 | 4 | 3 | 2 | 1 |

       |--------|-----------|-----|-----|-----|-----|-----|-----|
       |        | Parameter |     |     |     |     |     |     |
       |        | Encoding  |     |     |     |     |     |     |
       |Octet 1 | Type      |  0  |  0  |  0  |  0  | 0   |  1  |
       |        |           |     |     |     |     |     |     |
       |        |           |     |     |     |     |     |     |
       |        |           |     |     |     |     |     |     |
       |--------|-----------------------------------------------|
       |Octet 2 |           Invoke Reference Number             |
       |--------|-----------------------------------------------|
       |Octet 3 |                                               |
       |...     |             Result-parameter                  |
       |Octet N |                                               |
        ---------------------------------------------------------
       ESRO-RESULT-PDU Type Code = 1.
                       Table 18:  ESRO-RESULT-PDU format
                       _______________________
                       | Value |   Meaning    |
                       |_______|______________|
                       | 0     |    Basic     |
                       |_______|______________|
                       | 1     |   Packed     |
                       |_______|______________|
                       | 2     |     XDR      |
                       |_______|______________|
                       | 3     |  Reserved    |
                       |_______|______________|
        Table 19:  Parameter Encoding Type for ESRO-RESULT-PDU

4.4.2 ESRO-RESULT-PDU Format

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

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

4.4.3 ESRO-ERROR-PDU Format

 Bit string format of the ESRO-ERROR-PDU is represented in Table 20
 and Table 21.
  1. ——————————————————–

|Bit | 8 | 7 | 6 | 5 | 4 | 3 | 2 | 1 |

       |--------|-----------|-----|-----|-----|-----|-----|-----|
       |        | Parameter |     |     |     |     |     |     |
       |        | Encoding  |     |     |     |     |     |     |
       |Octet 1 | Type      |  0  |  0  |  0  |  0  | 1   |  0  |
       |        |           |     |     |     |     |     |     |
       |        |           |     |     |     |     |     |     |
       |        |           |     |     |     |     |     |     |
       |--------|-----------------------------------------------|
       |Octet 2 |           Invoke Reference Number             |
       |--------|-----------------------------------------------|
       |Octet 3 |               Error Value                     |
       |--------|-----------------------------------------------|
       |Octet 4 |                                               |
       |...     |             Error parameter                   |
       |Octet N |                                               |
        ---------------------------------------------------------
       ESRO-ERROR-PDU Type Code = 2.
                        Table 20:  ESRO-ERROR-PDU format
                       _______________________
                       | Value |   Meaning    |
                       |_______|______________|
                       | 0     |    Basic     |
                       |_______|______________|
                       | 1     |   Packed     |
                       |_______|______________|
                       | 2     |     XDR      |
                       |_______|______________|
                       | 3     |  Reserved    |
                       |_______|______________|
             Table 21:  Parameter Encoding Type for ESRO-ERROR-PDU

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

  1. ——————————————————–

|Bit | 8 | 7 | 6 | 5 | 4 | 3 | 2 | 1 |

          |--------|-----------------------|-----|-----|-----|-----|
          |        | ESRO-ACK-PDU Type     |     |     |     |     |
          |Octet 1 |                       |  0  |  0  | 1   |  1  |
          |--------|-----------------------------------------------|
          |Octet 2 |           Invoke Reference Number             |
           ---------------------------------------------------------
           ESRO-ACK-PDU Type Code = 3.
                          Table 22:  Fields of ESRO-ACK-PDU
             _____________________________________________
             | ESRO-ACK-PDU Type |        Meaning         |
             |___________________|________________________|
             | 0                 |Complete 3-way handshake|
             |___________________|________________________|
             | 1                 |        Hold on         |
             |___________________|________________________|
                Table 23:  Encoding of ESRO-ACK-PDU Type

4.4.4 ESRO-ACK-PDU Format

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

4.4.5 ESRO-FAILURE-PDU Format

 Bit string format of the ESROS-FAILURE-PDU is represented in Table 24
 and Table 25.
 The first nibble of the first octet of ESRO-FAILURE-PDU shall be set
 to zero.

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

4.4.6 ESRO-INVOKE-SEGMENTED-PDU Format

 Bit string format of the ESRO-INVOKE-SEGMENTED-PDU is represented in
 Table 25 and Table 26.
 Note:  Invoker SAP = Performer SAP - 1.
  1. ——————————————————–

|Bit | 8 | 7 | 6 | 5 | 4 | 3 | 2 | 1 |

   |--------|-----------------------|-----|-----|-----|-----|
   |Octet 1 |         Not used      |  0  |  1  |  0  |  0  |
   |--------|-----------------------------------------------|
   |Octet 2 |           Invoke Reference Number             |
   |--------|-----------------------------------------------|
   |Octet 3 |                Failure Value                  |
   ----------------------------------------------------------
   ESRO-FAILURE-PDU Type Code = 4.
                   Table 24:  ESRO-FAILURE-PDU format
             ________________________________________
             | Failure Value |       Meaning         |
             |_______________|_______________________|
             | 0             |Transmission failure   |
             |_______________|_______________________|
             | 1             |Out of local resources |
             |_______________|_______________________|
             | 2             | User not responding   |
             |_______________|_______________________|
             | 3             |Out of remote resources|
             |_______________|_______________________|
               Table 25:  Encoding of failure value

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

  1. —————————————————————-

|Bit | 8 | 7 | 6 | 5 | 4 | 3 | 2 | 1 |

   |--------|---------------------------|------|------|------|------|
   |Octet 1 | Performer Service         |  0   |  1   |  0   |  1   |
   |        | Access Point Selector     |      |      |      |      |
   |--------|-------------------------------------------------------|
   |Octet 2 |            Invoke Reference Number                    |
   |--------|-------------------------------------------------------|
   |        | Parameter   |                                         |
   |        | Encoding    |                                         |
   |Octet 3 | Type        |        Operation Value                  |
   |        |             |                                         |
   |        |             |                                         |
   |        |             |                                         |
   |--------|-------------------------------------------------------|
   |Octet 4 |First/|                                                |
   |        |Other |                  Segment Number                |
   |--------|-------------------------------------------------------|
   |Octet 5 |                                                       |
   |...     |             Operation Information                     |
   |Octet N |                                                       |
    ----------------------------------------------------------------|
    ESRO-INVOKE-PDU Type Code = 5.
              Table 26:  ESRO-INVOKE-SEGMENTED-PDU format
                        _______________________
                        | Value |   Meaning    |
                        |_______|______________|
                        | 0     |    Basic     |
                        |_______|______________|
                        | 1     |   Packed     |
                        |_______|______________|
                        | 2     |     XDR      |
                        |_______|______________|
                        | 3     |  Reserved    |
                        |_______|______________|
   Table 27:  Parameter Encoding Type for ESRO-INVOKE-SEGMENTED-PDU
    o For the first segment, the first/other bit is set to one, and
      the segment number field contains the total number of segments.
    o For segments other than the first one, the first/other bit is
      set to zero, and the segment number field has the sequence
      number of the segment.

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

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

4.4.7 ESRO-RESULT-SEGMENTED-PDU Format

 Bit string format of the ESRO-RESULT-SEGMENTED-PDU is represented in
 Table 28 and Table 29.
    o For the first segment, the first/other bit is set to one, and
    the
      segment number field contains the total number of segments.
    o For segments other than the first one, the first/other bit is
    set
      to zero, and the segment number field has the sequence number of
      the segment.
 The values of the Parameter-Encoding-Type field of the first segment
 is used by invoker and this field is ignored in the segments other
 than the first one.
  1. —————————————————————-

|Bit | 8 | 7 | 6 | 5 | 4 | 3 | 2 | 1 |

   |--------|-------------|------|------|------|------|------|------|
   |        | Parameter   |      |      |      |      |      |      |
   |        | Encoding    |      |      |      |      |      |      |
   |Octet 1 | Type        |  0   |  1   |  0   |  0   |  0   |  1   |
   |        |             |      |      |      |      |      |      |
   |        |             |      |      |      |      |      |      |
   |        |             |      |      |      |      |      |      |
   |--------|-------------------------------------------------------|
   |Octet 2 |            Invoke Reference Number                    |
   |--------|-------------------------------------------------------|
   |Octet 4 |First/|                                                |
   |        |Other |                  Segment Number                |
   |--------|-------------------------------------------------------|
   |Octet 5 |                                                       |
   |...     |                 Result Parameter                      |
   |Octet N |                                                       |
    ----------------------------------------------------------------|
   ESRO-RESULT-SEGMENTED-PDU Type Code = 1.
              Table 28:  ESRO-RESULT-SEGMENTED-PDU format

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

                        _______________________
                        | Value |   Meaning    |
                        |_______|______________|
                        | 0     |    Basic     |
                        |_______|______________|
                        | 1     |   Packed     |
                        |_______|______________|
                        | 2     |     XDR      |
                        |_______|______________|
                        | 3     |  Reserved    |
                        |_______|______________|
   Table 29:  Parameter Encoding Type for ESRO-RESULT-SEGMENTED-PDU
  1. —————————————————————-

|Bit | 8 | 7 | 6 | 5 | 4 | 3 | 2 | 1 |

   |--------|-------------|------|------|------|------|------|------|
   |        | Parameter   |      |      |      |      |      |      |
   |        | Encoding    |      |      |      |      |      |      |
   |Octet 1 | Type        |  0   |  1   |  0   |  0   |  1   |  0   |
   |        |             |      |      |      |      |      |      |
   |        |             |      |      |      |      |      |      |
   |        |             |      |      |      |      |      |      |
   |--------|-------------------------------------------------------|
   |Octet 2 |            Invoke Reference Number                    |
   |--------|-------------------------------------------------------|
   |Octet 3 |First/|                                                |
   |        |Other |                  Segment Number                |
   |--------|-------------------------------------------------------|
   |Octet 4 |                    Error Value                        |
   |--------|-------------------------------------------------------|
   |Octet 5 |                                                       |
   |...     |                 Error Parameter                       |
   |Octet N |                                                       |
    ----------------------------------------------------------------|
   ESRO-ERROR-SEGMENTED-PDU Type Code = 2.
                  Table 30:  ESRO-ERROR-SEGMENTED-PDU

4.4.8 ESRO-ERROR-SEGMENTED-PDU Format

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

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

    o For the first segment, the first/other bit is set to one, and
      the segment number field contains the total number of segments.
    o For segments other than the first one, the first/other bit is
      set to zero, and the segment number field has the sequence
      number of the segment.
 The values of the Parameter-Encoding-Type field of the first segment
 is used by invoker and this field is ignored in the segments other
 than the first one.

4.5 Concatenation and Separation

 The procedure for concatenation and separation conveys multiple
 ESRO-PDUs in one TSDU. This is accomplished by ESRO-CONCATENATED-PDU.
                        _______________________
                        | Value |   Meaning    |
                        |_______|______________|
                        | 0     |    Basic     |
                        |_______|______________|
                        | 1     |   Packed     |
                        |_______|______________|
                        | 2     |     XDR      |
                        |_______|______________|
                        | 3     |  Reserved    |
                        |_______|______________|
    Table 31:  Parameter Encoding Type for ESRO-SEGMENTED-ERROR-PDU
 An ESRO-CONCATENATED-PDU can contain one or more of the following
 PDUs:  INVOKE, RESULT, ERROR, FAILURE, and ACK.
 The ESRO-PDUs within a concatenated set may be distinguished by means
 of the length indicator.  A one byte length indicator comes before
 each ESRO-PDU.
 The number of ESRO-PDUs in an ESRO-CONCATENATED-PDU is bounded by the
 maximum length of TSDU.

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

4.5.1 Procedures

 Concatenation
 The ESROS provider concatenates PDUs as follows:
    o PDU type code 8 is used.
    o The length indicator which is the total length of first ESRO-PDU
      (header and data) in octets is placed after PDU type code in
      length indicator field of ESRO- CONCATENATED-PDU (see Section
      4.5.2).
    o The first PDU (header and data) is placed after the length
      indicator field and in the ESRO-PDU field of ESRO-CONCATENATED-
      PDU (see Section 4.5.2).
    o For any additional ESRO-PDU, the length indicator and PDUs are
      concatenated.
 Separation
 When the ESRO service provider receives a PDU with PDU type code 8,
 it separates the concatenated PDUs as described below:
    o Length indicator field coming after type code field (see
      Section 4.5.2) specifies the total length of the first PDU in
      octets.
    o The first PDU is in the ESRO-PDU field after the length
    indicator
      field (see Section 4.5.2).
    o Any additional PDU has its length indicator field specifying the
      total length of PDU, followed by PDU itself (see Section 4.5.2).
    o PDUs are separated until the end of the ESRO-CONCATENATED-PDU is
      reached.

4.5.2 ESRO-CONCATENATED-PDU format

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

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

 Length Indicator field
 This field is contained in one octet and comes before each ESROS-PDU
 in the concatenated PDU. The length indicated is total length of the
 ESRO-PDU (including header and data) coming after it in octets.
 ESRO-PDU field
 This field contains an ESRO-INVOKE-PDU, ESRO-RESULT-PDU, ESRO-ERROR-
 PDU, ESRO-FAILURE-PDU, or ESRO-ACK-PDU.
 The length of this field is specified by the length indicator field
 coming before it.

4.6 ES Remote Operations Protocol Parameters

4.6.1 PDU size

    o CLRO_SMALL_PDU_MAX_SIZE:
       ________________________________________________________________
      |Bit     |  8   |  7   |  6   |  5   |  4   |  3   |  2   |  1   |
      |--------|---------------------------|------|------|------|------|
      |Octet 1 |         Not used          |  1   |  0   |  0   |  0   |
      |--------|-------------------------------------------------------|
      |Octet 2 |                  Length Indicator                     |
      |--------|-------------------------------------------------------|
      |Octet 3 |                                                       |
      |...     |                     ESRO-PDU                          |
      |Octet N |                                                       |
      |--------|-------------------------------------------------------|
      |Octet   |                                                       |
      | N+1    |                  Length Indicator                     |
      |--------|-------------------------------------------------------|
      |Octet   |                                                       |
      | N+2    |                     ESRO-PDU                          |
      | ...    |                                                       |
      |--------|-------------------------------------------------------|
      | ...    |                        ...                            |
      |________|_______________________________________________________|
      ESRO-CONCATENATED-PDU Type Code = 8.
                   Table 32:  ESRO-CONCATENATED-PDU format

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

      The value of this parameter should be chosen based on the
      specifics of the subnetwork in use.  For example, in CDPD the
      maximum size of SN-Userdata size can be up to 2048 bytes (see
      part 404-2.b of CDPD Specification V1.1).  Based on this value
      and IP and UDP protocol information fields, the value of
      CLRO_SMALL_PDU_MAX_SIZE may be determined for CDPD. Again based
      on the specifics of the subnetwork, the optimum value of
      CLRO_SMALL_PDU_MAX_SIZE may best be determined based on field
      experience and may be smaller than the maximum size that the
      subnetwork supports.
    o CLRO_SEGMENTED_PDU_MAX_SIZE
      The value of this parameter should be chosen based on the
      specifics of the subnetwork in use.  The optimum value of
      CLRO_SEGMENTED_PDU_MAX_SIZE may best be determined based on
      field experience.
    o CLRO_MAX_PDU_SEGMENTS
      The value of this parameter should be chosen based on the
      specifics of the subnetwork in use.  The optimum value of
      CLRO_MAX_PDU_SEGMENTS may best be determined based on field
      experience.  In any case, this value should be smaller than 127.

4.6.2 Timers

    o INVOKE_PDU_RETRANSMISSION_INTERVAL:
      The INVOKE_PDU retransmission interval should be specified and
      optimized based on the characteristics of the network in use.
    o RESULT_ERROR_PDU_RETRANSMISSION_INTERVAL:
      The RESULT and ERROR-PDU retransmission interval should be
      specified and optimized based on the characteristics of the
      network in use.
    o MAX_RETRANSMISSIONS:
      The maximum number of retransmissions should be specified and
      optimized based on the characteristics of the network in use.
    o INACTIVITY_TIME:
      The minimum waiting time during which no duplicate PDU is
      received should be specified and optimized based on the
      characteristics of the network in use.

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

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

4.6.3 Use of lower layers

 ESRO protocol uses UDP port number 259.

5 ACKNOWLEDGMENTS

 Development of this specification was funded by AT&T Wireless
 Services (AWS). This protocol specification has been derived from
 AT&T Wireless Services' document titled:  "Limited Size Remote
 Operation Services (LSROS)", Revision 0.8, dated April 20, 1995.
 This specification is technically consistent with CDPD Forum's
 Implementor's Guidelines Part 1028, Release 1.03, June 21, 1996.

6 SECURITY CONSIDERATIONS

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

7 AUTHORS' ADDRESSES

 Mohsen Banan
 Neda Communications, Inc.
 17005 SE 31st Place
 Bellevue, WA 98008
 EMail: mohsen@neda.com
 Mark S. Taylor
 Director of Strategic Engineering
 AT&T Wireless Services
 Wireless Data Division
 10230 NE Points Drive
 Kirkland, WA 98033-7869 USA
 EMail: mark.taylor@airdata.com

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

 Jia-bing Cheng
 AT&T Wireless Services
 Wireless Data Division
 10230 NE Points Drive
 Kirkland, WA 98033-7869 USA
 EMail: jcheng@airdata.com

References

 [1] Remote Operations:  Model, Notation and Service Definition, March
      1988. Recommendation X.219.
 [2] Remote Operations:  Protocol Specification, March 1988.
      Recommendation X.229.
 [3] Specification of Abstract Syntax Notation One, 1988.
      Recommendation X.208.
 [4] Information Processing --- Open Systems
      Interconnection --- Specification of Packed Encoding Rules for
      Abstract Syntax Notation One (ASN.1). International Standard
      8825-2.
 [5] Information Processing --- Open Systems
      Interconnection --- Specification of Basic Encoding Rules for
      Abstract Syntax Notation One (ASN.1), 1987. International
      Standard 8825.
 [6] Srinivasan, R., "Binding protocols for onc rpc version 2".
      RFC 1833, Sun Microsystems Inc, August 1995.
 [7] Srinivasan, R., "Rpc:  Remote procedure call protocol
      specification version 2". RFC 1831, Sun Microsystems Inc, August
      1995.
 [8] Srinivasan, R., "Xdr:  External data representation standard".
      RFC 1832, Sun Microsystems Inc, August 1995.

Banan, et. al Informational [Page 57]

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