Computer Standards & Interfaces 19 ( 1998) tO5- 1 I7

sis of protocol converters: an annotated

Kassem Saleh * , Mansour Jaragh Kuwait (/nice&y, Dept. of Electrical and Computer Engineering P.O. Box 5969, 13060 S&t. Kuw~r

Accepted 13 July 1997

Abstract

The interoperability between heterogeneous, distributed computer networks requires the use of protocol converters. The need for constructing reliable and efficient communication protocol converters was due to the proliferation of such networks. Over a decade of intensive research and development has contributed greatly to the field of protocol conversion. Various formal approaches have been introduced to derive a converter in a systematic manner, the three most popular approaches being the service-level approach, the PDU-level approach and the hybrid approach. In this paper, we introduce protocol conversion and converter synthesis, and we briefly describe the main features of many proposed convener synthesis methods. Tables summarizing this bibliography are also provided. 0 1998 Etsevier Science B.V. All rights reserved.

Ke~rvordst Converter synthesis; Distributed computer networks; Protocol converters

1. Hntroduetio

The proliferation of heterogeneous, distributed network architectures has made the communication between users residing on different networks often impossible. Although proprietary communication ar- chitectures often offer similar services, the protocols implementing them are incompatible for trivial or non-trivial reasons. Current and projected future trends in the information technology market are favourable for the provision and implementation of value-added services and applications across various communication architectures, networks and plat- forms. To ensure the universality of such services,

* Corresponding author. Fax: +965 481 7451; e-mail: ksaleh@eng.kuniv.edu.kw

there is a need to interconnect the nesworks provid- ing such services. This can be achieved using proto- col converters. The development of such converters should be considered as a short to mid-term solution in contrast to the long term solution involving the standardization of network interfaces over which value-added services can be provided.

A network provides an infrastructure to facilitate the communication among distributed computer sys- tems. A protocol consists of a specified set of rules that govern the orderly exchange of messages among the users of these distributed systems. The underly- ing network providing the communication services to the users is organized as a series of layers, so as to reduce the design complexity. Each layer offers ser- vices to the higher layers. The active elements in

each layer are called entities. An entity in layer N provides services to the entity in layer (N-I- I).

0920-54&9/W/$19.00 0 1998 Elsevier Science B.V. All rights reserved Pll SO920-5489(97)00034-2

106 K. Soleh, M. Jaragh/ComputerStandards & Inteflaces 19 (1998) 105-117

or Underlying Service

Pig. 1. A refinement of a communication service.

SAPS (service access points) of layer N are the logical places where layer (N + 1) can access the services offered by layer N.

The relationship between protocols and services can be described at two levels of abstraction. At a high level of abstraction, a communication system can be viewed as a service provider which offers some specified communication services to a number of service users (Ul, U2,. . , Un) who access the system through many geographically distributed ser- vice access points @API, SAP2,. , SAPn) (Fig. I). At a lower level of abstraction, the communica- tion system can be seen to consist of a number of cooperating protocol entities (PEs) which exchange protocol messages, called protocol data units (PDUs), that are not observable to the users at the upper access points. The PEs exchange these messages over a reliable communication medium according to a FIFO discipline. These PEs have their own service access points for accessing the FIFO medium called lower SAPs (Fig. 1).

The communication service specification de- scribes the distributed functions provided by the communication system to its service users, whereas, the communication protocol specification describes the behaviour of the protocol entities, each servicing a particular access point. A protocol entity specifica- tion describes the behaviour of that entity with re- spect to its upper interface (SAPS) and with respect to its lower interface with the underlying service provider.

Due to the incompatibility between protocols, pro- tocol conversion has become a necessity for the

internetworking in a heterogeneous netwok environ- ment. The term ‘protocol conversion’ implies con- structing a converter as a mediator between two given protocols. It accepts messages from either protocol, interprets them and delivers appropriate messages to other protocol. Green [lS] analyzed different internetworking architectures and situations in which conversion has to be performed. Suppose we have two networks based on different layered communication architecture. Two distributed users, each belonging to a different network, will not be able to interconnect and communicate because of the incompatibilities at either or both the protocol or service levels. A protocol converter can be used here to reconciliate between the messages exchanged at the boundaries of both networks. A converter com- municates with each network in its own language and translates between incompatible networks, Thus it provides a common communication service throughout the heterogeneous interconnected system. As the different networks may have different proto- cols, a converter performs protocol conversion be- tween incompatible protocols. The design and func- tion of the converter depends mainly on the extent of disagreement between the protocols at which the conversion is made.

Protocol engineering, a term first coined by Pi- atkowski [6], is an interdisciplinary engineering field which emphasizes the use of formal methods and sound engineering principles for the development of reliable communications software. The areas of pro- tocol engineering include the: (i) formal protocoi and service specifications to allow a concise and unam- biguous description of both services and protocols, (ii) protocol validation to ensure that a formally described protocol is correct and complete, (iii) pro- tocol synthesis which attempts to automaticahy ob- tain a protocol specification from its intended service specification, (iv) protocol implementation to semi- automatically obtain a distributed software imple- mentation from the protocol specifications. (v) proto- col conformance testing to ensure that a protocol implementation conforms to its formal service and protocol specification, (vi) protocol conversion to obtain a protocol converter to allow the interopera- tion with existing protocols offering similar services. Other related issues involve protocol performance analysis, hardware/software protocol codesign, im-

K. Saleh, EA. Jaragh/Cornputer Standards & Metfaces 19 (19981 105-117 iQ7

Service Definition Capture

Protocol Design Validation &

I

I Protocol

Verification OR

Semi-Automatic Protocol Implementation 1 Testing

Protocol Conformance

tl . . . Prot plementation

Fig. 2. Areas and activities within the protocol engineering discipline.

plementation optimization quality assurance and en- gineering and project management. Various methods, techniques, tools and international standards have been developed to support the different phases of the protocol engineering process. Fig. 2 shows the phases and processes involved in protocol engineering. Since 1980, protocol engineering issues have been tackled extensively in the literature and at various interna- tional conferences.

The aim of this paper is to provide a comprehen- sive bibliography on the research work done on protocol converter synthesis. The protocol converter synthesis problem is formulated as: the design of a converter for the interworking between two incom- patible protocols, at layers N and M, given the formal specification of these protocols and/or the services they provide.

These special issues include a collection of arti- cles on the recent advances in protocol engineering [n-41.

This long paper overviews the recent advances achieved in the area of protocol engineering includ- ing protocol synthesis and protocol conversion [5].

This paper introduces the elements of an engineer- ing discipline for the protocol system design. It stresses the importance of the use of formal methods and tools in the development process [6].

This survey paper provides an overview of syn- thesis methods classified under the service-oriented and non-service-oriented approaches. It also criti- cally assesses these methods and provides some is- sues still to be resolved by synthesis methods 171.

This paper sets the philosophical grounds for the service-based development of protocols. Its main prescription is that, to design a protocol in a layered communications architecture, it is necessary to start from the specification of the service to be provided and the underlying service [$I.

2. Protocol converter synthesis overvie

The need for the development of forma! methods for protocol converter design was first pointed to by Green [15]. Since then, the issue of converter design and has been addressed extensively in the literature,

M+l

-k! M

Fig. 3. Service-level conversion in a global communication ser- vice.

and various formal design techniques have been proposed. Two major concerns are being addressed where interoperability is introduced. First, the archi- tectural concerns which deal with the identification of the layer at which a converter must exist, and second, the behavioral concerns which deal with the reconciliation between the different behaviours of protocols manifested by the different protocol mes- sage formats and their orderings. Many techniques address the second concern. These techniques are either top-down or bottom-up. Top-down service-ori- ented techniques attempt to generate communication gateways from formal service specification. Bottom- up protocol-oriented techniques do not consider the complete service specification, but attempt to recon- ciliate at the protocol messages level. Other service- based approaches use the concept of a service relay which translates each of the service primitive ema- nating from one network to a format acceptable by the otber network. However, this is not so simple since services can have different options, quality of services and classes. A new technique that has been recently introduced, namely the hybrid technique, has adopted the features of the top-down as well as the bottom-up approaches. Here, the converter syn- thesis procedure requires the formal service specifi- cations as well as the protocol specifications as input requirements in the initial stages of converter design. Other solutions are mainly based on local or global service complementation in which additional sub- layer(s) may be required to ensure service compati- bility.

The protocol converter can be designed according to two architecturally and fundamentally distinct de- sign approaches, namely, the service level and the protocol level. A third approach is the hybrid ap- proach, which merges the first two approaches.

In service level conversion, corresponding (com- mon) services from the two protocols are concate- nated. This method maps the service primitives af the two protocols. The conversion is performed at level N assuming that the protocols of the networks above that level are compatible. The gateway con- sists of the implementations of the two protocols hierarchies up to the conversion level, and includes a service interface adaptor at (N)-service level. Fig. 3 shows a service level conversion architecture. This method does not include the details of the PDU sequences and control associated with these message exchanges in the early design stage.

A drawback of the service-level approach is that the interactions of the protocol converter with the two networks is strictly confined to the service user level. Therefore, the converter can only play a pas- sive role in the conversion process, i.e., it can never have a choice of initiating an action by delivering a PDU from one network to the other, since this method does not involve transactions at the protocol level.

In protocol- or PDU-level conversion, the interop- erability is based on a conversion at the level of the protocols involved. The gateway function is defined explicitly in terms of the PDUs exchanged within the

Fig. 4. Protocol-level conversion in a global communication sys- tem.

K. Saleh, M. Jaragh / Computer Standards & Interfaces 19 (19981 105-I 17

Points of Observation

109

Fig. 5. Abstract representation of a global communication service.

two interconnected networks at protocol layer N, above which all protocols are compatible. Fig. 4 shows the protocol level conversion, in which a converter exists at layers N and M of Networks A and B respectively.

An equivalent abstract representation of the sys- tem, in which only the incompatible protocol layers appear, is shown in Fig. 5. This figure also shows various observation points which will be used later to synthesize a converter. This method may provide a more efficient and powerful conversion, but it is more complex to be specified and more difficult to im.plement. For more details on related issues and comparison between the two conversion approaches, refer to the work of Bochmann and Mondain-Monval i93.

One disadvantage of choosing a protocol-level converter synthesis method is that the generated converter may remain a passive one, i.e., it may fail to exhibit a dynamic behaviour. This is because the protocol-level.conversion method ignores all transac- tions between the two networks at the service user level, and as a result, the synthesized converter merely performs a ‘blind’, end-to-end data transla- tion and transfer between the two incompatible pro- tocols. Hence, such a converter may never be able to initiate an action during the conversion process.

Hybrid conversion is a merged product of the service level approach and the protocol level ap- proach. The converter synthesis process begins at the service level, but it continues down to the protocol level, and the generated converter reflects transaction details at the service level as well as the protocol

level. Hence, the drawbacks of ?he above two meth- ods can be overcome by adopting the hybrid ap- proach. Moreover, a converter synthesized using the hybrid approach will be able to function dynami- cally. This paper introduces a protocol converter design method that uses the hybrid approach.

3. Converter properties and desig

Irrespective of the design approach used, there are three general properties that must be guaranteed in a converter design.

(1) The safety properties of the protocol con- uerter: A safety property ensures that the converter never enters an undesirable state, that is, something bad never happens (the converter is safe). These properties include freeness from deadlocks, freeness from livelocks, and completeness (that is, absence of unspecified reception errors).

(2) The liveliness properties of the protocol con- verter: A liveliness property ensures that the con- verter will eventually enter a desirable state, that is, something good will eventually happen (the con- verter is live), meaning that the converter success- fully performs its intended functions.

(3) The timeliness properties of the converter: A timeliness property ensures that the converter re- spects the timing and response time requirements of the two protocol specifications. In this case, we say that the converter is responsive, i.e., the converter executes its functions efficiently within the specified time constraints, and thereby provides an effective

K. S&h, M. Juragh/ComputerStandards & Inre@ces 19 (1998) 105-117

and responsive service to the interacting incompati- ble protocols.

In order to classify and compare existing con- verter synthesis methods which appeared in the liter- ature, we introduce and define some general features that can be used to characterize those methods. These features are: the converter design approach, the modelling formalism, the assumptions and con- straints, the syntactic and semantic correctness of the synthesized converter, the comp!exity of the method, the flexibility of the converter and the performance of the converter.

or even a compiex Cartesian product in order to synthesize a converter. Other methods are less com- putationally complex and scale up to large protocols.

+ The converter design approach used in the method: As mentioned earlier, the converter can be designed according to two fundamentally and archi- tecturally distinct design approaches, namely, the service level top-down conversion approach and the rotocot level bottom-up conversion approach. Also,

a hybrid approach merging these two design ap- proaches can be used.

+ The flexibility of the synthesized converter: A protocol converter that is designed by strictly adher- ing to the rules of the converter synthesis design, may function correctly in an ideal, error-free envi- ronment. But in a real-life application, such a con- verter may be found too rigid. Depending on the kind of application involved and the kind of output expected, the converter design must be more or less flexible, allowing the use of different solutions to conveniently arrive at the final desired output, irre- spective of the design approach taken.

The modelling formalism used to describe the components of the conversion system: Various for- malisms can be used to model the heterogeneous protocols and services and the synthesized converter. This includes the communicating finite state ma- chines (CFSM), extended CFSMs or ECFSMs, Petri

+ The perjonnance of the synthesized converter: Ideally, the converter must be able to execute the conversion functions concurrently and bi-direction- ally. The aim of the conversion method is to an efficient converter that may have to strained by some real-time restrictions. Therefore, converter responsiveness is an important issue to consider.

References to formal models and languages used in the converter synthesis methods are from Refs. [IO-121.

and CCS-CSP-LOTOS-based models. The assumptions and constraints on the con-

version system: One of the assumptions on the con- verter is its reliability, i.e., being error-free and without loss. Other assumptions include the syn- chronous nature of the communication between the involved protocol entities and the converter, the availability of common services between the interop- erating protocols and the direction of communication in t converter.

fie syntactic and semantic correctness of the

4. Brief summary of protocol ~~~ve~~~~ syn methods

synthesized converter: Ideally, a protocol conversion method should guarantee that the synthesized con- verter is both syntactically and semantically correct. It is essential that the converter does not introduce any safety-related problems and guarantees the even- tual provision of its desirable conversion functions.

In this section, we provide a brief summary of the existing papers on protocol converter synthesis. We begin with a list of the introductory papers on proto- col converter synthesis, followed by a list of tutorials and survey papers on converter synthesis. The latter part of this section lists out the various methods proposed for converter synthesis. Papers related to the same method are grouped together. Moreover, methods are sorted in alphabetical order of the au- thor of the paper introducing the method.

4. I. Introductory papers

+ The complexity and scalability of the method: Identifies general principles for getaway design at The extent of complexity of a protocol converter the OS1 Transport layer. Different implementation design can vary depending on the design approach strategies for transport gateways are described. The and the modelling formalism chosen. Some methods possibilities of end-to-end recovery are also consid- may require computing a global reachability system ered [11,13].

K. Sale/i, M. Jaragh/ Computer Standards & interfaces 19 (1998 305-117 iif

Provides some definitions and terminologies on SNA/OSI integration techniques-direct, indirect protocol converters and communication gateways and mixed integration. Assesses possible areas for f14-171. future SNA/OSI connectivity [27].

4.2. Tutorial and survey papers 4.3. Formal methods

Covers the various aspects of network intercon- nection. Contains a list of papers dealing with inter- connection at different layers of the ISO-OS1 model [1X].

4.3.1. Ad hoc methods

Discusses principles that apply to the design of communication gateways. Emphasizes the impor- tance of the common communication service in in- terconnected systems. Conversion approaches at the service level and the protocol level are explored, and optimization techniques are suggested [ 191.

Presents an ad hoc approach for synt~es~ziRg protocol converters by adapting the labelled transi- tion system theory. Given the finite state representa- tion of the interface protocols, the state machine of the converter can be generated [28].

Defines principles involved in designing gate- ways, discusses the modification of services through subset selection and service concatenation, describes conversion at the service level and the PDU level. A method is suggested for deriving a protocol adaptor specification from two protocol specifications [9].

Introduces a ‘toolkit’ (a software function library) for synthesizing conversions between different trans- port interfaces. The toolkit is contrasted with other approaches, and shows how it can be used to solve various conversion problems [29].

Introduces TACT-a toolkit for software function library for synthesizing conversion between different transport interfaces. The toolkit is contrasted with other conversion approaches [30].

Describes and compares three different existing approaches to protocol conversion-Lam’s method of conversion via projection (a bottom-up approach), Okumara’s conversion method using the conversion seed (also a bottom-up approach) and Calvert’s Quo- tient method (a top-down approach). Architectural issues and problems concerning protocol mismatches involving multiple layers are discussed [20].

Presents a solution to the interoperability problem between the US Department of Defence Transmis- sion Control Protocol and the IS0 Class 4 Transport Protocol at the Transport layer [31--341.

Considers the design of a gateway for cormecting a Local Area Network and a remote computer through ISDN using the Petri Net model [35-371.

Briefly describes and compares some of the for- mal methods reported in the literature on protocol conversion and suggests further refinements of these models when being applied to practical protocols [Zl-231.

Explores methods for transition and coexistence of the US DOD Internet suite of protocols-TCPjIP -and protocols based on the OSI model [3&--411.

4.3.2. Service-level conversion methods (Top-down approach)

This paper contains a survey on various protocol conversion methodologies, analyses their underlying concepts, and discusses open problems specific to each approach. Suggestions about new research di- rections and more effective methods are included 24,25].

Starts with the FSM-based service specifications of the two interacting heterogeneous systems. A service interface adaptor at the layer above maps the service primitives relating the two services. A proto- col converter specification is modeled by composing

the specifications of the adaptor and the two service specifications [42].

Discusses two universal addressing and routing schemes emerged from the OS1 work to interconnect diverse networks while preserving their individual autonomy [26].

Discusses the concept and components of the SNA and QSI. Explains common interconnection devices, and discusses major characteristics of three

Demonstrates formal techniques for solving and reasoning about conversion systems and their cor- rectness. The protocol projection technique is used for verifying properties of complex protocols. The use of mappings, images and inverse images of properties for comparing semantic properties of pro- tocols is discussed and illustrated [43].

I!2 K. Saleh. M. Jaragh/ComputerStandards & Interlaces 19 (1998) 105-117

nown as the ‘Quotient Method’, this approach starts with the FSM-based specifications of the pro- tocols and the services to be provided by the con- verter. A converter machine is constructed such that the parallel composition of the converter machine and the intermediate protocol entities simulates the required service specification of the converter [44].

Proposes an architecture for protocol conversion based on the notion of adaptors. Using FSM model specifications, the adaptor enables the peer of one protocol to simulate the peer of another protocol. An enhancement of the ‘quotient method’ [45].

Uses the formal CFSM model. Starting with the service specifications, a service system graph is first generated, from which a conversion service specifi- cation is derived. The synchronization sets are then produced, which are composed to obtain the final protocol converter specification. The liveness, con- formity and transparency properties of the converter are confirmed [46].

Using CFSM model specifications, a conversion algorithm consisting of six sub-algorithms for syn- thesizing a prolocol converter is proposed. Starts with computing the Cartesian cross product of the state spaces of the intermediate protocol entities. All valid closed paths are found, merged and reduced to form a minimal CFSM, which represents the re- quired converter. Validity of the converter is verified [47-49’1.

Tabie 1 PDU-level conversion methods

Uses High-level Petri nets as the formal descrip- tion technique. The method works on the principle that for every service primitive, there is a coKe- sponding protocol operation in the protocol specifi- cation. A preliminary service converter is initially constructed and later transformed to a verter [so].

Generates a converter from a given service speci- fication of the internetworking system and two pro- tocol specifications, based on the concepts of con- trollability and observability. A reduction relation compares the desired service specification and the internetworking system. An algorithm is developed for optimizing the converter 1.511.

Using the CFSM model, a parallel model is pro- posed to construct a protocol converter with error-re- covery features. The mapping functions of the two target protocols are partitioned into separate entities,

from which two half-converters are derived. The method may be implemented in a single gateway architecture or a half-gateway architecture [52].

Proposes a five-step algorithm to construct a pro- tocol converter. Starts with the construction of a system graph, from which the final converter is derived. The converter satisfies the safety and con- formity properties [53].

Starts by computing the Cartesian product of the FSM-based service specifications of the two proto- cols and the total service specification, whereby/ a

[581 [571 Ml [551

Design approach Bottom-up Bottom-up Bottom-up Bottom-up

Modeling CFSM CFSM CFSM CFSM

formalism Assumptions Conversion Compatible Compatible Conversion

and constraints seed X intuitively messages messages seed assumed

Syntactic and Semantic Semantic Free from Syntactic

semantic correctness equivalency correctness guaranteed deadlocks correctness guararmteed

Complexity and State space Protocol projection Computational High computational

scalability explosion. High reduces complexity complexity reduced complexity, complex

computational complexity transformation

Performance Errors undetectable No systematic way Concatenation of partial Validation techniques for large conversion to compare images converters reduce perform verification

seeds performance

Flexibility Single-process Restricted to homogeneous Incompatible functions Passive conversion

conversion protocols not considered --

K. S&h, hf. Jaragh / Computer Standards & Intelfaces 19 (I 998) I&-ii’ 7 ! 13

Tabie 2 ‘PDU-level conversion methods

[601 k31 Ktl [491

Design approach Bottom-up Bottom-up Bottom-up Bottom-up Modeling formalism CFSM CFSM CFSM CFSM Assumptions and constraints Original protocols Mapping set with Conversion seed must Convertible messages constraints are correct significant messages be selected of the protocols

given defined Syntactic and Both guaranteed No buffer overflow or Syntactic correctness Deadlock-free, no semantic correctness improper termination guaranteed unspecified receptions

Complexity and Trace computation Low in time and Reduced state transition Depends on number of scalability increases complexity space computation graphs reduce complexity valid closed paths Performance Slows down with Validation procedures, No validation techniques Bidirectional conversion

excessive trace reduced reachability

computation Flexibility Trace-based technique Half-gateway Depends on conversion Unapplicable if conversion

makes approach more implementation seed selection requirements not given flexible adjustable

system graph is constructed to later obtain a prelimi- nary service converter. The involved entities are modified, and the Cartesian product of the new in- volved entities and the service converter generates the required protocol converter. A top-down analyti- cal approach 1541.

4.3.3. PDU-level comersion methods (Bottom-up ap- proach)

This conversion method involves protocol valida- tion techniques. Starts with CFSM-based specifica- tions of the protocols and a conversion seed. A reachable global state graph is constructed from the composite product of the protocols and the conver-

sion seed, from which the final converter is derived. Semantic and syntactic equivalence is guaranteed WI.

Demonstrates a formal technique of protocol con- version based on protocol complem~ntatio~, to achieve uniform protocol of the virtual layer. The method decides the feasibility of the protocol con- version L.561.

Using the CFSM model for protocol specification, the method proposes two types of converters-mem- oryless converters and finite state converters. Uses image protocols obtained by protocol projection to derive the converter. The method requires a heuristic search for an image protocol with useful properties [571.

Table 3 Service-level conversion methods

.-- WI 1451 [501

Design approach Top-down Top-down Top-down Modeling formalism FSM FSM High Level Petri Nets Assumptions and constraints Service definition Common service Every service primitive

describes global properties specification corresponds to a

protocol operation Syntactic :and semantic Not guaranteed Guaranteed Syntactically correct correctness

Complexity and scalability Too simple and generalized Exponential Exponential Perfommme Reduced in real applications Suited for multiprotocol Concurrent behaviors

conversions can be expressed Flexibility No formal method to get adaptor Applicable to all layers Protocol specificattons

need not be finite

114 K. Sale/z, M. Jaragh/Computer Standards & Interfaces 19 (1998) 105-117

Table 4

Service-level conversion methods

Design approach

Mode!ing formalism Assumptions and constraints

Syntactic and

semantic correctness

Compiexity and scalability

Performance

1541

Top-down FSM

t461

Top-down CFSM

I511

Top-down FLTS

Common service specification

Syntactically correct

Low computational

complexity, high time complexity

Verification procedures ensure validity

Original protocols are safe and live

Conformity, hveness

and transparency properties satisfied Exponential in time

and space

Depends on satisfying given service

Original protocols optimized

Controllability and observability features ensure correctness

Low computational complexity

Derived converter is optimized; gives

conversion requirement maximum performance

Service specifications may

be non-deterministics

Flexibility Concurrency and

dynamicity cannot be expressed

Handles conversion

between sequences of transition of different

protocols

Starts with CFSM models of the protocols and a conversion seed. An algorithm CONVERSION com- putes the composite product of the intermediate pro- tocol entities and the conversion seed to derive the converter. The use of the conversion seed enables the converter to satisfy semantic equivalency [58,59].

Known as the ‘trace method’; protocol entities are modeled by FSMs. A mapping relation is established between the execution traces of the intermediate

protocol entities. The final converter is a finite state machine which involves all the valid execution se- quences respecting the mapping relation [Xl].

This method is an improvement of the method of Okumara [58]. IJsing CFSMs, the algorithm first constructs the reduced state transition graphs of the protocols, and then follows Okumara’s technique to produce a reduced CFSM model of the converter. .ri low complexity, bottom-up approach [61,42].

Table 5

Hybrid conversion methods

[701 1691 [711 [721

Design approach

Modeling fomndism Assumptions and

constraints

Hybrid

CFSM Total service specification given

Syntactic and semantic correctness Complexity and

scalabilipj

Bounded queue length,

free from deadlocks Low computational

complexity

Performance

Flexibility

Efficiency depends on adaptor ADA An adaptor must be found to design converter

Hybrid CFSM

Protocol entities linked by half-duplex

communication channel Semantic correctness

guaranteed Polynomial time

algorithm reduces time complexity

State explosion in real-life protocols Dynamicity can be incorporated if required

Hybrid

CFSM Input protocols are live and correct

Syntactic and semantic

correctness guaranteed Large gcsd

increases complexity

Concurrency cannot be exhibited Conversion patterns introduced for different service combinations

Hybrid

Petri nets Timed and guarded

transitions satisfy timing constraints

Syntactic and semantic correctness guaranteed

No reachability compotation

Concurrent processing

Dynamic conversion;

timeliness behavior expressed

K. Saleh, M. Jaragh/ComputerStandards & Interfaces 19 (19981 10.5-117 !i5

Presents a formal model to analyse two aspects of the conversion problem: translation and synchroniza- tion. A converter synthesis algorithm is proposed to produce a valid converter that performs concurrent processing, easy debugging, and can easily be imple- mented for half-gateways [63].

Starting with FSM-based protocol specifications, the method proposes a PUT-GET mechanism to construct a converter with non-FIFO buffers. Mes- sages produced in one protocol are reordered and converted before being passed to the other protocol. Several techniques are proposed to optimize the computing time and space spent in validating the converter [64,65].

Known as the ‘modular method’, this method starts with the division of the protocols-specified by CFSM models into many phases, each realizing dif- ferent functions. A partial converter is constructed for each pair of compatible functions. The partial converters are grouped into a single converter. Com- putational complexity is reduced [66].

4.3.4. Hybrid conversion methods Protocols are modeled using FSMs. An interface

converter is first designed between the intermediate protocol entities to establish the mapping of related messages. The largest common subset of services is computed, which in turn is used to compute the converter. Conditions under which the converter can be stateless are also described [67:68].

An enhancement of Ref. [68]. Uses FSMs with FIFO queues to model the protocols. The largest common subset of services is then computed to generate a ‘trivial converter’, from which a pruned final converter is computed. The procedure exhibits polynomial complexity [69].

Starts with FSM-based protocol specifications and a total service specification of the two protocols. The method works on the principle that when a total system and a sub module are given, a second sub module-called the ‘complement’ sub module-can be constructed [701.

Using FSM as the formal definition technique, the greatest common service definition of the two proto- cols is computed, and two sets of traces are gener- ated that describe the events occurring at each of the networks. The trace sets are complemented and syn-

chronized to produce a minimal finite state machine representing the required converter [71].

An enhancement of Ref. [71]; the FSrYI model is replaced by the Petri net model to specify protocols and services. The method synthesizes a dynamic converter exhibiting timeliness features and concur- rent behaviours [72].

5. Summary Tables

In Tables l-5 we summarize some of the proto- col converter synthesis methods described briefly in Section 4, and we compare them according to the classification criteria listed in Section 3. We have selected the methods for comparison by giving prior- ity to the pioneering works in each design approach, the most recent methods proposed, as well as those methods accepted as most applicable to real-life protocols.

Acknowledgements

The authors would like to acknowledge the partial support of this work by Kuwait University Research Grant EE 064.

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professor. He is a mem Section. His research 1 tolic array architecture

Mansour H. Jaragh received his %.Sc. in electrical engine&rig from Tulane Uni- versity, and the M.Sc. and PhD with honors in Computer Engineering from New Mexico State University. in 1979 and 1983. resoectivelv. He *ioined the Ministry of Cbmmun&ation ;n Kuwait in July 1975 and was in charge of test- ing the International Telephone Ex- change (M.O.C.) in 1977. In 19&g, he joined the department of electrical and computer engineering at Kuwait Univer- sity where he is currently an associate

iber of the board of IEEE Region 8. Kuwait Interest include computer ar&tecture, sys- and protocol engineering.

Kassem Saleh received the %.Sc., MSc. degrees in Computer Science, and the PhD degree in Electrical Engineering from the University of Ottawa. Canada in 1985, 1986 and 1991, respectively. He worked as a computer systems spe- cialist at Bell Canada from 19S5 to 1991 then he joined Concordia University as an assistant professor. He is currently an associate professor in the Department of Electrical and Computer Engineering at Kuwait University. He was awarded the IBM telecommunications Software

Scholarship in 1988, the George Franklin Prize for the best student paper in 1990 from the Canadian Interest Group on Open Svstems (CIGOS). the Distineuished Young Researcher Award and the Distinguished Teacher Award from Kuwait University in 1994 and 1996, respectively. He is a frequent tutorial presenter at many international conferences. His research and teaching inter- ests include object oriented software engineering, distributed sys- tem design and communications protocol engineering.