Adding performance aspects to specification languages
Departement d'Informatique et RO, Universite de Montreal,
Email: Bochmann@videx.udem.cdn Adding performance aspects to specification languages6 Abstract
In the area of communication protocol design, so-called FormalDescription Techniques (FDTs) are used to describe the behavior of thesystem components executing the communication protocol. Such formalspecifications can be executed in a simulated mode in order to detect anyremaining logical errors in the specification. If a specification language isextended with performance primitives which allow the description of suchperformance aspects as time delays, resource usage and stochasticbehaviour, then simulation can be the basis for the performance evaluationof a complete system. The paper discusses this combination of logical andperformance aspects in a single specification, and the choice of appropriatelanguage elements for expressing the performance aspects. Such languageelements are presented for several FDTs, namely Estelle, SDL and Lotos. Emphasis is placed on the justification of the chosen language elements andtheir relation with other well-known performance models, such Markovmodels, queuing networks, timed Petri nets and simulation. 1. Introduction
In the area of communication protocol design, so-called Formal Description Techniques(FDTs) are used to describe the behavior of systems. Estelle [Este 87], Lotos [Loto87]and SDL [SDL87] are formal specification languages which have been proposed asstandards for the specification of OSI protocols and services [NBS 85]. SDL has alsobeen used for the description of switching systems. The basic goal of such formalspecifications is to ensure the correct specification and implementation of communicationprotocols. The formal nature of the specifications allows the application of partiallyautomated methods for the validation of the specifications, for the implementation process,and for the systematic testing of resulting implementations [Boch 87c].
These formal specifications are intended to describe precisely the "logical" behaviour ofsystems, that is, the possible order of interactions and allowed parameter values of these. Most properties relating to performance aspects are, however, not addressed. A completespecification system should also address these questions. What is the maximum
6 The work described here was funded by the Department of Communication of Canadathrough research contract OST83-0031 and by the Natural Sciences and EngineeringResearch Council Canada. Part of the notation described here was presented in 1984 tothe ISO TC97/SC21/WG1 ad hoc group on FDT.
throughput of a link, what is the end-to-end delay on transmission, what degradation ofservice is introduced by a given error rate, etc.? Furthermore, protocols usually havemany parameters which have to be tuned for optimum performance, such as duration oftime-outs or number of buffers (window size) to be used. For these reasons, it would beuseful to have operational specifications (simulations) that would allow the real-timebehaviour to be measured. To do this, a specification language must include performanceaspects which allow the description of such things as time delays, resource usage andstochastic behaviour.
The approach taken in this paper is to extend a given specification in order to address theperformance question. This is in contrast to most traditional approaches where a newperformance model is created in a different formalism to deal with performance issues. Inthat case, some of the "logical" properties of the system are often lost.
Estelle and SDL have many similarities, in particular, both use the concepts of Finite StateMachines (FSM) to describe systems, but they also have important differences, most ofwhich are related to the way system components can be created and interconnected. Theselanguages allow certain performance elements to be specified. First, they have some basicmeans for talking about time. In the case of SDL, a global TIME variable is accessibleand can be used for decisions and updating of variables. In the case of Estelle, so-called"delayed" transitions with minimum and maximum time limits can be defined. However,these primitives are insufficient for meaningful simulations.
The paper discusses the choice of appropriate language elements for expressingperformance. Such language elements are presented for several FDT's, namely Estelle,SDL and Lotos. Emphasis is placed on the justification of the chosen language elementsand their relation with other well-known performance models, such Markov models,queuing networks, timed Petri nets and simulation.The selection of language features forperformance specification follows similar objectives as the selection of programminglanguage features in general. On the one hand, one wants a small set of primitives withsimple semantics (meaning) and easily interpreted, and on the other hand, one needssufficient features to express the real system properties which are to be modelled. Onetherefore has to find the right compromise between these two objectives.
The paper is organized as follows. First, classical performance models are reviewed tobring out concepts useful in modelling performance. Then, the finite state machine modelwhich serves as the basis for the existing specification languages is explained and twopossible extensions to handle time duration are considered. Next, a complete set ofperformance primitives is proposed in the framework of an extension to Estelle. The useof these primitives is illustrated by the formal description of a simple network and itsprotocol. This is followed by a discussion of the application of similar primitives in thecontext of SDL and Lotos. We close with some comments on our experience with the useof an extended formal specification language. 2. Classical performance models
Models are often used to study the behaviour and performance of complex dynamicsystems. For simple idealized systems, analytical models are appropriate; for morecomplex situations, one must resort to simulation. These models indicate what languagefeatures are useful to describe performance aspects in a formal specification language.
The following are classical models for that have been used to describe the performance ofsystems:
a) Markov models and probabilistic finite state machines (FSM):
In such models, a system is characterized by a set of possible states in which thesystem can be and probabilistic state transitions that lead from state to state. Solvingthe model gives steady state probabilities of being in any given state.
These models are particularly suited for the description of the performance aspectsresulting from shared resources. A specified system is characterized by a number ofresources which process service requests. The execution of each service requesttakes a certain amount of time and each resource processes only one request at atime. When a resource is busy, further requests wait in a queue associated with theresource. Arrival of new requests and the service times may have randomdistributions.
Some real-time systems require guaranteed response times for certain requests. Toachieve this, these systems include so-called "time-outs" or timers. When started,timers will invoke some predefined action after a given "time-out" period, unlessthey are stopped by some other system activity. A simple model for this kind ofbehaviour is the "timed" Petri net [Merl 86] where the execution time for eachoperation has a minimum and maximum bound.
Each of performance models described above has a semantic which allows analyticalmodelling, at least for simple system descriptions. However, with real systems, often anexact analytical solution is not possible and simulation studies are required to obtainperformance data.
A simulation imitates system behaviour and system performance is estimated by measurements on the model. In computer simulation, the elements of a real system are represented by subroutines and records in a program in such a way that running the program produces results analogous to the behaviour of the system. In discrete event simulation, the activity of the entities in the system is viewed as a sequence of events (or instantaneous changes of state) separated by intervals of time. For instance, the loading of a truck would be modelled by a "start-load" event followed by a "stop-load" event after a delay representing the duration of the action. Parallel activity is imitated by interleaving the events of various entities and executing them in chronological order.
Simulation can be used to model systems of arbitrary complexity and size. However,simulation is expensive and simulation methods only provide approximate solutions (the
more precise a solution is sought, the more computer time is required). Often, simulationis not used to get precise estimates of system performance; rather simulation is used to getunderstanding of the system and to identify bottle-necks. Once a system is understood, thesimulation can be discarded and performance obtained from simple analytical models ofthe identified bottle-necks.
GPSS, one of the oldest simulation languages, introduced many concepts that are usefulin modelling [Schr 74]. GPSS conceives reality in terms of transactions (processes)moving through a system and requesting the use of resources. The passage of time ismodelled by an advance dt primitive. Transaction use a seize operation to try and obtainthe resources they need and they are blocked if the resources are busy. They hold them fora given service time and release them to be used by the next transaction. GPSS also hasfacilities to analyze performance: statistics pertaining to all resources are gatheredautomatically and transit times through various parts of the systems can be measured. Theconcepts of processes, resources and time advance are all very pertinent to protocolspecification languages. 3. Finite State Machines
Several formal description techniques are based on finite state machines (FSM). Weconsider here a FSM with one or several input and output streams, as shown in Figure 1. Each FSM is characterized by a finite set of internal states and sets of possible inputs oroutputs for each stream. Two kinds of transitions are considered: (a) An input transitionconsumes a particular input interaction from a particular input stream; it can only beexecuted if the given kind of input is at the head of the given stream and the machine is ina particular state. (b) A spontaneous transition consumes no input; it can be executed ifthe machine is in a particular state. Both kinds of transitions lead to a new state and mayproduce output over one or several output streams.
Figure 1b shows possible transitions for the machine of Figure 1a. There are threepossible states S1, S2 and S3. In the notation used here, "A:IN / B:OUT" means that atransition requires the input "A" to be present at the head of the input stream "IN" and as aresult of the transition, "B" will be output on stream "OUT". In the example, there are 2input streams IN1 and IN2 and one OUTput stream. The transition from S1 to S2 as wellas that from S3 back to S1 both require "a" to be present at the head of "IN1". Thetransition from S2 to S1 requires a "b" on stream "IN2". The transition from S2 to S3 isspontaneous, consuming no input. The transition from S1 to S2 produces "b" on theoutput stream; the one from S2 to S1 produces an "a" and none of the other transitionsproduces any output.
A set of FSMs becomes a system of interconnected FSMs if some of the output streamsare identified or connected with some of the input streams.
In the specification language Estelle, the machine of Figure 1 would be described as a module: module M1_type; state S1, S2, S3 ; trans from S1 to S2 when IN1 . a beginoutput OUT.b end; trans from S2 to S1 when IN2 . b ; trans from S2 to S 3 begin output OUT.a end; trans from S3 to S1 when IN1 . a begin end ;
First, the possible states and the interaction points (streams) are declared. Each transition is described by a trans statement with from and to clauses indicating initial and final states. A transition which is triggered by availability of input has a when clause; instantaneous transitions have none. Finally, output statements specify the creation of messages. The notation for IO is "interaction_point . message". The notation used above is based on that of Estelle and it will be extended to include performance aspects.
A set of FSMs becomes a system of interconnected FSMs if some of the output streamsare identified or connected with some of the input streams. For example, this would occurif the output stream OUT of M1 in Figure 2 were to be connected to the input of anotherFSM. Then an output as a result of a transition in M1 could trigger a further transition inthe connected machine. A possible notation for the creation and interconnection of theFSMs in the example could be:
connect M1. OUT to M2.In2 ; .etc.
3.1 Performance models for interconnected FSMs:
The first step in specifying performance is representing the passage of time. There seemsto be basically two ways in which an FSM model can be extended to do this: with Markovtransitions or lengthy transitions.
Here a transition is instantaneous but some time elapses between the instant when it couldoccur and that when it does occur. For each transition t of the machine, a distributionfunction Pt (T) defines the probability that the machine does this transition within T time
units after the transition has become possible, and assuming that no other transition hasbeen executed.
(2) Transition execution performance model:
Here, transitions start as soon as possible. However, a transition takes some amount oftime and the execution of one blocks the execution of others. To deal with the case whenseveral transitions become possible at the same time, each transition is assigned a
probability and one of the possible transitions is selected at random according to theprobabilities. The transition execution time may be a random variable, where a distributionfunction St (T) indicates the probability that the execution of the transition t will terminate
The first model is conceptually simpler. The second model has the advantage that it canbe used to naturally model shared resources with FIFO queuing of requests. The requestswait in the input stream until they are processed, and the processing is modelled by thetransition and its execution time. Only one request (transition) is processed at a time byany given FSM. Various other performance models have been described in the literature,e.g. [Moll82] and [Krit 86]. 4. Performance models for Estelle/SDL
Estelle and SDL can be considered as extensions of the "interconnected FSM model"described previously. The extensions are related to the definition of input/outputparameters, local variables (in addition to the STATE variable which identifies the FSMstate), data types and procedures/functions for defining the transition operations in moredetail. The two languages have many similarities, in particular the basic state transitionmodel, but also important differences, most of which are related to the way componentmodules are created and interconnected.
To illustrate the use of performance description and motivate the proposed extensions, weshall consider the specification of the simple system shown in Figure 2. A formaldescription of this system is given figure 3. In the system, several users are inter-connected via a Network Service Provider. The users send messages at random intervalsto other users. Several typical situations will be considered such as receiving messagesand responding with acknowledgements as well as retransmitting messages if noacknowledgement arrives within a specified time-out period. The description will expresstransmission delays, the possibility of message loss and the maximum throughput capacityof the user links.
In order to focus on the language aspects relevant to the present discussion, many detailsin both the model and the description are omitted: the messages have neither headers norcontent, message recipients are chosen at random and some declarations and initializationswill be missing. We describe first the aspects of the specification which do not concernperformance. The performance aspects will be discussed later when the relevantspecification primitives are introduced.
The specification (figure 3) start by declaring the message types (line 1) that can be exchanged over channels (line 2) between the modules in the system. Essentially, the system will be composed of one "NS_provider" module whose description starts at line 3 and several "users" described in lines 11-19. The actual creation of the modules and their connection is not shown. During this phase, the relevant "interaction points" or ip defined at lines 4 and 12 will be linked with statements such as: connect U1.Inq to Network.NS_out  ; connect U1.Outq to Network.NS_in  ;
The NS_provider module defines three transitions (lines 8,9,10). The first two deal with reception of a message from a user over a NS_in port: in the first case (line 8), the message is sent out over the network, in the second case (line 9), nothing is sent out and there is loss of the message. The expression in the when clause of a transition serves 2 purposes. First, it identifies the stream and message type that will trigger the transaction. Secondly, it allows extraction of information from the message into local variables. For the transactions considered, addr will be assigned the ip address on which the message was received and kind will be set to the actual parameter of the message. In Prolog parlance, one could consider the operation as a unification between the message and the when clause. The final transition (line 10) passes on a received message to the correct user.
The user module defines two states basic and waiting (line 13) and four transitions (lines16,17,18,19). The machine is in the waiting state after it has sent out a message and untilan acknowledgement is received; otherwise, it is in the basic state. The first transition (line16) handles the reception of unsolicited messages. These are acknowledged. The nexttransition (line 17) generates messages spontaneously. The third transition treatsacknowledgements returning to the basic state. The final transition specifies that messagesare retransmitted if no acknowledgement is received within a time-out period.
4.2. Performance parameters for Estelle specifications
This section defines some extensions to Estelle for defining performance parameters ofspecifications. These extensions are mainly based on the transition execution performancemodel described earlier.
An instance of an Estelle module is considered a resource. Input interactions arriving at aninteraction point of the module enter a "common" input queue or an individual queueassociated with the particular interaction point. The selection of the next transition to beexecuted is assumed to take no time. During the execution of a transition the moduleresource is held and no other transition may be performed by the same module. Theoutputs generated by the transition are available at the end of the transition. The executiontime of a transition is indicated by a HOLD clause (extension of Estelle) of the form
hold for <expression>
where <expression> is an real value expression in time units.
For certain applications, it was found convenient to introduce the concept of declaredresources. In this case the HOLD clause of the transition has the form
hold <resource> for <expression>
and <resource> is a variable access expression referring to a variable of type resource. The performance semantics of this clause is as follows: the transition has an additional
enabling condition, which requires that the <resource> must be free. If and when theexecution of the transition is decided, the transition is executed in zero time and theoutputs are produced; however, the resource remains occupied the amount of timespecified by <expression>. It is therefore possible that immediately after the execution ofthe transition, another transition associated with another resource could execute, while atransition associated with the same resource must wait.
For instance, the Network module defined in Figure 3 receives input packets over anumber of interaction points. In order to model the maximum throughput available for agiven interaction point, a corresponding resource (IP_resource[i]) is declared within themodule (line 6) and the input transitions receiving a packet hold the correspondingresource for the time proportional to the length of the packet received (lines 8,9). Thereception of packets over different interaction points may proceed in parallel.
If the resource exists in a certain number of identical units, it may be convenient toindicate for a given transition how many units of the resource are required for theexecution of that transition. This may be expressed by the notation
hold <number of units> units <resource> for <expression>.
As discussed in Section 3.1, it is sometimes necessary to indicate with which probabilitythe different transitions which are possible in a given system state will be executed. Due tothe extensions that Estelle provides in respect to the simple FSM model, a transition, inEstelle, has parameters: they include the parameters of the input (if any) and the presentvalues of the local variables. These transition parameters may influence whether thetransition is possible. In addition, if executed, they may also influence the values of outputparameters and updated variables. The present (FSM) STATE and available kinds ofinputs at the heads of the input streams do not completely determine which transitions arepossible. Therefore it is not clear how transition execution probabilities (as in thetransition execution performance model) can be associated with the transitions in astraightforward manner.
Since a given transition may "compete" with different sets of other transitions dependingon the available inputs and the module state, its probability of execution may be specifiedindirectly by assigning a WEIGHT to the transition through a clause of the form
weight ( <expression> )
where <expression> is a real value expression. The semantics of this clause is that theprobability of selection of this transition for a particular system state is equal to the valueof this <expression> divided by the sum of the weights of all transitions enabled in thatsystem state.
In the example, weight clauses (lines 8,9) are used to indicate that on the average one message out of 1000 is lost .
4.2.3. Interaction queues with transmission delays
For modelling the transmission delays in telecommunication networks, it is convenient tointroduce transmission delays for input/output streams. A similar approach is often takenin reachability analysis for protocol design validation where ad hoc models are used forthe communication medium between the two communicating protocol entities. Propertiessuch as FIFO discipline, and transmission error and loss possibilities are important notonly for the performance but also for the logical aspects of protocol operation.
The basic performance parameters of a transmission medium are the delay and maximumthroughput. The latter can be modelled by associating a resource with the input to themedium (sect 4.2.1.). Its service time will limit the number of transmission requests thatcan be handled. For the description of transmission delays, an extension to Estelle isintroduced by which additional properties can be defined for the input queue associatedwith a given interaction point. The syntax of the interaction point declaration becomes
<interaction point> <properties> ":" <interaction point type>
where <properties> can be of the forms
delay <expression> fifo delay <expression>.
The meaning of the first form is that an output generated for the given interaction point isdelayed by the amount specified by <expression> before it is entered into the input queueof the interaction point. In the case of a constant <expression> the implicit FIFO propertyof the Estelle queues remain valid. However, if the <expression> contains randomdistribution functions, the order of arrival of interactions in the queue may be differentfrom the order in which the outputs where generated. In other words, some interactionsmay overtake others. When the second form of the <properties> is used the delays will belengthened, if necessary, in order to maintain FIFO order.
In the example of figure 3, transmission delays are modelled by using a FIFOtransit_queue. Incoming messages are not sent out immediately to their receivers; rather,they are first placed in the transit_queue which has been declared to operate in FIFO modewith normally distributed random delays (line 5). Only on exit from the transit_queue arethe messages placed on outgoing streams through the transition of line 10.
The DELAY clause for spontaneous transitions is already defined in Estelle. The proposalhere is slightly different. It is assorted with an enabling condition of the form:
provided <enabling_condition> delay <expression>
where <expression> is a real value expression in time units. The semantics of this clauseis as follows: the transition is scheduled for execution when its <enabling_condition> hasbeen satisfied for at least <expression> time (if transitions are executed during this timeinterval, the condition must remain true in between the transition executions). An example
is shown at line 17 for the transition which generates the messages coming into the systemand at line 19 with a constant delay to model a time-out.
4.2.5. Use of random distribution functions
It is important to note that probability distributions may be used for describing non-deterministic behavior of the specified module. The simplest notation for suchdistributions seems to be the use of pseudo-random functions that return (random) valueswhich have a given distribution. For simulation studies, it is important to allow for theuse of independent streams of random numbers. Random functions are used at line 5 tospecify transmission delays and at line 17 to compute intervals between spontaneousincoming messages.
As mentioned above, SDL and Estelle are similar in many aspects. The followingdiscussion indicates to what extent the same performance concepts can be used in thecontext of SDL. Resources: An SDL process instance corresponds to a module instance in Estelle. An SDL transition corresponds to all Estelle transitions for a given STATE and type of input. Within an SDL transition, different cases (possibly depending on input parameters) may be considered. Like in Estelle, a resource may be associated with a process which can be held during a transition. However, the declaration of multiple resources seems to be less useful, since an SDL process has only a single common input queue (while an Estelle module instance may have individual input queues for all its interaction points). Therefore the parallelism in the system of Figure 3 cannot be directly obtained in the SDL context. Transition probabilities:
SDL has no possibility for implicit non-determinism,
and therefore there is always at most one SDL transition to be executed. Differentprobabilities for different branches of execution can, however, be introduced by definingdecisions which may depend on random functions, or which are not completely defined,leaving thus room for different decision outcomes. This is similar to sequentialprogramming languages where the conditions used in IF or CASE statements may not bedeterministic. Instead of introducing transition probabilities, like in Estelle, it maytherefore be useful to introduce the possibility of non-deterministic decisions withprobabilities for each of the possible decision outcomes. Transmission delays:
The same concept as for Estelle could be used. DELAY clause:
SDL does not have such a construct. Instead, as mentioned
previously, a global TIME variable can be read, and its value can be used to influence thesystem behavior. 5. Performance parameters in Lotos/CCS
In contrast to SDL and Estelle, Lotos uses rendezvous interactions. The rules for thesequential ordering of interactions in Lotos are largely based on CCS [Miln 80]; however,more than two processes may participate in a single rendezvous interaction. In Lotos,there are no implicit queues associated with interaction points. The Lotos "gates" play therole of interaction points, and an interaction at a gate can only be executed if all Lotos"processes" coupled to the gate are ready for that interaction. For example the processsimple defined below uses the gates a, b, x, and y for its interactions. process simple [a, b, c, x, y] : noexit := y ; ( a ; suite_a [x, y]  b ; suite_b [x, y]  i ; suite_c [x, y]  i ; suite_d [x, y]  i ; x ; simple [a, b, x, y] ) endprocess
The body of this process definition indicates that the simple process will first execute theinteraction y and then may either execute in rendezvous with its environment theinteractions a or b, in which cases it will continue with the behavior defined by suite_a orsuite_b, respectively (the "suite" behaviors will only involve the visible interactions x andy), or it will make an internal transition, indicated the action i. In the case that it choses thelast alternative, the process will execute the interaction x and thereafter start again with theinteraction y.
Lotos also has facilities for defining data types as well as process and interactionparameters. Algebraic data type definitions can be written, similar to [ACT ONE]. Thenotation for interaction parameters is similar to CSP [Hoar 78] and not further explainedhere.
It seems only two performance concepts are sufficient in Lotos to express most practicalperformance questions. These concepts correspond to the execution time of transitions andtransition probabilities. The following notation could be used. The notation
can be associated with an internal action "i" and means that the interaction requires the timeperiod specified by the <expression>. A similar notation has also been used in [Quem 87]. The notation
can be associated with an internal action which introduces an alternative of a choice. The<expression> defines the weight of that alternative (similar as described in Section 4.2.2)among all those alternatives that start with an internal action "i". For example the behaviorexpression
( a; suite_a [x, y]  b; i wait 50; suite_b [x, y]
 i weight 2; suite_c [x, y]  i weight 1; suite_d [x, y]  i weight 1 wait 100; x ; simple [a, b, x, y] )
defines a process which may participate in actions a or b (depending on its environment)or may choose one of the last three alternatives. Among the cases that one of the latter arechosen, suite-c will be executed with probability 1/2, suite_d and suite_e with probability1/4. In case that the last alternative is chosen, a delay of 100 units is introduced before theexecution of the simple process starts again. A delay is also introduced if action b isexecuted, such that suite_b can only start 50 time units later. In the other cases, thesubsequent actions would start immediately provided, however, that the environment ofthe process does not introduce additional delays.
These basic performance primitives can be used, together with the normal features of theLotos language, to construct processes that behave like resources with queuing delays orlike communication media, as shown below. Therefore the above basic performancefeatures seem to suffice for most typical applications.
A resource which remains reserved for t time units can be written as a process of the form
process resource [G] : noexit := G ; i wait t ; resource [G] endprocess
which participates in the action G, then waits t time units and starts again. It can be used tolimit the speed of execution of a very fast process executing interactions at the gate a byinvoking the resource process in parallel, coupled with the former. This can be written as
A transmission medium with random transmission delay can be written as
process medium [In, Out] : noexit := hide Middle in delay [In, Middle] || queue [Middle, Out] endprocess
where queue is defined as a normal FIFO queue; In and Out are the gates where themessages are entered into the medium and received, respectively. The gate Middle isexternally not visible and is used to transfer the messages from the process delay to theprocess queue. The latter keeps the messages in FIFO order until the user gets them. Theprocess delay may be defined as follows
process delay [In, Out] : noexit := delay_a_message [In, Out] ||| delay [In, Out] where process delay_a_message [In, Out] : noexit := In ?x:message ; i wait <expression> ; Out !x; stop endprocess endprocess
This definition shows that a delay_a_message process instance is available for eachmessage that is entered. The process waits a specified delay and then presents it at its Out
gate. This gate is in fact the Middle gate through which the message is entered into thequeue process and available for the user.
In the case that the medium loses messages occasionally, the delay process body could bedefined by the body
In ?x:message ; ( (i weight 99 wait <expression> ; Out !x )  i weight 1 (* loss *) ) ; stop endprocess
Using the concepts introduced in Section 5.2, it is not difficult to write a Lotosspecification of the Network example discussed in Section 4. Figure 4 gives the definitionof the user process and Figure 5 shows the interconnection of the different system parts,similar to the structure given in Figure 2. This specification is believed to be equivalent tothe one given in Figure 3, not only concerning the logical behavior of the system, but alsofor its performance aspects.
The definition of Figure 4 indicates that the user process remains in the basic state until anormal message is output (first line of body definition). The second line of the definitionintroduces a delay for this output to occur. In the basic state, messages that are not of type"acknowledgement" are acknowledged. If the process receives an acknowledgement in thewaiting state, it goes back to the initial state; however, after a time-out delay it will send a"retransmission" message. 6. Discussion
The performance concepts described above are closely related to the performance modelsof simulation languages such as GPSS [Schr 74] and Simula [Dahl 71]. The conceptshave, however, been adapted to the particular context of the FDT's used for thedescription of communication systems. Similar approaches can be used for addingperformance aspects to other specification languages.
The characteristic feature of the performance extensions to the FDTs described here is thepossibility of combining the analysis of logical correctness of a specification with theevaluation of its performance. A case study has been done for the OSI class 0/2/4Transport protocol [Boch 87e]. The same Estelle specification of the protocol was usedfor both the simulation and a semi-automatic implementation [Boch 87i]. For thesimulation studies, an Estelle compiler generated Pascal code that was linked to asimulation package also written in Pascal [Vauch84b].
User processes and an underlying Network service similar (but more complex) to Figure 3were also written in Estelle to provide an environment in which the Transport protocolprocesses could be simulated. Simulation runs were compared to the real Transportprotocol running on our VAX-VMS environment, and all experimental results could bereproduced. These simulations were useful for several reasons:
(a) Some errors in the specification (and therefore in the implementation) were found
(b) The simulation showed that the performance bottleneck was CPU usage; this had
(c) We could find optimal values for certain protocol parameters, like the number of
credits allowed for each user and some retransmission time-outs.
Our experience has shown that the extensions described here are both practical and useful. The simulations helped to improve both the reliability and the performance of protocolimplementations. 7. References
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[Vauc 84b] J. Vaucher, "Process-oriented simulation in standard
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[Viss 86 ] C.Vissers, "Formal description techniques for OSI", Proc. IFIP Congress
Figure 3: Simplified Network Service Specification
To test and demonstrate Simulation extensions to FDT - Channels with delays - Spontaneous transitions - Resources and the Hold construct - Delay for random times in spontaneous transitions
Definition of channel and messages type message_kind = (normal, retransmission, acknowledgement); channel NSAP_primitives ( provider, user ); by Service provider module
3. module NS_provider; 4.
NS_in,NS_out : array [ N_address_type ] of NSAP_primitives ( provider );
transit_queue fifo delay normal(avg_transit, std_dev)
IP_resource : array [N_address_type] of resource;
IP_resource := newresource (' User-1 channel',1);.etc.etc.transwhen hold IP_resource[addr] forK1 * message length output transit_queue . MESSAGE (kind) trans when hold IP_resource[addr] forK1 * message length trans when transit_queue . MESSAGE (kind) end NS_provider; USER DEFINITION: a module
11. module user; 12. state basic, waiting; initializeto basic; transfrom basic
then output Outq . MESSAGE (acknowledgement) transfrom basic to waiting provided true output Outq . MESSAGE (normal); from waiting to basic when provided transfrom waiting to same provided true
begin o u t p u t end user ; process user [In, Out] : noexit :=
|| ( i wait uniform (a,b,U); Out !normal ) )
; waiting [In, Out] where process basic [In, Out] : noexit :=
In ? m:message_type ;( [ m <> acknowledgement ] -> Out !acknowledgement [ m = acknowledgement ] -> i )basic [In, Out]
endprocess endprocess process waiting [In, Out] : noexit :=
( In !acknowledgement ; user [In, Out] ) ( i wait timeout ; Out !retransmission ; waiting [In, Out] )
endprocess Figure 4: Specification of the User Process in LOTOS
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