097230 Methodologies in Information Systems Development

Model-Based Protocol Engineering:

Specifying Kerberos with Object-Process Methodology

Yaniv Mordecai & Dov DoriTechnion – Israel Institute of Technology, Haifa, Israel

November 3, 2015

Model-Based Protocol Engineering 2

Multiple engineering professionals talk different

languages

Mechanical Engineers Civil Engineers

Software EngineersElectronics Engineers

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What is a Model?• A physical, mathematical, or otherwise logical representation of a system, entity,

phenomenon, or process (DoD 1998).• A representation of one or more concepts that may be realized in the physical

world (Friedenthal, Moore, and Steiner 2009).• A simplified representation of a system at some particular point in time or space

intended to promote understanding of the real system (Bellinger 2004).• An abstraction of a system, aimed at understanding, communicating, explaining,

or designing aspects of interest of that system (Dori 2002).• A selective representation of some system whose form and content are chosen

based on a specific set of concerns (Object Management Group 2010).

• Source: http://www.sebokwiki.org/wiki/What_is_a_Model%3F

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Systems Engineering Languages

– Systems Modeling Language – SysML• OMG Standard since 2007

– Object-Process Methodology – OPM • OPM book published in 2002• ISO Standard 19450 as of Aug. 2014(formally: 19450 Publically Available Specification)

OPM software: OPCAT, freely downloadable from http://esml.iem.technion.ac.il/

Along with papers and other resources

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Model-Based Protocol Engineering

The idea behind conceptual modeling

5

conceived reality modeled reality

Is modeled by

Bus

Aircraft

Vehicle

Gas Filling

Is modeled by

Is modeled by

Using graphical symbols, the model expresses physical things – objects and processes – and relations among them.

is a

is a

affects

Object

Process

Energy Replenishing

is

Car

affects

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OPM Entities – the bricks: Things and States

• Object: A thing that exists or might exist physically or informatically.– Objects are stateful:

• Objects can have states• At each point in time a stateful object is

– at one of its states - static, or – in transition between two states – undergoing change

• Process: A thing that transforms an object.– Transforming an object is:

• creating it, • consuming it, or • changing its state.

Object

Processing

State 1 State 2

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OPM unifies the system’s structure and behavior throughout the analysis and design of the system within one frame of reference using a small alphabet:

– Two types of things: (1) stateful objects (2) processes

– Two families of links:(1) structural links: connect objects with objects(2) procedural links: connect processes with

objects

Compact Ontology: A Minimum Length OPM alphabet

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Hierarchical Decomposition Illustration

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• OPM is now approved ISO standard 19450.

• ISO 19450 is meant to provide a basis for a new generation of model-based standards.

• Official standards need formal semantics to ensure the standard’s consistency and integrity.

• Several standards we’ve tested were found to include contradictions and ambiguities that could be avoided, had a model-based approach been used to author and specify the standard.

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ISO Standardization

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Unblocking the Paradigm Shift

• “Standards/protocols are not systems” – But they are processes, procedures, or sets of definitions, eventually pertaining to

a system at some level.• “Standards should be solution-independent”

– But their underlying semantics must be well defined.– You don’t have to design the system that implements the standard with OPM, but

you know that it is well-defined thanks to OPM. • “Standards should be defined in natural language text rather than

conceptual modeling languages”– But we want to make sure that the text is valid and verified.– OPM lets you generate semi-natural language text based on the model.

• “You can’t capture everything in the standard with a model”– Right! Focus the model on the core procedural, functional, and/or structural

aspects being standardized.

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Kerberos

• Common, widely-accepted protocol for computer user and device authentication across insecure computer networks.

• Based on trusted 3-rd party authentication service.• Invented and developed at MIT with the support of

Microsoft, Google, Apple, Oracle, and others. • MIT Kerberos Consortium has not provided a well-

formed model-based protocol representation.

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Why Kerberos

• It’s simple• It’s standardized• It’s well known to domain professionals• It consists of several components, services,

information objects, and processes.

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Kerberos Procedure

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UML Sequence Diagram of Kerberos

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UML Sequence Diagram of Kerberos

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UML Sequence Diagram of Kerberos

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Problems with the UML Sequence Diagram

• Notation problems.• Mixture sequence diagram, activity diagram, and some informal

diagram semantics.• Confusing use of rectangular blocks for different types of entities

(processes and objects).• Extensive use of in-diagram free text.• Lack of capability to execute the visualized model.• The diagram does not show branches of the process, i.e., what

happens when one of the subsequences fails. • The diagram captures only the first two levels of the internal

processes conducted by each participant.November

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Why Did Those Problems Occur?

• UML has 13 different diagrams to capture various aspects, each highlighting a different aspect.

• Each diagram has different notation for similar concepts (e.g. Use Case, Activity, Method, Process)

• UML does not generate a formal textual specification.

Eventually, the analyst works for the model more than the model works for the analyst!

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Kerberos in OPM

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Summary

• Model-based approach to standard authoring.• Object-Process Methodology (ISO 19450) as a

conceptual modeling framework for protocol specification.

• OPM-based specification and simulation of Kerberos, a well-known protocol.

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Protocol Modeling Assignment

• Model a formal procedure (a standard or a protocol) with Object-Process Methodology (OPM) – ISO 19450 for complex system and process modeling and simulation.

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Important Dates

• Preliminary submission Dec. 19, 2015, Sat, 23:59.– Preliminary version of report

• Introduction and literature review.• Description of the protocol. • A preliminary high-level OPM model of your protocol.

– Preliminary version of the model• Presentation Jan. 12, 2016, Tue, 10:30.• Final submission Feb. 27, 2016, Sat, 23:59.

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Assignment Grading

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MODULE TASK WEIGHTAssignment (77%)

Preliminary report 10%Preliminary version of OPM model 10%Presentation 12%Final report 15%Final version of OPM model 30%

Coursework (23%)

Attendance 13%Participation, involvement, and consultation 10%

TOTAL 100%

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Submission Guidelines

• All submissions are to be made through Moodle.• Submit your report in the form of an

IEEE Conference Paper (up to 8 pages).• Submit your model file separately.• The report should contain at most 3-4 OPDs for

introductory and impression purposes. • In the text, refer to model views and diagrams in the file.• Focus on the most relevant and applicable parts of the

protocol or standard that you’re modeling.

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Modeling Guidelines• Build the OPM model iteratively, using the detail decomposition approach.• Keep your model executable – this is a mandatory and significant!• Make assumptions and abstractions, employ modeling patterns and techniques as

you see fit. Record your decisions and references.• Find issues in the technical specification (two contradicting sequences of the

stages of the procedure; mismatch between the total time required for the protocol to end, and the sum of minimal durations of the individual steps, unclear or ambiguous specification…). Use the model and reasonable assumptions to resolve them.

• Your OPM model should eventually contain 20-30 processes and subprocesses across at least four levels (0..3).

• All processes must be connected to objects, states, or other processes. • All objects must be connected to processes (directly or through a state) or other

objects.

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Further Reading• Alex Blekhman and Dov

Dori, Model-Based Requirements Authoring. INCOSE 2011 – the 6th International conference on System Engineering. March, 2011.

• Alex Blekhman and Dov Dori, Tesperanto – A Model-Based System Specification Methodology and Language. Proc. 23 rd

Annual INCOSE International Symposium, Philadelphia, PA, USA, June 24-27, 2013• Yaniv Mordecai and Dov

Dori, Conceptual Modeling of System-Based Decision-Making. Proc. 24th

Annual INCOSE International Symposium, Las Vegas, NV, USA, June 30 – July 3, 2014.• Juan Wachs, Boaz Frenkel, and Dov

Dori, Operation room tool handling and miscommunication scenarios: An Object-Process Methodology conceptual model. Artificial Intelligence in Medicine, Nov. 2014.

• Yaniv Mordecai and Dov Dori, Model-Based Protocol Engineering: Specifying Kerberos with Object-Process Methodology. IEEE 28th Convention of Electrical and Electronics Engineers in Israel, Eilat, Israel, Dec. 2014.

• ISO/PDPAS 19450 - Automation systems and integration — Object-Process MethodologyNovember

2015

Questions:

Yaniv Mordecai, yanivmor@technion.ac.il