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Internet Security CSCE 813 Communicating Sequential Processes. Project. Related Work Need to know by now: What is the problem domain? What is the specific problem you’re addressing? What solutions are out there (if there is any)? What are the limitations of these solutions? - PowerPoint PPT Presentation
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Internet Security Internet Security CSCE 813CSCE 813
Communicating Sequential Communicating Sequential ProcessesProcesses
CSCE 813 - Farkas 2
ProjectProject Related Work Need to know by now:
– What is the problem domain?– What is the specific problem you’re addressing?– What solutions are out there (if there is any)?– What are the limitations of these solutions?– How your proposed approach overcome some of these
limitations?
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Related WorkRelated Work
Format:1. Problem Overview2. Related work
2.1 Research on problem domain2.2 Research on specific problem2.3 Limitation of existing research
References
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Related WorkRelated WorkOrganize existing work into categories, e.g.,
on what specific problem they solve, what is the nature of the proposed solution, etc.
Don’t just list the different papers in a sequential order!
Briefly explain what problems they address and what the main contributions are.
Be critical!
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ReferencesReferences Be precise! Use full references, with authors, title, where it
was published, when, and the page numbers If you supply URLs, list when the URL was
downloaded Organize references in alphabetical order Use one of the accepted bibliography format See
http://www.asij.ac.jp/middle/lib/BibliographyFormat/Bibliography%20Format.htm for more formatting on references
Back to CSPBack to CSP
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ReadingReading
Today: – Modelling and analysis of security protocols:
Chapter 1
Next Class: – Modelling and analysis of security protocols:
Chapter 1 and 2
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CSP ObjectivesCSP Objectives
Model dynamicsModel and analyze concurrency
– E.g., calculation intensive systems, distributed applications
Support parallelism
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PrefixPrefix
Offering a single actionOffering of choice: any set of visible
actions– If A , ?x : A → P(x) represent all the
actions in A– x is the parameter of P -- parameters can be
used in events or manipulated– When a A is chosen, it behaves like P(a)
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Choice OperatorChoice Operator Choice operator:
– Gives the option between the actions of two processes then
– Behaves like the one chosen Revisit: if A = B C then
?x : A → P(x) = (?x : B → P(x)) (?x : C → Q(x) )
If B and C are disjoint: together they give all the choices in A
What happens if B and C overlap?
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Non-Deterministic Non-Deterministic ChoiceChoice
P Q – behaves like P or like Q– User has no control over which– Can be implemented using two internal actions– Implementer is not required to implement this way (can
choose either P or Q or (P or Q)) Useful for model degree of unpredictability, like
communication medium that transmits data correctly or loose it.
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Time-Based ChoiceTime-Based Choice
P t Q– Chose choices offered by P for t time units and – If nothing is chosen, it behaves like Q
Similar traces than other choice if no time is recorded
Can be P Q where t is non-deterministic
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Conditional ChoiceConditional Choice If-then-else Choice is based on condition
– if b then P else Q Example:
FW(s) = in?x → (if valid(x,s) then out!x → FW(newstate(s,x)) else FW(newstate(s,x)) )
Revisit non-deterministic machine:NDM = in?x → (NDM out!x → NDM)
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Parallel OperatorsParallel OperatorsPut sequential processes parallelSystem state: state of each component
– Number of possible states increases exponentially with the size of the network
How to put processes together for parallel network?
How to check whether such a network satisfies a specification?
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Parallel CombinationParallel Combination
Just an other process to which any of the previous operators can be applied.
Each parallel process is equivalent to a sequential one (with infeasibly large number of states)
CSP processes influence each other by affecting what communications they can perform.
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Parallel CombinationParallel Combination
Synchronize all visible actions– P || Q can perform a only when P and Q can – (?x : A → P(x)) || (?x : B → Q(x)) =
?x : A B → (P(x) || Q(x))
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Parallel CombinationsParallel Combinations
Interfaces parallel operator: P ||X Q– Synchronize all events in X
Example: – P = ?x : A → P’(x)– Q = ?x : B → Q’(x)– P ||X Q = ?x : X A B → (P’(x) || Q’(x))
?x : A \ X → (P’(x) ||X Q) ?x : B \ X → (P||X Q’(x))
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General InterleavingGeneral Interleaving
P ||| Q when P ||Ø QP and Q use disjoint sets of events
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Alphabet ControlledAlphabet Controlled
P X ||Y QEach process is given control of a particular
set of eventsNo process is ever permitted to
communicate outside of its own alphabetInterface between two processes:
intersection of their alphabet
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Use of Parallel Use of Parallel OperatorsOperators
Achieve a particular overall behaviorFor example, build constraints on traces
– P ||X Q, where P is any process, and all Q’s processes belong to X => P is only allowed to do things in X that Q permits.
– E.g., example on page 54
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Hiding and RenamingHiding and Renaming Hiding:
– Internal details are not visible to outsiders– If X in and P is a process than P \ X behaves like P but all
events in X are hidden (turned into invisible actions) Renaming:
– Alphabet replacement (relation)– P[[R]] behaves like P but all visible events a from P are
renamed by whatever R associates a with– Use to make copies– e.g., P[[a,a/b,c]] – both b and c are mapped to a– e.g., P[[b,c/a,a]] – both a is mapped to b and c (offers the
choice of b and c to the environment but the state after either of these choices is the same
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Additional operatorsAdditional operators Sequential composition P ; Q
– Does whatever P does until terminates and then does what Q does
Process Skip : successful termination– Special event: -- always the final event– e.g., a → b → Skip, terminates successfully after
events a and b– e.g., (a → Skip) ; P same external behavior as a → P
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CSP OperatorsCSP Operators Stop process does nothing a → P event prefix ?x:A → P event prefix choice P Q choice between two processes P Q nondeterministic choice P || Q lockstep parallel P ||X Q interface parallel P X ||Y Q synchronizing parallel
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CSP OperatorsCSP OperatorsP \ X event hidingP[[R]] process relation
renamingSkip successful terminationP ; Q sequential composition
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Process BehaviorProcess Behavior Concurrent processes may lead to: Deadlock: each process is willing to do something
but the entire system cannot agree on any action Livelock: infinite sequence of internal (hidden)
communication occur between the components. Similar external appearance to deadlock
Non-determinism: both processes P1 and p2 are willing to talk to a third one Q which has to make a choice.
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TracesTraces Sequences of visible events until an arbitrary finite
time E.g.,
– traces(Stop) = { < > }– traces(a → P b → Skip) =
{ <a > n, <a > n^ <b >, <a > n^ <b, > n in N } Traces model
– Nonempty– Prefix closed (if s^t is in trace, so is s)
We can calculate traces(P) for any CSP P
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Next Class: Next Class: Modeling security protocols in CSPModeling security protocols in CSP