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# Calculus of Communicating Systems

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### Calculus of Communicating Systems

The Calculus of Communicating Systems (CCS) is a process calculus introduced by Robin Milner around 1980 and the title of a book describing the calculus. Its actions model indivisible communications between exactly two participants. The formal language includes primitives for describing parallel composition, choice between actions and scope restriction. CCS is useful for evaluating the qualitative correctness of properties of a system such as deadlock or livelock.[1]

According to Milner, "There is nothing canonical about the choice of the basic combinators, even though they were chosen with great attention to economy. What characterises our calculus is not the exact choice of combinators, but rather the choice of interpretation and of mathematical framework".

The expressions of the language are interpreted as a labelled transition system. Between these models, bisimilarity is used as a semantic equivalence.

## Syntax

Given a set of action names, the set of CCS processes is defined by the following BNF grammar:

P ::= \emptyset\,\,\, | \,\,\,a.P_1\,\,\, | \,\,\,A\,\,\, | \,\,\,P_1+P_2\,\,\, | \,\,\,P_1|P_2\,\,\, | \,\,\,P_1[b/a]\,\,\, | \,\,\,P_1{\backslash}a\,\,\,

The parts of the syntax are, in the order given above

empty process
the empty process \emptyset is a valid CCS process
action
the process a.P_1 can perform an action a and continue as the process P_1
process identifier
write A \overset{\underset{\mathrm{def}}{}}{=} P_1 to use the identifier A to refer to the process P_1 (which may contain the identifier A itself, i.e., recursive definitions are allowed)
choice
the process P_1+P_2 can proceed either as the process P_1 or the process P_2
parallel composition
P_1|P_2 tells that processes P_1 and P_2 exist simultaneously
renaming
P_1[b/a] is the process P_1 with all actions named a renamed as b
restriction
P_1{\backslash}a is the process P_1 without action a

## Related calculi and models

• Communicating Sequential Processes (CSP), developed by Tony Hoare, is a language that arose at a similar time to CCS.
• The pi-calculus, developed by Milner in the late 80's, provides mobility of communication links by allowing processes to communicate the names of communication channels themselves.
• PEPA, developed by Jane Hillston introduces activity timing in terms of exponentially distributed rates and probabilistic choice, allowing performance metrics to be evaluated.

Some other languages based on CCS:

Models that have been used in the study of CCS-like systems:

## References

• Robin Milner: A Calculus of Communicating Systems, Springer Verlag, ISBN 0-387-10235-3. 1980.
• Robin Milner, Communication and Concurrency, Prentice Hall, International Series in Computer Science, ISBN 0-13-115007-3. 1989
1. ^ Herzog, Ulrich, ed. (May 2007). "Tackling Large State Spaces in Performance Modelling". Formal Methods for Performance Evaluation. Lecture Notes in Computer Science 4486. Springer. pp. 318–370.
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