inf-mcs-notes/03-ccs.tex

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\begin{document}
\paragraph*{CCS}
Define the set of operations and semantics: 
\begin{itemize}
    \item {
        $0$ (inaction): 
        
        $0$ represents a graph with 1 (initial) state, 0 transitions.
    }
    \item {
        $a, \bar{a}$ (complementary actions): 

        Complementary actions are assumed to communicate / synchronize with one another. 
    }
    \item {
        $a.P$ (action prefix) for each action $a$, process $P$, which:  
        \begin{enumerate}
            \item Define new initial state $\mathsf{i}$.
            \item Creates transition $\mathsf{i} \xrightarrow{a} I_P$.
        \end{enumerate}
    }
    \item {
        $P + Q$ (summation / choice / alternative composition), where: 
        \begin{itemize}
            \item Define new initial state $\mathsf{root}$.
            \item $\mathrm{States}(P + Q) \coloneqq \mathrm{States}(P) \cup \mathrm{States}(Q) \cup \{\mathsf{root}\}$
            \item Replace all $I_P \xrightarrow{a} s$ with $\mathsf{root} \xrightarrow{a} s$. 
            \item Replace all $I_Q \xrightarrow{a} s$ with $\mathsf{root} \xrightarrow{a} s$.
        \end{itemize}
    }
    \item {
        $P | Q$ (parallel composition).

        This takes the cartesian product of the states of $P, Q$, such that: 
        \begin{itemize}
            \item $s \xrightarrow{a} s^{'} \in P \implies \forall t \in Q: (s, t) \xrightarrow{a} (s^{'}, t)$
            \item $t \xrightarrow{a} t^{'} \in Q \implies \forall s \in P: (s, t) \xrightarrow{a} (s, t^{'})$
            \item $(s \xrightarrow{a} s^{'} \in P) \wedge (t \xrightarrow{\bar{a}} t^{'} \in Q) \implies (s, t) \xrightarrow{\tau} (s^{'}, t^{'})$
        \end{itemize}

        Note that \textbf{CCS adheres strictly to a handshaking communication format} -- this differs from ACP which gives greater leeway to implementation, via the use of $\gamma$ operator.
    }
    \item {
        $P{\backslash}a$ (restriction) for each action $a$. 

        This produces copy of $P$ such that all actions $a, \bar{a}$ are omitted. This is useful to remove unsuccessful communication. 
    }
    \item {
        $P[f]$ (relabelling) for each function $f: A \rightarrow A$.

        This replaces each label $a, \bar{a}$ by $f(a), \overline{f(a)}$.
    }
\end{itemize}

\paragraph*{Recursion}
\begin{definition}[process names and expressions]
    Suppose we bind names $X, Y, Z, \dots$ to some expression in the CCS language: 
    \begin{equation*}
        X = P_X
    \end{equation*}
    Here, $P_X$ represents ANY expression in the language, possibly including $X$. 

    It is trivial to see this can cause recursive definitions: 
    \begin{equation*}
        X = a.X
    \end{equation*}
\end{definition}

\begin{definition}[recursive specification]
    Define \textbf{recursive specification} as partial function $s: X \rightarrow E$: 
    \begin{itemize}
        \item $X$: \textbf{recursion variables}. 
        \item $E$: \textbf{recursion equations} of form $x = P_x$.
    \end{itemize}

    In general, recursive spec.s are written as follows: 
    \begin{equation*}
        \langle x | s \rangle
    \end{equation*}
    which reads as ``process $x$ satisfying equation $s$''. 
\end{definition}

\begin{definition}[guarded recursion]
    A recursion is \textbf{guarded} if each occurrence of a process name in $P_X$ occurs within the scope of a subexpression $a.P^{'}_X$. 
    
    Think of it as being unwind-able such that progress is guaranteed.
\end{definition}

\paragraph*{Structural Operational Semantics (CCS)}
\begin{center}
    \begin{tabular}{cc}
        $
        \begin{prooftree}
            \infer0{a.E \xrightarrow{a} E}
        \end{prooftree}
        $   &
        $
        \begin{prooftree}
            \hypo{E_j \xrightarrow{a} E_j^{'}}
            \infer1{\sum_{i \in I}E_i \xrightarrow{a} E_j^{'}}
        \end{prooftree}\ (j \in I)
        $   \\
        \\
        $
        \begin{prooftree}
            \hypo{E \xrightarrow{a} E^{'}}
            \infer1{E | F \xrightarrow{a} E^{'} | F}
        \end{prooftree}
        $   &
        $
        \begin{prooftree}
            \hypo{E \xrightarrow{a} E^{'}}
            \hypo{F \xrightarrow{\overline{a}} F^{'}}
            \infer2{E | F \xrightarrow{\tau} E^{'} | F^{'}}
        \end{prooftree}
        $   \\
        \\
        $
        \begin{prooftree}
            \hypo{E \xrightarrow{a} E^{'}}
            \hypo{a \notin L \cup \overline{L}}
            \infer2{E{\backslash}L \xrightarrow{a} E^{'}{\backslash}L}
        \end{prooftree}
        $   &
        $
        \begin{prooftree}
            \hypo{E \xrightarrow{a} E^{'}}
            \infer1{E[f] \xrightarrow{f(a)} E^{'}[f]}
        \end{prooftree}
        $   \\
        \\
    \end{tabular}
\end{center}

\end{document}