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#if 0
Cambridge Ring Protocol [Needham 82]
basic protocol - no LTL properties
LOSS RELAXED states depth
0 0 482,061 9,232 no reduction
1 0 993,547 10,234 no reduction
0 1 629,079 18,268 no reduction
1 1 1,252,660 17,117 no reduction
#endif
#define LOSS 1
#define RELAXED 1
#if RELAXED==1
#define stimeout empty(sender)
#define rtimeout empty(recv)
#else
#define stimeout timeout
#define rtimeout timeout
#endif
#define QSZ 6 /* length of message queues - should be 6 */
mtype = {
RDY, NOTRDY, DATA, NODATA, RESET
};
chan sender = [QSZ] of { mtype, byte };
chan recv = [QSZ] of { mtype, byte };
active proctype Sender()
{ short n = -1; byte t,m;
xr sender;
xs recv;
I: /* Idle State */
do
#if LOSS==1
:: sender?_,_ -> progress2: skip
#endif
:: sender?RESET,0 ->
ackreset: recv!RESET,0; /* stay in idle */
n = -1;
goto I
/* E-rep: protocol error */
:: sender?RDY,m -> /* E-exp */
assert(m == (n+1)%4);
advance: n = (n+1)%4;
if
/* buffer */ :: atomic {
printf("MSC: NEW\n");
recv!DATA,n;
goto E
}
/* no buffer */ :: recv!NODATA,n;
goto N
fi
:: sender?NOTRDY,m -> /* N-exp */
expected: assert(m == (n+1)%4);
goto I
/* Timeout */
/* ignored (p.154, in [Needham'82]) */
:: goto reset
od;
E: /* Essential element sent, ack expected */
do
#if LOSS==1
:: sender?_,_ -> progress0: skip
#endif
:: sender?RESET,0 ->
goto ackreset
:: sender?RDY,m ->
if
:: (m == n) -> /* E-rep */
goto retrans
:: (m == (n+1)%4) -> /* E-exp */
goto advance
fi
:: sender?NOTRDY,m -> /* N-exp */
goto expected
/* Timeout */
:: stimeout ->
printf("MSC: STO\n");
retrans: recv!DATA,n /* retransmit */
:: goto reset
od;
N: /* Non-essential element sent */
do
#if LOSS==1
:: sender?_,_ -> progress1: skip
#endif
:: sender?RESET,0 ->
goto ackreset
:: sender?RDY,m -> /* E-rep */
assert(m == n) /* E-exp: protocol error */
-> recv!NODATA,n /* retransmit and stay in N */
:: recv!DATA,n; /* buffer ready event */
goto E
:: goto reset
/* Timeout */
/* ignored (p.154, in [Needham'82]) */
od;
reset: recv!RESET,0;
do
#if LOSS==1
:: sender?_,_ -> progress3: skip
#endif
:: sender?t,m ->
if
:: t == RESET -> n = -1; goto I
:: else /* ignored, p. 152 */
fi
:: timeout -> /* a real timeout */
goto reset
od
}
active proctype Receiver()
{ byte t, n, m, Nexp;
xr recv;
xs sender;
I: /* Idle State */
do
#if LOSS==1
:: recv?_,_ -> progress2: skip
#endif
:: recv?RESET,0 ->
ackreset: sender!RESET,0; /* stay in idle */
n = 0; Nexp = 0;
goto I
:: atomic { recv?DATA(m) -> /* E-exp */
assert(m == n);
advance: printf("MSC: EXP\n");
n = (n+1)%4;
assert(m == Nexp);
Nexp = (m+1)%4;
if
:: sender!RDY,n; goto E
:: sender!NOTRDY,n; goto N
fi
}
:: recv?NODATA(m) -> /* N-exp */
assert(m == n)
/* Timeout: ignored */
/* only receiver can initiate transfer; p. 156 */
:: empty(recv) -> sender!RDY,n; goto E
:: goto reset
od;
E:
do
#if LOSS==1
:: recv?_,_ -> progress1: skip
#endif
:: recv?RESET,0 ->
goto ackreset
:: atomic { recv?DATA(m) ->
if
:: ((m+1)%4 == n) -> /* E-rep */
printf("MSC: REP\n");
goto retrans
:: (m == n) -> /* E-exp */
goto advance
fi
}
:: recv?NODATA(m) -> /* N-exp */
assert(m == n);
goto I
:: rtimeout ->
printf("MSC: RTO\n");
retrans: sender!RDY,n;
goto E
:: goto reset
od;
N:
do
#if LOSS==1
:: recv?_,_ -> progress0: skip
#endif
:: recv?RESET,0 ->
goto ackreset
:: recv?DATA(m) -> /* E-rep */
assert((m+1)%4 == n); /* E-exp and N-exp: protocol error */
printf("MSC: REP\n");
sender!NOTRDY,n /* retransmit and stay in N */
:: sender!RDY,n -> /* buffer ready event */
goto E
/* Timeout: ignored */
:: goto reset
od;
progress:
reset: sender!RESET,0;
do
#if LOSS==1
:: recv?_,_ -> progress3: skip
#endif
:: recv?t,m ->
if
:: t == RESET -> n = 0; Nexp = 0; goto I
:: else /* ignored, p. 152 */
fi
:: timeout -> /* a real timeout */
goto reset
od
}
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