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Mohamed G. Gouda						CS 356
Summer 2008							Midterm 1
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75 minutes							Open Book
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1. (10 points)
Consider a protocol that consists of two symmetric processes p and q. The 
execution of this protocol proceeds in an infinite sequence of "rounds". In 
each round, each process generates a random bit b and sends it in a data(b) 
message to the other process in the protocol. After generating one bit and 
receiving another bit, each process sums up the two bits and stores the 
resulting bit in the next available element of its array "round". (Note that 
the bit sum of 1+1 is 0.)

An incomplete specification of process p is as follows.

	process p
	var	round	: array [integer] of 0..1,
		x	: integer,	{index of round; init. 0}
		b,z	: 0..1,		{init. 0}
		gen	: boolean	{init. false}
	begin
		~gen	          --> 	b := random;
					z := z +2 b;
					send data(b) to q;
					gen := true	
	[]	rcv data(b) from q --> 	<statement S>
	end

Specify statements S.
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2. (10 points)
In a protocol involving two processes p and q, assume that the messages sent
from p to q can be corrupted, lost, or reordered but the messages sent by
q remain in the channel from q to p unchanged until they are received by p.
An incomplete specification of p and q is as follows.

	process p
	inp 	data	:	array [integer] of integer
	var	x	:	integer,	{index of data, init.0}
		ready	:	boolean 	{init. true}
	begin
		...
	end

	process q
	var	rcvd	:	array [integer] of integer,
		y	:	integer,	{index of rcvd, init.0}
		z	: 	integer
	begin
		rcv msg(z) from p -->	<statement S.0>
	[]	rcv error  from p -->   skip
	end

Complete the specifications of processes p and q such that p sends the 
elements of its input array data one by one in messages of the form 
msg(data[x]) to process q which then stores the received elements in its
variable array rcvd. Note that eventually the two arrays data and rcvd 
become identical.
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