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Mohamed G. Gouda						Midterm 1
CS 356: Computer Networks					Spring 2007
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Open Book							80 Minutes
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1. (10 points)

Design an array of two processes p[i:0..1] that exchange data messages. The 
exchange of data messages is assumed to be error-free At each instant the 
number of data messages sent by p[0] is no more than 5 plus the number of data 
messages sent by p[1], and the number of data messages sent by p[1] is no more 
than 7 plus the number of data messages sent by p[0]. A sketch of the process 
array is as follows: 

	process p[i:0..1]
	inp n : integer		{n.p[0]=5 and n.p[1]=7}
	var ...

	begin
	...

	end

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2. (10 points)

Consider a network that consists of two processes p and q. Process p sends an 
infinite sequence of data messages to process q, and q sends back an ack 
message for each data message that it receives from p. Data and ack messages 
can be lost but are neither corrupted nor re-ordered. The data messages have 
alternating sequence numbers in the range 0..1. Thus the sequence of data 
messages sent by p is as follows:

	data(0); data(1); data(0); data(1); ...

After process p sends a data(b) message to process q, p waits until one of the 
following two events happens: 

	either p receives an ack message from q, and in this case p sends the 
	next data(1-b) message to q and the cycle repeats
	
	or p detects that the sent data(b) message or the corresponding ack 
	message is lost and in this case p re-sends the data(b) message to q,
	and the cycle repeats.

Specify processes p and q in this protocol. 

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