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Mohamed G. Gouda						CS 356
Summer 2008							Midterm 2
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60 minutes							Open Book
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1. (10 points)

Consider an IP header that consists of 20 Bytes and has a checksum that 
consists of 2 Bytes. Assume that this checksum is computed using the first
corruption detection protocol in Section 8.1. 

a. Can this checksum be used to detect a corruption where every bit in the IP
   header is corrupted (from 0 to 1 or from 1 to 0)? Explain your answer. 

b. Can this checksum be used to detect a corruption where every bit, except
   one, in the IP header is corrupted? Explain your answer.

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2. (10 points)

A process p has an infinite input array d whose integer elements are to be 
sent one by one from p to process q. Each sent message from p to q is of the 
form data(s, t), where s is a sequence number in the range 0..1 and t is a
value of an integer element in array d. Assume that the messages sent from p 
to q can be lost but neither corrupted nor reordered. Assume also that the 
message loss is 1-bounded. When process q receives a data(s, t) message and 
checks that this message is the expected one, process q stores t in the next 
available element in array e. An incomplete specification of processes p and 
q is as follows.

	process p
	inp	d	:	array [integer] of integer
	var     x	:	integer		{index of d; init. 0}
	begin
		true	--> 	<statement S.0>
	end

	process q
	var	e	:	array [integer] of integer,
		y	:	integer,	{index of e; init.0}  
		exp, s	:	0..1,		{init. 0}
		t	:	integer
	begin
		rcv data(s, t) from p --> <statement S.1>
	end

Specify the two statements S.0 and S.1.

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