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Mohamed G. Gouda                                        	CS 356
Spring      2007                                        	Homework 2
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1. (4 points)
Design a full-duplex connection protocol between two processes p and q. Each of 
the two proceses has two boolean variables conr and cons, where conr stands for
connection-to-receive (data messages) from the other process and cons stands 
for connection-to-send (data messages) to the other process. Initially the two
conr variables and the two cons variables are false. When process p sends a 
crqst message to q and receives back crply, it assigns its conr and cons the 
value true and starts to send and receive data messages. Later process p 
assigns cons the value false, stops sending data messages (but still accepts 
received data messages), and starts sending drqst messages to q. When p receives
 drply messages from q, it assigns its conr and cons the value false. When 
process q receives a crqst message from p, it can assign its conr the value 
true and send a crply message to p. It can also assign its conr the value false
 and assign its conr and cons the value false. When q receives a data message 
from p and its conr has the value true, it assigns its cons the value true. 
When cons in process q has the value true, q can send data messages to process 
p. Finally, when q receives a drqst message from p, q assigns its conr and 
cons the value false and sends a drply message to p.

Specify processes p and q in this protocol.
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2. (3 points)
The corruption detection protocol in section 8.1 is bit-based. It is required 
to modify this protocol to make it byte-based. For example, in the modified 
protocol, a data block consists of m*n data bytes and the parity added to each 
data block consists of n parity bytes. Also, each message sent from p to q is 
of the form byte(u), where u is in the range 0..255. The data and parity arrays 
in process p in the modified protocol are specified as follows: 

inp  dp  : array[integer] of 0..255
var  pp  : array[0..n-1] of 0..255, 

Both p and q use a function xor defined as follows: u := xor(v,w) where each of 
u,v and w is in the range 0..255 and xor returns the bit-wise exclusive-or of 
the two bytes v and w. Specify process p in the modified protocol.
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3. (3 points) 
Design a protocol for forward recovery from n-bounded re-order. Process p in 
this protocol sends data(t,s) messages to process q where t is an integer that 
represents the message text and s is a sequence number in the range 0..(2n-1). 
The data structure of process p in this protocol is as follows: 

process p
const  n
inp    txt : array[integer]of integer 
var    i   : integer, 			{index of txt, init. 0}
       s   : 0..(2n-1)			{seq. number of next message, init. 0}

The data structure of process q in this protocol is as follows

process q
const  n
var    txt  : array[integer]of integer,
       i    : integer, 			    {index of txt, init. 0}
       exp  : 0..(2n-1), 		    {seq. # of exp. message init. 0}
       bff  : array[0..(2n-1)] of boolean,  {init. false}
       rcvt : array[0..(2n-1)] of integer,  {received text out of order}
       t    : integer, 			    {received text}
       s    : 0..(2n-1)			    {received seq. number}

Specify processes p and q in this protocol. 
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