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Mohamed G. Gouda						Homework 1
CS 356: Computer Networks					Spring 2007
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1. (3 Points):

In the resource allocation protocol discussed in class, each resource can 
be accessed by at most one user at a time (and each user can access at most one
resource at a time). Modify this protocol such that each resource can be 
accessed by at most three users at a time (but each user can still access at 
most one resource at atime). Specify the controller process c in the modified
protocol.

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2. (3 Points):

Consider a network that consists of a process p and a process array 
q[i : 0..n-1]. The communications in this network go through consecutive 
"rounds". In each round,

	first, p sends n rqst messages, one to each process q[i],

	then, p waits until for each i, i = 0..n-1, either p receives
	a rply message from q[i] or p is certain that either the rqst 
	message from p to q[i] is lost, or the corresponding rply message
	from q[i] to p is lost.

Process p in this network is specified as follows.

	process p
	const 	n
	var	ready : boolean,		{initially true}
		i     : 0..n-1
	begin
		ready --> S.0
	[]	rcv rply from q[i] --> S.1
	[]      timeout ~ready ^ (#ch.p.q[i] + #ch.q[i].p = 0) --> S.2
	end

Specify the three statements S.0, S.1, S.2 in process p.
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3. (4 Points):

A network has m+n+1 processes: p[i : 0..m-1], q[j : 0..n-1], r. In this 
network, process r acts as a "switch" between the p[i] processes and the 
q[j] processes. At any time, a process p[i] sends a rqst(j) message to process
r, then waits to receive a rply message from r before it can send another 
rqst(j') message to r. When r receives a rqst(j) message from p[i], r sends a 
rqst(i) message to q[j]. When q[j] receives a rqst(i) message from r, q[j] 
sends a rply(i) message to r. When r receives a rply(i) message from q[j], r
sends a rply message to p[i]. Specify the proceses in this network.

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