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Mohamed G. Gouda						CS 356
Spring 2017							Homework 1
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1. Consider an ISP tree that has
	one Tier-1 ISP: T1,
	two Regional ISPs: R1 and R2, and
	four Access Networks: A1, A2, A3, and A4

Connections in this tree are as follows. T1 is connected to both R1 and R2,
R1 is connected to both A1 and A2, and R2 is connected to both A3 and A4.

In Exercise 1, we show that if any one of the ISPs in this tree, namely T1, 
R1, or R2, fails and can no longer receive or send messages, then there is 
a message whose original source is in an Access Network Ai and whose ultimate 
destination is in another Access Network Aj and this message cannot be routed 
from its original source to its ultimate destination.

Show that three more ISPs, namely T2, R3, and R4, can be added to this ISP 
tree such that the following condition holds. If any one ISP (whether 
original ISP or added ISP) fails, then every message whose original source
is in an access network Ai and whose ultimate destination is in another
Access Network Aj can still be routed from its original source to its
ultimate destination.

(Hint: The added ISP T2 is a Tier-1 ISP, and the added ISPs R3 and R4 are 
Regional ISPs.)

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2. Explain how a company, whose domain is x.com, can be registered in DNS.
This company has an "A" DNS server, a web server, and an email server.

The "A" DNS server has the following properties
		Canonical Name:		Chicago.center.us.x.com
		IP Address:		128.83.15.7

The web server has the following properties
		Alias Name:		x.com
		Canonical Name:		www.x.com
		IP Address:		128.83.201.49

The email server has the following properties
		Canonical Name:		mail.x.com
		IP Address:		128.83.12.129

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3. Identify all the errors in following scenario of the Cookie protocol, 
where C stands for a client and S stands for a server:

	C --> S:	HTTP rqst1 ( )
	C <-- S:	HTTP rspn1 (Cookie: 125)
	C --> S:	HTTP rqst2 (Set-cookie: 125)
	C <-- S:	HTTP rspn2 (Cookie: 125)

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4. In the Bit-Torrent protocol, when a peer P registers with the tracker of
a torrent T, P receives IP adresses of 50 peers out of the 1000 peers that 
the tracker keeps track of. 

(a) Why does not P receive the IP adresses of all 1000 peers?

(b) Why does not P receive the IP address of only one peer?

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Solutions
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1. Add the three ISPs T2, R3, and R4 along with the following connections to
the original ISP tree:
	T2 is connected to both R3 and R4
	R3 is connected to both A1 and A2
	R4 is connected to both A3 and A4

It is straightforward to show that if any of the six ISPs, namely T1, T2, R1,
R2, R3, and R4, fails, then every message whose original source is in an 
access network Ai and whose ultimate destination is in another access network 
Aj can still be routed from its original source to its ultimate destination.

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2. To register this company in DNS, the following three steps are executed:

(a) Publish the following two RRs in every com DNS server in the world:
		
		(x.com, Chicago.center.us.x.com, NS, TTL)
		(Chicago.center.us.x.com, 128.83.15.7, A, TTL)

(b) Publish the following two RRs in the "A" DNS server of the company:
	
		(x.com, www.x.com, CNAME, TTL)
		(www.x.com, 128.83.201.49, A, TTL)

(c) Publish the following RR in the "A" DNS server of the company:

		(mail.x.com, 128.83.12.129, A, TTL)

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3. The Cookie header for a response message from S to C is Set-cookie rather
than Cookie. Thus the second and fourth messages in the given scenario are
erroneous.

The cookie header for a request message from C to S is Cookie rather than
Set-cookie. Thus the third message in the given scenario is erroneous.

The first message in the given scenario can be correct.

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4. (a) If P receives the IP addresses of all 1000 peers, then P needs to
communicate with all 1000 peers to get all chunks of any file F that P 
needs, even though the probability, that all chunks of F can be obtained 
from any 50 peers out of the 1000 peers, can be very high.

(b) If P receives the IP address of only one peer Q, then P needs to get 
all chunks of any file F that P needs from Q, even though the probability
that Q has all chunks of F can be very small.

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