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Mohamed G. Gouda					CS 356
Spring 2017						Midterm 1
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1. (a) Is it possible that at any instant multiple cookies, that
are generated by the same server, co-exist in the same cookie 
file? 

(b) Is it possible that at any instant multiple cookies, that
are generated by distinct servers in the same cookie file, are
identical?

(Hint: The correct answers to (a) or (b) are either Yes or No.)

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2. 
(a) What DNS data is stored in a local DNS server?

(b) What DNS data is stored in a root server?

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3. In the following HTTP scenario, a client C requests and gets
from a server S a web page consisting of one HTML Base file and 
Video clip:

	C --> S:	GET <Base file>
	C --> S:	GET <Video clip>
	C <-- S:	Base file
	C <-- S:	Video clip

Eplain what is wrong with this scenario. Write down a correct
scenario.

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4. Let P denote the URL of a web page. Assume that at some
instant t, the version of P that is stored in the original server
is last-modified at time T, and the version of P that is stored
in the proxy server is last modified at time T'. Which of the
following two statements is true:

	(a) T >= T'
	(b) T < T'

Explain your answer.
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Solutions
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1.
(a) No
(b) Yes
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2.
(a) A local DNS server stores the IP address of a root server.

(b) A root server stores the names and IP addresses of all TLD 
servers.
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3.
The mistake in this scenario is that client C does not know the URL
for the Video clip and so can not ask for the Video clip until after
it receives the Base file from server S.

The corrected scenario is as follows: 

	C --> S:	GET <Base file>
	C <-- S:	Base file
	C --> S:	GET <Video clip>
	C <-- S:	Video clip
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4. 
Statement (a), namely T >= T', is true.

Explanation:
When page P is modified, the version of P in the original server is 
modified first, making T > T'. Later this modification is reflected 
in the version of P in the proxy server, making T = T'. Therefore,
at each instant t, T >= T'.
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