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Mohamed G. Gouda					CS 356
Spring 2018						Midterm 1
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Solve any four of the following five problems.
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1.
What is the main difference between a client-server application
and a P2P application?

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2.
The link layer of a host H receives a frame f from the subnetwork 
attached to H. Then the link layer of H transforms f into a
packet p and forwards p to the network layer of H. Then the
network layer of H transforms p into a segment s and forwards s 
to the transport layer of H.

(a) What are the headers in p?
(b) What are the headers in s?

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3.
Where in DNS does one store the set of RRs that can be used to 
compute the IP address of the "A" DNS server of the s.edu domain?

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4. 
When a user activates the email application on a client host ch,
the user informs its agent (e.g. "outlook") on ch of the user 
name. What does the user agent do with the user name?

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5.
What is the smallest number of distinct DNS servers that need to
be visited in order to resolve the host name (www.ab.com) into
the corresponding IP address. Explain your answer?

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Solutions
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1.
To request a service in a client-server application, a client 
sends requests to and gets back replies from a well-known always-
running server.

To request a service in a P2P application, a client called peer
sends requests to and gets back replies from other peers who 
happen to be running in the application.

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2.
(a) The headers in p are
    network header, transport header, and application header.

(b) The headers in s are
    transport header and application header.

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3.
In DNS, the set of RRs, that can be used to compute the IP
address of the "A" DNS server of the s.edu domain, is stored in
every edu server.

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4.
The user agent uses the user name to know the email server in 
which the user is registered. Then the agent communicates with 
this email server to inform it that the user is currently active
on the client host ch.

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5.
The smallest number of distinct DNS servers that need to be
visited in this case is "one" when the IP address of the host
name (www.west.ab.com) is currently stored in the cache of the
local DNS server.

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