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Mohamed G. Gouda						CS 356
Spring 2018							Midterm 2
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1. 
How many checksums (used to detect end-to-end or hop-by-hop message
corruption) in a frame? Explain your answer.
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2.
Explain the fact that the block of IP addresses
	223.1.1.128/25
has the IP address
	223.1.1.255
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3.
A TCP connection between a client and a server has two sides. One side is
the pair (ch1, j1) and the other side is the pair (sh, i), where
																																																																																																																																																																																																																																																																																																																																																																																																																																																																																																																																																																																																																																																																																												
	ch1 is a client host
	j1  is a TCP port in ch1
	sh  is a well-known server host
	i   is a well known TCP port in sh

This connection is identified by one entry in the TCP socket table in ch1
and by one entry in the TCP socket table in sh. Specify these two entries.

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Solutions
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1.
There are three checksums in a frame:

	One, in the transport header in the frame, 
	to detect end-to-end message corruption

	One, in the network header in the frame,
	to detect hop-by-hop message corruption

	One, in the link header in the frame,
	to detect hop-by-hop message corruption

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2. 
The block of IP addresses
	223.1.1.128/25
has each IP address of the form
	223.1.1.1xxxxxxx, where x is either 0 or 1
and so the block has the IP adress
	223.1.1.11111111
which can be rewritten as
	223.1.1.255

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3.
The TCP socket table in ch1 has the entry
	(src=(sh,i), dst=(ch1,j1), cs1)

The TCP socket table in sh has the entry
	(src=(ch1,j1), dst=(sh,i), ss1)

where cs1 is the socket associated with TCP port j1 in ch1
      ss1 is the socket associated with TCP port i in sh
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