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Mohamed G. Gouda						CS 356
Spring 2017							Midterm 2
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1. Consider an application server aps that is deployed on top of TCP port
i in host sh. What are the steps that aps needs to execute before it
starts to receive TCP segments from any application client.
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2. In a version of the TCP sliding window protocol, one side S sends DATA 
segments and receives ACK segments and the other side R receives the sent 
DATA segments and sends back the ACK segments. 

Side S maintains three variables: na, ns, w.

Side R maintains a buffer bff for storing the received data bytes until
they are read by the application. Side R also maintains four variables:
|bff|, nd, nr, and wr.	

Consider a state of this protocol where w = 89, nd = 81, and nr = 92.

Compute the size of buffer bff.
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3. Consider a data byte stream from one side S to another side R in a 
sliding window protocol in TCP. Assume that the initial seq# for the stream
is 14. Also assume that the values of variables na and ns in S are 15 and
17, respectively. Which of the data bytes, whose seq#'s are 14, 16, or 18,
have already been sent by S and which of them have already been acknowledged
by R.
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4. Consider a data byte stream from one side S to another side R in a 
sliding window protocol in TCP. Assume that the initial seq# for the stream
is 14. Also assume that the values of variables nd and nr in R are 15 and
16, respectively. Which of the data bytes, whose seq#'s are 15, 16, or 17,
have already been read by the application and which of them have already 
been received by R.
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Solutions
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1.
Server aps needs to execute the following steps before it starts to receive 
TCP segments from any application clients:

	1. aps allocates TCP port i in sh.
	2. aps allocates any TCP "welcome socket" ss in sh.
	3. aps binds socket ss to port i in sh.
	4. aps adds the following entry to TCP socket table in sh:
	   (src = any, dst(sh, i), ss)
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2.
	wr = w provided w >= 1
	wr = 89

	wr = (|bff| - (nr - nd))
	89 = (|bff| - (92 - 81))
	89 = (|bff| - 11)
	Therefore |bff| equals 89+11 which equals 100
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3.
	Only the data bytes whose seq#'s are 14 or 16 have already been sent
	by S

	Only the data byte whose seq# is 14 has already been acknowledged by
	R.
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4.
	None of the data bytes whose seq#'s are 15, 16, and 17 have already been
	read by the application.

	Only the data byte whose seq# is 15 has been received by R.
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