CS 640 - Midterm Exam - October 24, 1995

(10) 1. Let M  = 101100010 and G(x) = x**5 + x**4 + x+ 1
a. Give the shift register corresponding to G(x).
b. Compute T, the bit string to transmit for the data M and generator polynomial G(x).

(9) 2. Assume the following communication between two TCP entities. Give the values 
indicated by "??" for the indicated TCP segments.

Seq # = 50
Ack # = 400               ----------->		Arrive
Window = 250
120 bytes data
						Seq# = 400
Arrive			<-----------		Ack# = ?? a.
						Window = 200
						350 bytes data
Seq# = ?? b.
Ack# = ?? c.		 ----------->
30 bytes data

 (24) 3. True or False 
a. Sliding windows are used to provide a connectionless service interface.
b. There is transport layer functionality in network switching elements.
c.  Data link protocols usually have variable size receive windows.
d. Transport protocols can have receive or send windows whose size exceeds the size 
of the sequence number space.
e. When the timestamp option is used to estimate round trip time, the timestamp 
value sent as TS.ECR is always taken from the incoming segment that last advanced 
the left edge of the receive window.
f. If two TCPs that support window scaling are communicating, both have the same 
receive window scale factor?
g. Byte-oriented data link protocols are preferable to bit-oriented data link 
protocols because they depend on a fixed character set representation.
h. When a data link frame arrives with incorrect FCS, its Ack field is still 
processed if the frame sequence number is that of the next expected frame. 

 (15 ) 4. a. Give one way in which TCP is dependent on IP.
b. Suppose we want 16mpbs TCP throughput on a connection with RTT 50ms. What is 
the minimal window scale factor required?
c. If the TCP data flow is momentarily idle and all data sent has been 
acknowledged (and all acknowledgments have been correctly received), what 
can you say about the relationship of the threee variables SND.NXT at the 
sender and RCV.NXT  and SND.UNA at the receiver to each other?
d. Give two LAP B commands that are used for flow control.
e. Give the principal benefit of NAKs in data link protocols.

(20) 5. a. Explain virtual circuit and connection-oriented service interface.
b. Explain how a transport protocol would provide a connection-oriented service 
interface on top of a datagram network.

(20) 6. a. Explain the 3-way handshake.
b. Why is it not needed by link protocols? 
c. What problem does it solve?