4007 CNT Computer Network Fundamentals, Spring 2012
Assignment 3
given by
Jonathan C.L. Liu
Out: Mar. 30 (Friday), 2012
Due: Beginning of the lecture on Apr. 06 (Friday), 2012
The problem sets form an important part of the learning in this course. Thus you are
required to do them in order to pass.
Try to be as clear and precise as possible in your presentation. Sloppy answers may be at
a disadvantage, i.e., likely to receive fewer points.
No late assignments are accepted !!
Problem 1
In Section 3.5.4, we saw that TCP waits until it has received three duplicate ACKs before
performing a fast retransmit. Why TCP designers chose not to perform a fast retransmit after
the rst duplicate ACK for a segment is received?
Problem 2
In Section 3.5.4 we discussed the doubling of the timeout interval after a timeout event. This
mechanism a is form of congestion control. Why does TCP need a window-based congestioncontrol mechanism (as studied in Section 3.7) in addition to this doubling-timeout-interval mechanism?
Problem 3
Consider sending a large le from a host to another over a TCP connection that has no loss.
(a) Suppose TCP uses AIMD for its congestion control without slow start. Assuming
CongWin increases by 1 MSS every time a batch of ACKs is received and assuming approximately constant round-trip times, how long does it take for CongWin to increase
from 1 MSS to 6 MSS (assuming no loss events)?
(b) What is the average throughput (in term of RTT and MSS) for this connection up
through time = 5 RTT?
Problem 4
Explain the dierence(s) between routing and forwarding? Three types of switching fabrics are
discussed in Section 4.3. Explain each type.