Due April 14, 2010
Note that homeworks should be entirely individual effort. You should not
discuss them with anybody, other than the TA or the instructor. The answers
should be succinct and not long-winded (say, about 1-2 paras per
usually sufficient). Examples/figures will work well, where applicable. All
sources that you got substantial help from must be cited.
TCP over wireless links: What
is the major difference between M-TCP and Freeze-TCP? [Freeze-TCP is here.
M-TCP is a part of readings.]
Study the Fast Recovery
algorithm in TCP Reno carefully, particularly how the congestion window is
adjusted. Assume that one single packet is lost and dupacks will arrive at
the sender as expected. Assume that right before the 3rd dupack, the window
size is W. What will be the window size at the 4th and 5 dupack?
Suppose, now the ack due to the retransmitted packet comes back. What will
be the window size?
Can new data be sent at all
before the retransmitted packet is acked? Explain.
As a follow up to the
previous question, it turns out that TCP Reno works well when there is a
single packet loss inside a window. But with multiple losses within the same
window, it is not quite as effective. Argue why? A modification TCP
NewReno has been proposed to
counter this and is implemented in many modern TCP stacks. Study the NewReno
modification. Argue why it could be effective with multiple losses within
the same window.
Solve P33 from Chapter 3 of
Textbook. Copied below.
In Section 3.5.4, we saw that TCP waits until it has received
three duplicate ACKs before performing a fast retransmit. Why do you think the
TCP designers chose not to perform a fast retransmit after the first duplicate
ACK for a segment is received?
Solve P49 from
Chapter 3 of Textbook. Copied below.
Consider a modification to TCP’s congestion control algorithm.
Instead of additive increase, we can use multiplicative increase. A TCP sender
increases its window size by a small positive constant a ( 0 < a <
1) whenever receives a valid ACK. Find the functional relationship between
loss rate L and maximum congestion window W. Argue that for
this modified TCP, regardless TCP’s average throughput, a TCP connection always
spends the same amount of time to increase its congestion window size from
W/2 to W.
Solve P51 from Chapter 3 of
Textbook. Copied below.
In our discussion of TCP
congestion control in Section 3.7, we implicitly assumed that the TCP sender
always had data to send. Consider now the case that the TCP sender sends a
large amount of data and then goes idle ( since it has no more data to send)
at t1 . TCP remains idle for a relatively long period of time and then wants
to send more data at t2 . What are the advantages and disadvantages of
having TCP use the cwnd and ssthresh
values from t1 when starting to send data at t2 ? What alternative would you