CNT 4704: Analysis of Computer Communication Networks
Fall 2012
Homework 2
(assigned 10/11; due: 10/19 midnight, submitted via
webcourse)
1.
Suppose Client A initiates a Telnet session
with Server S. At about the same time, Client B also initiates a
Telnet session with Server S. Provide possible source and
destination port numbers for
a). The segments sent from A to S.
b). The segments sent from B to S.
c). The segments sent from S to A.
d). The segments sent from S to B.
e). If A and B are different hosts, is
it possible that the source port number in the segments from A
to S is the same as that from B to S?
f). How about if they are the same
host?
2. Suppose we have packets that use 8
bits as their checksum:
a). Suppose you have the following 2 bytes in
a packet: 11110100 and 01101001. What is the 1's complement
checksum?
b). Suppose you have the following 2 bytes in
a packet: 11110101 and 01101001. What is the 1's
complement checksum?
3.
Consider the rdt 3.0 protocol. Draw a diagram
showing that if the network connection between the sender and
receiver can reorder messages (that is, that two messages
propagating in the medium between the sender and receiver can be
reordered), then the alternating-bit protocol will not work
correctly (make sure you clearly identify the sense in which it
will not work correctly). Your diagram should have the sender on
the left and the receiver on the right, with the time axis
running down the page, showing data (D) and acknowledgement (A)
message exchange. Make sure you indicate the sequence number
associated with any data or acknowledgement segment.
4. Justify your answer even if the
answer is true. For statement (a)(b), you should justify it by
drawing a protocol running example.
Answer true or false to the following
statements and briefly justify your answer:
a). With the SR protocol,
it is possible fo rthe sender to receive an ACK for a packet
that falls outside of its current window.
b). With GBN,
it is possible for the sender to receive an ACK for a packet
that falls outside of its current window.
c). The alternating-bit protocol is the
same as the SR protocol with a sender and receiver window size
of 1.
d). The alternating-bit
protocol is the same as the GBN protocol with a sender and
receiver window size of 1.
5. For qustion (b), assume that the
network is perfect and there is no error or retransmissions.
Consider transferring an enormous file of L
bytes from Host A to Host B. Assume an MSS of 1,400 bytes.
a). What is the maximum value of L such that
TCP sequence numbers are not exhausted? Recall that the TCP
sequence number field has 4 bytes.
b). For the L you obtain in (a), find how
long it takes to transmit the file. Assume that a total of 60
bytes of transport, networwk and data-link header are added to
each segment before the resulting packet is sent out over a
10Mbps link. Ignore flow control and congestion control so A can
pump out the segments back to back and continuously.
6. TCP congestion control:
7. Consider the cross-country example shown in lecture notes Chapter3-part2.ppt (page 9-12). How big would the window size have to be for the channel utilization to be greater than 80 percent?