Description
Questions:
You are required to answer the following questions from the attached document:
Question Question
P.6 P.27
P.15 P.37
P.17 P.40
P.22 P.44
P.25
P6. Consider our motivation for correcting protocol rdt2.1. Show that the
receiver, shown in Figure 3.57, when operating with the sender shown in Figure 3.11, can lead the sender and receiver to enter into a deadlock state, where
each is waiting for an event that will never occur.
P15. Consider the cross-country example shown in Figure 3.17. How big would
the window size have to be for the channel utilization to be greater than 98
percent? Suppose that the size of a packet is 1,500 bytes, including both
header fields and data.
P17. Consider two network entities, A and B, which are connected by a perfect bidirectional channel (i.e., any message sent will be received correctly; the
channel will not corrupt, lose, or re-order packets). A and B are to deliver
data messages to each other in an alternating manner: First, A must deliver a
message to B, then B must deliver a message to A, then A must deliver a message to B and so on. If an entity is in a state where it should not attempt to
deliver a message to the other side, and there is an event like
rdt_send(data) call from above that attempts to pass data down for
transmission to the other side, this call from above can simply be ignored
with a call to rdt_unable_to_send(data), which informs the higher
layer that it is currently not able to send data. [Note: This simplifying
assumption is made so you don’t have to worry about buffering data.]
Draw a FSM specification for this protocol (one FSM for A, and one FSM for
B!). Note that you do not have to worry about a reliability mechanism here;
the main point of this question is to create a FSM specification that reflects
the synchronized behavior of the two entities. You should use the following
events and actions that have the same meaning as protocol rdt1.0 in
Figure 3.9: rdt_send(data), packet = make_pkt(data),
udt_send(packet), rdt_rcv(packet), extract
(packet,data), deliver_data(data). Make sure your protocol
reflects the strict alternation of sending between A and B. Also, make sure to
indicate the initial states for A and B in your FSM descriptions.
P22. Consider the GBN protocol with a sender window size of 4 and a sequence
number range of 1,024. Suppose that at time t, the next in-order packet that the
receiver is expecting has a sequence number of k. Assume that the medium
does not reorder messages. Answer the following questions:
a. What are the possible sets of sequence numbers inside the sender’s window at time t? Justify your answer.
b. What are all possible values of the ACK field in all possible messages currently propagating back to the sender at time t? Justify your answer.
P25. We have said that an application may choose UDP for a transport protocol
because UDP offers finer application control (than TCP) of what data is sent
in a segment and when.
a. Why does an application have more control of what data is sent in a segment?
b. Why does an application have more control on when the segment is sent?
P27. Host A and B are communicating over a TCP connection, and Host B has
already received from A all bytes up through byte 126. Suppose Host A then
sends two segments to Host B back-to-back. The first and second segments
contain 80 and 40 bytes of data, respectively. In the first segment, the
sequence number is 127, the source port number is 302, and the destination
port number is 80. Host B sends an acknowledgment whenever it receives a
segment from Host A.
a. In the second segment sent from Host A to B, what are the sequence number, source port number, and destination port number?
b. If the first segment arrives before the second segment, in the acknowledgment of the first arriving segment, what is the acknowledgment number,
the source port number, and the destination port number?
c. If the second segment arrives before the first segment, in the acknowledgment of the first arriving segment, what is the acknowledgment
number?
d. Suppose the two segments sent by A arrive in order at B. The first acknowledgment is lost and the second acknowledgment arrives after the first timeout interval. Draw a timing diagram, showing these segments and all other
segments and acknowledgments sent. (Assume there is no additional packet
loss.) For each segment in your figure, provide the sequence number and
the number of bytes of data; for each acknowledgment that you add, provide the acknowledgment number.
P37. Compare GBN, SR, and TCP (no delayed ACK). Assume that the timeout
values for all three protocols are sufficiently long such that 5 consecutive data
segments and their corresponding ACKs can be received (if not lost in the
channel) by the receiving host (Host B) and the sending host (Host A) respectively. Suppose Host A sends 5 data segments to Host B, and the 2nd segment
(sent from A) is lost. In the end, all 5 data segments have been correctly
received by Host B.
a. How many segments has Host A sent in total and how many ACKs has
Host B sent in total? What are their sequence numbers? Answer this question for all three protocols.
b. If the timeout values for all three protocol are much longer than 5 RTT,
then which protocol successfully delivers all five data segments in shortest
time interval?
P40. Consider Figure 3.58. Assuming TCP Reno is the protocol experiencing the
behavior shown above, answer the following questions. In all cases, you
should provide a short discussion justifying your answer.
a. Identify the intervals of time when TCP slow start is operating.
b. Identify the intervals of time when TCP congestion avoidance is
operating.
c. After the 16th transmission round, is segment loss detected by a triple
duplicate ACK or by a timeout?
d. After the 22nd transmission round, is segment loss detected by a triple
duplicate ACK or by a timeout?
e. What is the initial value of ssthresh at the first transmission round?
f. What is the value of ssthresh at the 18th transmission round?
g. What is the value of ssthresh at the 24th transmission round?
h. During what transmission round is the 70th segment sent?
i. Assuming a packet loss is detected after the 26th round by the receipt of a
triple duplicate ACK, what will be the values of the congestion window
size and of ssthresh?
0
0 2 4 6 8 10 12
Transmission round
14 16 18 20 22 24 26
5
10
15
20
25
Congestion window size (segments)
30
35
40
45
Figure 3.58 ! TCP window size as a function of time
j. Suppose TCP Tahoe is used (instead of TCP Reno), and assume that triple
duplicate ACKs are received at the 16th round. What are the ssthresh
and the congestion window size at the 19th round?
k. Again suppose TCP Tahoe is used, and there is a timeout event at 22nd
round. How many packets have been sent out from 17th round till 22nd
round, inclusive?
P44. Consider sending a large file 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 cwnd 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 cwnd increase from 6 MSS to 12 MSS (assuming no loss events)?
b. What is the average throughout (in terms of MSS and RTT) for this connection up through time = 6 RTT?
Submission Materials:
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