Telecom & Comp Ntwkg 2

Assignment#2 MMIS 653 Online (Winter 2013)

Total points: 70

Due date: 2/19/2013 11:59PM

Questions: (50 points)

1, (4 points) Consider a TCP connection between Host A and Host B. Suppose that the

TCP segments traveling from Host A to Host B have source port number x and

destination port number y. What are the source and destination port numbers for

segments traveling from Host B to Host A?

2, (4 points) Suppose a process in Host C has a UDP socket with port number 6789,

Suppose both Host A and Host B each send a UDP segment to Host C with destination

port number 6789. Will both of these segments be directed to the same socket at Host C?

If so, how will the process at Host C know that these two segments originated from two

different hosts?

3, (4 points) Suppose that a Web server runs in Host C on port 80. Suppose this Web

server uses persistent connections, and is currently receiving requests from two different

Hosts, A and B. Are all of the requests being sent through the same socket at Host C? If

they are being passing through different sockets, do both of the sockets have port 80?

Discuss and explain.

4, True or false? (10 points)

a, Host A is sending Host B a large file over a TCP connection. Assume Host B has no

data to send Host A. Host B will not send acknowledgements to Host A because Host B

cannot piggyback the acknowledgements on data.

b, The size of the TCP RcvWindow never changes throughout the duration of the

connection.

c, Suppose Host A is sending Host B a large file over a TCP connection. The number of

unacknowledged bytes that A sends cannot exceed the size of the receive buffer.

d, Suppose Host A is sending a large file to Host B over a TCP connection. If the

sequence number for a segment of this connection is m, then the sequence number for the

subsequent segment will necessarily be m+1.

e, The TCP segment has a field in its header for RcvWindow.

5, (4 points) Suppose Host A sends two TCP segments back to back to Host B over a

TCP connection. The first segment has sequence number 115; the second has sequence

number 190.

a, How much data is in the first segment?

b, Suppose that the first segment is lost but the second segment arrives at B. In the

acknowledgement that Host B sends to Host A, what will be the acknowledgement

number?

6, (8 points) UDP and TCP use 1s complement for their checksums. Suppose you have

the following three 8-bit bytes: 01010101, 01110000, 01001100. What is the 1s

complement of the sum of these 8-bit bytes? (Note that although UDP and TCP use 16-

bit words in computing the checksum, for this problem you are being asked to consider 8-

bit sums.) Show all work. Why is it that UDP takes the 1s complement of the sum; that is.

Why not just use the sum? With the 1s complement schema, how does the receiver detect

errors? Is it possible that a 1-bit error will go undetected? How about 2-bit error?

7, (16 points) Consider the following plot of TCP window size as a function of time.

Assume 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, Identify the intervals of time when Fast Recovery is operating.

d, After the 16 th

transmission round, is segment loss detected by a triple duplicate ACK

or by a timeout?

e, After the 22 nd

transmission round, is segment loss detected by a triple duplicate ACK

or by a timeout?

f, What is the value of Threshold at the 10th transmission round?

g, What is the value of Threshold at the 25th transmission round?

h, Assume a packet loss is detected after the 26 th

round by timeout event, what will be the

values of the congestion window size and of Threshold?

Wireshark Lab: TCP (20 points)

See Wireshark_TCP.pdf. Submit your answers to question 1-12 (except for question #7

and #11) in this lab.

Question #3: 0 point

Question #5: 4 points

All the rest: 2 points each