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2. (7 pts) Sequence numbers
Fill out all missing sequence numbers in the following timing diagram.
Host A
Host B
Sega100, 40 byles
Segz a, 20 bytes
Timeout Interval
ACK= b
Seq= d, e bytes
Timeout Interval
Sequf, 40 bytes
ACK= 9.
2= 140 6= 140 0= 160
d= 100
*= 40 # 160 8 200
3. (8 pts) Propagation, transmission, and queuing delays
Consider sending 3 packets back to back, each of length L bits, from Host A to Host B. Assume, as shown
in the figure below, that there is a Router between A and B. Suppose that the distance and the transmission
rate between Host A and the Router are d meters and RI bps, and those between the Router and Host B
are d meters and R2 bps. Suppose the speed of propagation is s meters per second. Ignore all processing
delays. Assume that all queues are of infinite capacity, and are all initially empty.
Host A
Host B
(a) (2 pts) Express the total delay Dj it takes to send the Ist packet from A to B as a function of L, d, s,
RI and R2.
Total delay D,
12 RX s


(2pts) Suppose Al - 12. Express the total delay Da it takes to send the three packets from A to B as
a function of L, d, s, RI and R2.
3 6 + 20
(c) (2 pts) Suppose RI > R2. Express the queueing delay Q2 the 2nd packet experiences at the Router as a
function of L, d, s, RI and R2.
I seconds
12 2
(d) (2 pts) Suppose RI > R2. Express the queueing delay Q3 the 3rd packet experiences at the Router as a
function of L, d, s, RI and R2.
2 4
12 2


4. (5 pts) TCP slow start
Consider sending a large file over a TCP connection that uses a link of R = 3.2 Mops. Suppose that this
connection is the only TCP connection that the link is supporting, and that this link is the only congested
link between the sender and the receiver. Assume that the round-trip time is 0.4 seconds (RTT=0.4 seconds),
the maximum segment size is 10000 bits (MSS-10000 bits), and the receiver's receive buffer is much larger
than the congestion window. Suppose that this TCP connection is in Slow-Start phase, and that the window
size is initially set to 1 segment.
(a) (1.5 pts) What is the maximum window size (in segments) that this TCP connection can achieve?
Wx Ass = 3.2 Mbps
W x 10000 = 3. 2 x 10 6
1 .S
0 .4
W = 120 xsegments
(b) (1.5 pts) How long does it take (in seconds) for the window to reach its maximum size? Recall that in
Slow-Start, the window size is doubled every round-trip time.
12 02 x ( 0. 4 ) =/ 2 4 seconds
(c) Suppose when the maximum window size is reached, a 'timeout event' occurs.
i. (1 pt) What will be the value of the congestion window if TCP Tahoe is used?
The value is doubled if TCP Tahoe is used
Cong wind = 1 segment.
ii. (1 pt) What will be the value of the congestion window if TCP Reno is used?
The value is halfed if TCP Reno is use


that you find easy.
1. (10 pts) Short-Answer Questions.
(a) (2 pts) Name the 5 layers of the 5-layer Internet Protocol Stack.
i) application it) Transport
in) Network iv) Physical v) link
(b) (2 points) List two differences between packet-switching and circuit-switching.
Difference 1: In packet switching directly data transfer takes place
Difference 2: In circuit - Switching Jata is processed at source system only.
(c) (2 pts) What does 'store-and-forward' mean? Explain in one sentence.
packet must
arrive at the router before it can
be transmitted on next link
(d) (2 pts) Recall that EstimatedRTT(") = (1 - o)EstimatedRTT("-1) + aSampleRTT("). What problem will
we have when a is too close to 1? Explain?
Transient fluctuations / changes in the network load affects
the Estimated RFT and it makes it unstable when it
should not when of is too close to q.
(e) (2 pts) What does TCP Reno use as a sign of congestion?
- 3 Jup Acks indicate network
still capable of
delieviring some segments after loss.
- Timeout indicates more alarming congestion scenario.

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