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課程名稱︰計算機網路
課程性質︰系必修
課程教師︰逄愛君
開課學院：電資學院
開課系所︰資工系
考試日期（年月日）︰2010.6.23
考試時限（分鐘）：180
是否需發放獎勵金：是
（如未明確表示，則不予發放）
試題 :
Question 1:"Quickies"(30%) Answer each of the following questions briefly,
e.g., in at most a few sentences.
a)(5%)Where can queueing occur in a router? Briefly explain the conditions that
lead to such queueing.
Queueing can occur at both the input ports and the output ports of a
router. Queueing occurs at the ourpur port when the arriving rate of
packets to the outgoing link exceeds the link capacity. Queue occurs on
and input port when the arriving rate of packets exceeds the switch
capacity; head-of-the-line blocking can also cause queueing at the
input ports.
b)(5%)What is HOL (Head-of-Line) blocking? Does it occur in input ports or
ouput oirts?
The switch fabric can only switch the packets at the head of the buffer
per clock, HOL arises in input ports when packets arriving at different
input ports are destined for the same output port. If the HOL packet
of a certain buffer at the input cannot be switched to an output port
because of contention, the rest of the packets in that buffer are
blocked by that Kead-of-Line packet, even if there is no contention
at the destination output ports for those packets.
c)(5%)What is meant by the term "tunneling"?
In tunneling, an IP datagram (e.g., an Ipv6 datagram) is carried as the
patkiad if another IP datagram.
d)(5%)Briefly describe TCP's slowstart algorithm. What causes the TCP slowstart
algorithm to end, and TCP congestion avoidance begin?
When connection begins, TCP's congestion window is 1 MSS. For each
ACK received, the congestion window size is double. This causes the
number of segments sent per RTT to increase exponentially. Slowstart
ends when a threshold window size is reached.
e)(5%)Two hosts simultaneously send dat through a link of capacity 1Mbps. Host
A generates data with a rate of 1Mbps and uses TCP. Host B uses UDP and
transmits a 100bytes packet every 1ms Which host will ontain higher
throughput?
f)(5%)What are different inter-AS and intra-AS protocols used in the Internet
g)(5%)What is the difference between a group-shared tree and a source-based
tree in the context of multicast routing?
Problem 2: Congestion Control and TCP (40%)
a)(5%)What is the difference between congestion control and flow control?
Flow control is about matching the speed of a sender to the
capabilities of the receiver.
Congestion occurs when senders overutilize the resources within the
network.
b)(5%)It is said that a TCP connection "probes" the network path it uses for
available bandwidth. What is meant by that?
TCP keeps increasing its send rate (by inceasing its window size)
until loss occurs (at which point congestion has set in).
TCP then sets its rate lower but again begins increasing its send
rate to again determine the point at which congestion sets in.
In this sense, TCP is contantly
probing the network to see how much bandwidth it can use.
c)Suppose that in TCP, the sender window is of size N, the base of the window
is at sequence number x, and that the sender has just sent a complete
window's worth of segments. Let RTT be the sender-to-receiver-to-sender
round trip time, and let MSS be the segment size.
i.(10%)Is it possible that there are ACK segments in the receiver-to-
sender channel for segments with sequence numbers lower than x?
Justigy your answr.
ii.(5%)Assuming no loss, what is the throughput(in packets/sec) of
the sender-to-receiver connection?
iii.(5%)Suppose TCP is in its congestion avoidance phase. Assuming no
loss, what will the window size be after the N segments are ACKed?
N+1
d)(10%)Consider the use of TCP over a wireless channel, which is much more
prone to bit-level errors than a wire or optical fiber. Comment on how well
suited TCP's(i)error detection mechanism, and (ii)its window-based congestion
control algorithm are for such a bit-error-prone environment.
Question 3: Routing Algorithms(20%)
a)(10%)Consider the network shown below. Show the operation of Dijkstra's
(Link State) algorithm for computing the least cost path from D to all
destinations.
b)(10%)Consider a node Z, which has only two beighbors -X and Y. The link cost
from Z to X is 2 and the link cost from Z to Y is 3. Suppose X and Y have
the distance tables shown below, which they send to Z.
Complete the following table in node Z after it receives the destance tables
from its neighbors X and Y:
Question 4:NAT:Network Address Translation(15%)
a)(5%)Suppose host A is behind a NAT, and host B has a public IP. The TCP
connection between host A and B has been established. While host A sends
datagram to host B and host B replies datagram to host A, what does the NAT
router do?
b)(10%)NATs on P2P applications: Suppose a peer A discovers through quety that
peer B has a file it wants to download. Also, suppose that A and B are both
behind a basic NAT which does not support any other fuctions. Is there a
way to allow A to establish a TCP connection with B directly without
application-specific NAT configuration? Discuss why and how to do it
(if it is possible).
Question 5:Subnets(10%)
Considr the topology shown below. Denote the three subnets with hosts
(starting clockwise at 12:00) as Networks A, B, and C. Denote the subnets
without hosts as Networks D, E, and F.
(a)Assign network addresses to each of these six subnets, with the following
constraints: All addresses must be allocted from 214.97.254/17; Subnet A
should have enough addresses to support 250 interfaces; Subnet B should have
enough addresses to support 120 interfaces; and Subnet C should have enough
addresses to support 120 interfaces. Of courese, subnets D, E and F should
each be able to support two interfaces. For each subnet, the assignment
should each be able to support two interfaces. For each subnet, the
assignment should take the form a.b.c.d/x or a.b.c.d/x - e.f.g.h/y.
(b)Using your answer of part (a), provide the forwarding tables (using
longest prefix matching) for each of the three routers.
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