课程名称︰计算机网络
课程性质︰资讯大三必修
课程教师︰周承复
开课学院：电资
开课系所︰资讯
考试日期（年月日）︰103/11/13
考试时限（分钟）：120
是否需发放奖励金：Y
（如未明确表示，则不予发放）
试题 :
Question 1: "Quickies"
Answer each of the following questions briefly, i.e. in at most a few sentences
a) (10%) What does it mean for a protocol to be stateful? What does it mean for
a protocol to be stateless? Give an example one stateful protocol and
one stateless protocol.
b) (10%) What does it mean when we say that control messages are "in-band"?
What does it ean when we say that control messages are "out-of-band"?
Give an example of a protocol that has in-bond control messages and
one example of a protocol that has out-of-bond control messages.
c) (10%) List four factors that contribute to the end-to-end delay in a
packet-switched network. Which of these are constant and which of
these depend on the load in the network?
d) (10%) Suppose you would like to urgently deliver 20 terabytes data from
Boston to Los Angeles. You have available a 500 Mbps dedicated link
for data transfer.
Would you prefer to transmit the data via this link or instead use
FedEx overnight delivery? Explain.
e) (10%) Suppose a web server has 399 ongoing TCP connection. How many
server-side sockets are used? How many server-side port number are
used? Briefly(two sentences at most each)explain your answer.
f) (10%) For each question, please circle a single best answer.
(1) What are three common HTTP 1.1 message method fields?
(a) GET HTML POST
(b) GET PUT HTML
(c) GET UPLOAD HTML
(d) GET POST PUT
(e) none of the above
(2) Given that the requested information is not available at any
intermediate databases, a purely recursive DNS query from a requesting
host would follow the path:
(a) Root name server, local name server, authoritative name
server
(b) authoritative name server, root name server, host name
server
(c) local name server, root name server, local name server,
authoritative name server
(d) local name server, root name server, TLD name serve,
authoritative name server.
(e) none of the above.
Question 2: "DHT"
(a) ( 5%) What are two fundamental advantages of peer-to-peer(P2P) systems over
client-server systems?
(b) (20%) Consider a circular Distributed Hash Table(DHT) with node identifiers
in the range[0;15]. Suppose there are seven peers with identifiers
1,3,6,9,12,14 and 15.
(i) Suppose that the following(key,value) pairs should be stored in the
DHT: (2,0),(6,5),(7,13)and(15,7). Which peers will store which
(key,value) pairs? Fill in the table below.
(key,value) Identifier of the responsible peer
( 2, 0)
( 6, 5)
( 7,13)
(15, 7)
(ii) Suppose that peer 6 learns that peer 9 has left the DHT. How does
peer 6 updates its successor state information? Which peer is now its
first successor? Its second successor?
(c) (10%) Explain how DHT improves search performance from O(n) to O(log n),
where n is the total number hosts in DHT method.
Question 3: "Homework"
(a) (10%) Suppose users share a 3Mbps link. Also suppose each user requires
150kbps when transmitting,but each user transmits only 20 percent of
the time. Suppose there are 100 users. Find the probability that there
are 21 or more users transmitting simultaneously.(Write the
equation only)
(b) (15%) Consider sending a large file of F bits from Host A to Host B. As
shown in the following diagram, there are four links and three
switches between A and B, and the links are uncongested(that is,
no queuing delays). Host A segments the file into segments of
S bits each and adds 95 bits of header to each segment forming packets
of L = 95 + S bits. Each links has a transmission rate of R bps. Find
the value of S that minimizes the delay of moving the file from Host A
to Host B.
Disregard propagation delay.