川大软件学院2012计算机网络_A闭_

1. Multiple Choice (30 points, 1.5 points for each question)

（1） Which of the following protocol layers is not explicitly part of the TCP/IP architecture?

A) application B) session C) data link D) Transport

（2） In the 1-persistent approach in CSMA, when a station finds an idle line, it _______.

A) sends immediately B) waits 0.1 s before sending

D) waits a time equal to (1 - p) seconds before sending C) waits 1 s before sending

（3） Transport-layer packet is called ______.

A) message B) segment C) datagram D) frame

（4） Among the following applications, which one is not suitable for P2P architecture.

A) le sharing B) video streaming C) instant message D) electronic banking

（5） A Web cache _______.

A) can help prevent DoS attacks.

B) is a network entity that guarantees anonymity of Internet traffic.

C) responds to HTTP requests on the behalf of a Web server.

D) makes use of cookies to reduce the response time for a client request.

（6） In TCP, the timeout interval is a function of ______:

A) estimated RTT at the sender

C) the size of buffer at the receiver B) MSS and the overhead of a segment D) the size of sending window

（7） In an Ethernet frame, the preamble is responsible for ______

A) collision detection

B) error detection

C) synchronization of the receiver’s clock to the sender’s clock.

D) multiplexing/ demultiplexing

（8） What is the main difference between stop-and-wait and pipelined reliable data transfer protocol?

A) The pipelined protocol uses the NAK packets, whereas in the stop-and-wait protocol

senders always wait for ACK packets.

B) With the pipelined protocol, the sender can send several packets in row, whereas in the

stop-and-wait protocol the sender cannot send the packets in row.

C) With the pipelined protocol, the receiver must send one ACK for several packets

(cumulative ACK), whereas in the stop-and-wait protocol the receiver can not send the cumulative ACK.

D) The pipelined protocol uses timeouts, whereas the stop-and-wait protocol does not use the

timeout.

（9） 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; TLD name server; local name server; authoritative name server.

B) authoritative name server; TLD name server; Root name server; local name server

C) TLD name server; Root name server; local name server; authoritative name server;

D) local name server; Root name server; TLD name server; authoritative name server;

（10） Which of the following nodes belongs to the network core? ______

A) a Web Server B) a Host with Win2003 Server

D) a Supernode on Skype Network C) a Router with NAT service

（11） When a user retrieve his email from mail server, which of following protocols can’t be used?

A) POP3 B) HTTP C) IMAP D) SMTP

（12） In the _______ random-access method collision is avoided.

A) CSMA/CD B) CSMA/CA C) ALOHA D) token-passing

（13） ____ is an interdomain routing protocol using path vector routing.

A) BGP B) RIP C) OSPF D) None of the choices are correct

（14） The use of hierarchy in routing tables can ________ the size of the routing tables.

A) reduce

B) increase D) None of the choices are correct C) neither reduce nor increase

（15） IP is a _________ protocol.

A) connection-oriented unreliable

C) connectionless unreliable B) connection-oriented reliable D) connectionless reliable

（16） During an FTP session the control connection is opened _______.

A) once B) twice C) many times D) none of the choices are correct

（17） DHCP is a (an) ___________ layer protocol.

A) application B) transport C) network D) data-link

（18） In a centralized P2P network, the directory system uses the ____ paradigm; the storing and

downloading of the files are done using ____ paradigm.

A) client-server; client-server

C) client-server; peer-to-peer

（19） RIP uses the services of _______.

A) TCP B) IP C) UDP D) None of the choices are correct

Ttrans B) peer-to-peer; client-server D) peer-to-peer; peer-to-peer （20） In the CSMA/CD protocol, what condition on the transmission delay

propagation delay prophas to be satis ed to guarantee that a node always detects a collision?

T TpropT 2TT 2TpropT TproppropA) trans B). trans C) trans D) .trans

2. Fill in the blank ( 15 points, 1.5 point for each blank )。

（1） Suppose host A sends host B one TCP segment with sequence number 800, acknowledgement

number 580, and 40 bytes of data. Then the sequence number in the acknowledgement to this segment is [ ① ]

（2） Let’s assume there is 8-bit piece data 11001001, and the CRC is applied to it with generator

1001. Thus the CRC bits should be [ ② ]

（3） The flag [ ③ ] in TCP segment header is used during the shutdown of a TCP connection.

（4） In a TCP connection , there is a timeout event when the value of threshold is 32 and the size of

congestion window is 16. According to the TCP congestion control policy, the new value of threshold should be [ ④ ] and the new size of congestion window should be [ ⑤ ].If there is a duplicate ACK event, the new value of the threshold should be [ ⑥ ] and the new size of congestion window should be [ ⑦ ].

（5） Suppose the IP address of a host is 192.168.5.121, and its subnet mask is 255.255.255.248, then

the netid of the host is [ ⑧ ], and the broad cast address of the network is [ ⑨ ]

（6） In CSMA/CD, after the 4th collision, the probability that a node chooses K=4 is [ ⑩ ]

3. True or False ( 10 points, 1 point for each statement )。

（1） Switches will decrease the TTL field in the IP header.

（2） In the 5-layer Internet reference model, network layer handles point-to-point functions while

transport layer handles end-to-end functions.

（3） Wireless networks can perform collision detection Tand the

（4） A drawback of distance vector routing algorithm is count-to-infinity problem.

（5） Congestion control reduces the transmission rate at the sender when the receiver is overloaded

（6） Network node means to end host or router or switch

（7） TCP waits until it has received two duplicate ACKs before performing a fast retransmit.

（8） When an Ethernet sender detects that the media is idle, it sends a jam signal onto the media to

tell other devices not to transmit, and then it sends its packet.

（9） If there are some errors in the routing tables at some routers, then, with IPv4, it is possible that

a packet loops forever.

（10） With the SR protocol, it is possible for the sender to receive an ACK for a packet that falls

outside of its current window.

4. Please answer the following questions briefly (20 points)

(1). [6 points] The following shown the processes between Host A and Host B to communicate to

transfer a mail message, answer the questions based on the conversation.

A: 220 beta.gov simple mail transfer service ready

B: HELO alpha.edu

A: 250 beta.gov

B: MAIL FROM: smith@alpha.edu

A: 250 mail accepted

B: RCPT TO: jones@beta.gov

A: 250 recipient accepted

B: RCPT TO: green@beta.gov

A: 250 no such user here

B: RCPT TO: brown@beta.gov

A: 250 recipient accepted

B: DATA

A: 354 start mail input; end with <CR><LF>.<CR><LF>

B: Date: Sat 12 May 2012 13:26:31

B: From: smith@alpha.edu

B: …….

B: ...…

B: .

A: 250 OK

B: QUIT

A: 221beta.gov service closing transmission channel.

QUESTIONS:

I. What is the name of the host sending mail? And what is the name of the user sending the mail?(2 points)

II. How many users to whom the sender wants to send mail? And what is the name for each user?(2 points)

III. What is the name of the host receiving the mail?(1 point)

IV. in order to receive the mail, what is the port number to which the receiving host is waiting for being connected?(1 point)

(2). [5 points] Suppose you open a startup company “starwar” and want to set up your company

network. Your network has the following servers:

DNS server: “” with IP as “128.119.12.40”

Web server: “” with two IP as “128.119.12.55” and “128.119.12.56”. The web server also has a name as “”.

Email server: “” with IP as “128.119.12.60”

Your company’s email address is “username@”.

QUESTIONS

I. What resource records (RRs) do you need to provide to the top-level domain “.com” Registrar? (2 points)

II. What RRs do you need to put in your company’s DNS server? (3 points))

(3). [5 points] A router has the following (CIDR) entries in its routing table

Address/mask Next Hop

135.46.56.0/22 interface 0

135.46.60.0/22 interface 1

192.53.40.0/23 router 1

Default router 2

For each of the following IP addresses, what does the router do if a packet with that address arrives?

I. 135.46.63.10

II. 135.46.57.14

III. 135.46.52.2

IV. 192.53.40.7

V. 192.53.56.7

(4). [4points]. Consider the BGP protocol, an autonomous system (AS) A, and some destination

network X. How does A control whether or not other autonomous systems route traffic destined to X through A?

5. Application (25 points)

(1). [13 points] Consider the network scenario shown below, assume the MAC address of the host is

00-15-c5-c1-5e-28, and its IP address is 10.2.128.100 (private address), figure 5-1 is the network topology, and figure 5-2 is the first 80 bytes of the frame which encapsulated the http request message when the host request a web page which is located in a web server somewhere in the

Internet, and the frame is presented in hexadecimal(十六进制) and ASCII. Figure 5-3 is the Ethernet frame structure and figure 5-4 is the format of the header of

IPv4 datagram.

Figure 5-1 Network Topology

Figure 5-2 the First 80 bytes of an Ethernet Frame

Figure 5-3 Ethernet Frame Structure

Figure 5-4 IPv4 Datagram Header

Answer the following questions based on figure 5-1 and 5-2

I. What is the IP address of the web server visited by the host? what is the MAC address of the default gateway(router) of the host? (4 points)

II. When the host constructs the frame shown in figure 5-2, what protocol was used to determine the destination MAC address? And what is the destination MAC address of the Ethernet frame which encapsulated the request of the protocol? (4 points)

III. Assume HTTP /1.1 uses persistent connections without pipelining, and there are 5 jpeg images referenced in rfc.html, how many RTTs will it take to obtain all the objects from sending the request message? (2 points)

IV. When router R forwarded the datagram encapsulated in the frame, which fields in the header of the datagram would be updated? (3 points)

(2). [5 points]Suppose that x bits of user data are to be transmitted over a k-hop path in a

packet-switched network as a series of packets. Each containing p data bits and h header bits, with x>>p+h, the transmission rate of the channel is b bps and the prorogation delay is negligible. What value of p minimizes the total delay?

(3). [7 points]Suppose a network runs RIP, the routing table in router B shown as below

Destination network distance next-hop

N1 7 A

N2 2 C

N6 8 F

N8 4 E

N9 4 F

NOW, Router B receives d-v vectors from Router C, shown as below

Destination network distance

N2 4

N3 8

N6 4

N8 3

N9 5

What is C’s new routing table?

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