COSC 6377 Spring 2002 Midterm

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COSC 6377 Spring 2002Midterm 2 (With Solution)

• Student Name
• Student ID
• Student Signature

2

• Assume the flag used in HDLC consists of the byte
  10000001. Bit stuffing is used to prevent the
  occurrence of the flag inside the HDLC frame.
  The transmitter examines the contents inside the
  frame and inserts an extra 1 after each instance
  of five consecutive 0s.
• (a) (3 points) Show the binary sequence after
  performing the bit stuffing procedure described
  above for the following binary sequence
• 1000001000000000000001
• Answer
• 1000001100000100000100001
• (b) (2 points) Assume that the bit stuff
  described above is used by the transmitter. Show
  the binary sequence after the receive performs
  the bit destuffing procedure to the following
  binary sequence generated by the transmitter
• 0000011000000100000100000
• Answer
• 00000100000010000000000

3

• 2. An virtual-circuit packet-switching network
  and its routing tables are provided below.
• (2.5 points)
• Which host will be reached by a packet sent by
  host A with virtual circuit identifier (VCI) 2?
• Answer Host D
• (2.5 points)
• Which host will be reached by a packet sent by
  host E with virtual circuit identifier (VCI) 1?
• Answer Host B

D
C
B
2
Port 2
2
Port 1
1
3
3
1
Port 3
S1
S3
S2
A
E
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• (10 points)
• Consider a packet-by-packet fair queuing system
  with three logical queues and with service rate
  of two units per second. Show the sequence of
  transmission for this system for the following
  packet arrival pattern.
• Queue 1 packet 1 of length 2 arrives at time t
  0 packet 2 of length 1 arrives at t 4
• Queue 2 packet 1 of length 8 arrives at time t
  1 packet 2 of length 1 arrives at t 2
• Queue 3 packet 1 of length 1 arrives at time t
  3 packet 2 of length 5 arrives at t 4
• Calculate the order of transmission.
• Answer

5

• Use the TCP state transition diagram provided in
  the next page to answer the following questions
• (a) (5 points)
• A TCP connection is in the ESTABLISHED state.
  The application sends a Close message. After
  the Close message is successfully processed, what
  is the clients state?
• Answer FIN WAIT-1
• And what is the clients action?
• Answer FIN to server TCP
• (b) (3 points)
• In the TIME-WAIT state, Timer is used to guard
  the consequences caused by the ACK segment not
  received correctly by (Select all correct ones)
• (a) Client TCP
• (b) Server TCP
• (c) Both client TCP and Server TCP
• Answer (b)
• (c) (2 points)
• Server TCP is in the SYS-RCVD state. When the
  server TCPs retransmission timer timed out.
  What is the action?
• Answer The server TCP will send a Reset (RST)
  segment to the client.
• What is the consequence of this action on the
  client TCP?

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BF Figure 12-24 State Transition Diagram
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5. The state transition diagram of
point-to-point protocol (PPP) is provided below.
(a) (2.5 points) If one end of the PPP
connection sends an LCP packet with code 0216,
what is the state of the connection after this
event? Answer Note to TA The answer can be
either Authenticating state or Networking
state. (b) (2.5 points) When an initiator of
the PPP connection sends a LCP packet with code
0D16 to the responder, what is the code of the
packet that responders will send back to the
initiator? Answer 0716
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• 6. (5 points)
• The frame delimiter used by the point-to-point
  protocol (PPP) consists of the byte 0x7E
  (01111110). PPP uses the byte stuffing method to
  prevent the occurrence of the frame delimiter
  inside the PPP frame. The transmitter examines
  the contents inside the frame and performs the
  following replacements (note 0x7D is the escape
  character)
• - Replace 0x7E by 0x7D 0x5E
• - Replace 0x7D by 0x7D 0x5D
• The following byte sequence is generated by the
  transmitter after the transmitter performed the
  byte stuffing method to a PPP frame
• 0x7D 0x5E 0x7D 0x5E 0x5E 0x24 0x7D 0x5D 0x7D
  0x5D 0x5D 0x7D 0x5E
• Show the byte sequence after the receive
  performs the byte destuffing method to the above
  byte sequence.
• Answer
• 0x7E 0x7E 0x5E 0x24 0x7D 0x7D 0x5D 0x7E

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• (5 points)
• A router with IP address 195.5.2.12 and Ethernet
  physical address AA25AB1F67CD has received a
  packet for a destination with IP address
  185.11.78.10.
• When the router checks its routing table, it
  finds out the packet should be delivered to a
  router with IP address 199.5.2.6.
• Complete the entries in the following ARP
  request packet sent by the router. Assume there
  is no subnetting.
• Answer

16 bits
16 bits
10

• 8. Given the topology in the following figure,
  and the following specifications
• Bridge B1s ID represented by its MAC address
  (in HEX) is 0C00C8110000
• Bridge B2s ID represented by its MAC address
  (in HEX) is 0C00C8222222
• Bridge B3s ID represented by its MAC address
  (in HEX) is 0C00C8111111
• Costs assigned to each LAN are assumed to be
  equal.
• Please answer questions 8(a), 8(b), 8(c), and
  8(d) on the next page based on the spanning tree
  algorithm.

Port 0 of B2
Port 1 of B2
LAN 2
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• 8. (a) (2.5 points)
• Which bridge will be chosen as the root bridge?
• Answer
• B1
• 8. (b) (2.5 points)
• Which are the root ports of bridge B2 and bridge
  B3 respectively?
• Answer
• The root port of bridge B2 port 0
• The root port of bridge B3 port 0
• 8. (c) (2.5 points)
• For LAN2 , which bridge is the designated
  bridge, and which port of the designated bridge
  is the designated port?
• Answer
• B3/port 1
• 8. (d) (2.5 points)
• Which port of which bridge will be in blocking
  state?
• Answer
• B2/port 1

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• 9. (5 points)
• The IPV4 strict source route option is provided
  in the next page for easy reference.
• In the figure below, Host A wants to send a
  datagram to Host B using a specific route A to
  R1 to R2 to B
• Please complete the strict source route option
  fields at R1 and R2.
• Answer

138.6.25.40
142.6.25.42
140.6.25.40
138.6.25.42
142.6.25.40
140.6.25.42
138.6.0.0
140.6.0.0
142.6.0.0
A
R2
B
R1
R1
R2
137
11
4
137
137
11
11
8
12
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BF Figure 7-15 Strict source route option
Note The pointer is the pointer into the route
data. It indicates the byte which begins the
next source address to be processed. The pointer
is relative to this potion, and the smallest
legal value for the pointer is 4.
4 bytes
1 byte
1 byte
1 byte
1 byte
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• (5 points)
• Router R1 receives an Ethernet frame generated
  by Host A on Network 1. The frame carries 1400
  bytes of data.
• Network 2s MTU is 620 bytes consisting of 20
  bytes of header and 600 bytes of data.
• R1 performs the fragmentation described below
  Fragment 1 consist of 600 bytes of data Fragment
  2 consists of 600 bytes of data Fragment 3
  consists of 200 bytes of data. Please complete
  the following table
• Answer

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• 11 (a) (2 point)
• ICMP type 3 means destination unreachable.
  Under type 3, different codes are defined
• Code 0 network unreachable
• Code 1 host unreachable
• Code 2 protocol unreachable
• Code 3 port unreachable
• Code 4 fragmentation needed and DF flag (do
  not fragment) has been set
• Code 5 source routing failed
• Which codes can be generated from a host? And
  which codes can be generated from a router?
• Answer
• Codes that can be generated from a host are
  Code 2, and Code 3
• Codes that can be generated from a router are
  Codes 0,1,4,5
• 11 (b) (3 points)
• Host A sends an ICMP timestamp request to Host
  B. Host A receives the corresponding ICMP
  timestamp reply from Host B at 13,567,000
  milliseconds.
• The values of the original timestamp field, the
  receive timestamp field, and the transmit
  timestamp field are 13.560,000, 13,562,000, and
  13,564,300 milliseconds respectively. What is
  the sending trip time? What is the receiving trip
  time? And what is the round-trip time?
• Answer
• Sending trip time 13,562,000 13,560,000
  2,000 milliseconds
• Receiving trip time 13,567,000 13,564,300
  2,700 milliseconds
• Round-trip time 2,000 2,700 4,700
  milliseconds

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• 12 (a) (5 points)
• A network administrator has a class C IP
  address. Based on the requirements of his TCP/IP
  network, he realizes that he must create 5
  subnets, and each subnet needs to have up to 25
  host IDs. Which subnet masks should he choose
  for his network? (Choose the correct one)
• a. 255.255.255.224 (corresponding binary
  representation 11111111.11111111.11111111.11100000
  )
• b. 255.255.255.240 (corresponding binary
  representation 11111111.11111111.11111111.11110000
  )
• c. 255.255.255.248 (corresponding binary
  representation 11111111.11111111.11111111.11111000
  )
• d. 255.255.255.252 (corresponding binary
  representation 11111111.11111111.11111111.11111100
  )
• Answer a
• (b) (5 points)
• Subnets are created using the subnet mask
  255.255.255.252 (corresponding binary
  representation 11111111.11111111.11111111.11111100
  ) for a class C network 221.45.71.0
  (corresponding binary representation 11011101
  00101101 01000111 00000000).
• Network 221.45.71.0 receives a packet with the
  following destination IP address 221.45.71.15
  (corresponding binary representation 11011101
  00101101 01000111 00001111). The destination IP
  address is a direct broadcast address for a
  subnet. Which host Ids on the subnet will
  receive the packet? (Select all correct ones)
• a. 221.45.71.9 (binary representation 11011101
  00101101 01000111 00001001)
• b. 221.45.71.14 (binary representation 11011101
  00101101 01000111 00001110)
• c. 221.45.71.13 (binary representation 11011101
  00101101 01000111 00001101)
• d. 221.45.71.6 (binary representation
  11011101 00101101 01000111 00000110)
• Answer b,c

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• 13 (a) (3 points)
• Select all correct statements related to
  pseudoheader
• a. Pseudoheader is transmitted as part of the
  TCP datagram because TCP is a connection-oriented
  protocol while UDP pseudoheader is not
  transmitted as part of the UDP datagram because
  UDP is a connectionless protocol.
• b. UDP length field is included twice in the UDP
  checksum calculation while TCP length field is
  included only once in the TCP checksum
  calculation.
• c. For TCP datagram, pseudoheader fields include
  fields from the IP header.
• Answer b,c
• (b) (2 points)
• The time when Host A receives a TCP
  acknowledgement from Host B is 13,800 seconds.
• The value in the timestamp field of the
  acknowledgements timestamp option is 13,780
  seconds.
• The value in the timestamp echo reply field of
  of the acknowledgements timestamp option is
  13,770 seconds.
• What is the round-trip time based on the
  information provided above?
• Answer 13,800 seconds 13,770 seconds 30
  seconds.

BF Figure 12-11Timestamp option
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• 14. (a) (1 points) A wide area network (WAN) uses
  the OSPF protocol that connects two routers. What
  type of OSPF link is it?
• a. Stub link
• b. Point-to-point link
• c. Transient link
• Answer b
• (b) (1 points) An Ethernet LAN uses the OSPF
  protocol with five attached routers. What type
  of OSPF link is it?
• a. Stub link
• b. Point-to-point link
• c. Transient link
• Answer c
• (c) (1 point) Which one of the following two
  routing protocols uses reliable flooding to
  exchange information among routers?
• a. RIP
• b. OSPF
• Answer b
• (d) (1 point) Which one of the following two
  routing protocols is more suitable for large
  networks?
• a. RIP
• b. OSPF
• Answer b
• (e) (1 point) RIP uses hop counts as a
  criteria to select routing path while OSPF uses
  cost metric as a criteria to select routing
  path. Which criteria is more flexible?s

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• (5 points)
• A router has the following RIP routing table
• What would be the contents of the table if the
  router receives the following RIP message from
  router C before necessary increments
• Answer

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RIP Updating Algorithm
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• (10 points)
• The leaky bucket algorithm shown in the next
  page assumes that the leaky bucket drains at a
  continuous rate of 1 packet per packet time. In
  order to apply the leaky bucket algorithm to the
  problem described below, one block of the leaky
  bucket algorithm needs to be modified.
• Given I 4, L 6, and the arrival times of
  packets, complete the following table using the
  leaky bucket algorithm.
• Note
• 1.The unit of all numbers in this problem is
  packet time
• 2. Packets are assumed to be of fixed length (I
  4)
• 3. The leaky bucket will drain at a continuous
  rate of 2 packets per packet time

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LG Figure 7.54 Leaky bucket algorithm used for
policing
The parameters are defined below ta packet
arrival time LCT last conformance time X value
of leaky bucket counter X auxiliary variable I
packet length LI Depth of the bucket
Arrival of a packet at time ta
X X - (ta - LCT)
Yes
X lt 0?
No
X 0
Yes
Nonconforming
X gt L?
packet
No
X X I
LCT ta
conforming packet