CMPT 371 Summer 2022
Midterm
Q1. (8 marks) Consider the extended LAN connected using bridges B1 and B2 in the figure below
and suppose the hash tables in the two bridges are initially empty. For each bridge, list all ports on
which a packet will be forwarded for the following sequence of data transmissions:
i. E sends a packet to F.
ii. F sends a packet to E.
iii. D sends a packet to E.
iv. A sends a packet to F.
Transmission B1 ports B2 ports What B1 learns at the end What B2 learns at the end
forwarding forwarding of this transmission of this transmission
i 1,2,3 2,3,4 E is on port 4 E is on port 1
ii N/A 1 N/A F is on port 2
iii N/A N/A N/A D is on port 1
iv 2,3,4 2 A is on port 1 A is on port 4
Q2. (8 marks) While in transit, a 400-octet IP packet (20-octet header included) arrives at a
network with an MTU of 220 octets. Hence, the router responsible for that network fragments
the IP packet into fragments. Find:
a) the number of fragments
data length = 400-20=380 octets
maximum data that can be sent = 220-20=200 octets, which is already a multiple of 8 octets
(200=25×8)
380
number of fragments = 200 = 1.9, so we need 2 fragments.
b) for each fragment, what is:
i. the total length field (in octets)
ii. the offset field (in 8 octets)
iii. the more flag (binary)
Fragment total length (octets) Offset (8 octets) more flag
1 200+20=220 0 1
2 180+20=200 25 0
This study source was downloaded by 100000853497421 from CourseHero.com on 06-27-2023 03:05:52 GMT -05:00
https://www.coursehero.com/file/172910228/MidtermSolutionpdf/
Midterm
Q1. (8 marks) Consider the extended LAN connected using bridges B1 and B2 in the figure below
and suppose the hash tables in the two bridges are initially empty. For each bridge, list all ports on
which a packet will be forwarded for the following sequence of data transmissions:
i. E sends a packet to F.
ii. F sends a packet to E.
iii. D sends a packet to E.
iv. A sends a packet to F.
Transmission B1 ports B2 ports What B1 learns at the end What B2 learns at the end
forwarding forwarding of this transmission of this transmission
i 1,2,3 2,3,4 E is on port 4 E is on port 1
ii N/A 1 N/A F is on port 2
iii N/A N/A N/A D is on port 1
iv 2,3,4 2 A is on port 1 A is on port 4
Q2. (8 marks) While in transit, a 400-octet IP packet (20-octet header included) arrives at a
network with an MTU of 220 octets. Hence, the router responsible for that network fragments
the IP packet into fragments. Find:
a) the number of fragments
data length = 400-20=380 octets
maximum data that can be sent = 220-20=200 octets, which is already a multiple of 8 octets
(200=25×8)
380
number of fragments = 200 = 1.9, so we need 2 fragments.
b) for each fragment, what is:
i. the total length field (in octets)
ii. the offset field (in 8 octets)
iii. the more flag (binary)
Fragment total length (octets) Offset (8 octets) more flag
1 200+20=220 0 1
2 180+20=200 25 0
This study source was downloaded by 100000853497421 from CourseHero.com on 06-27-2023 03:05:52 GMT -05:00
https://www.coursehero.com/file/172910228/MidtermSolutionpdf/
