COMPUTER NETWORKING EXAM 2025
QUESTIONS AND ANSWERS
R1. Suppose the network layer provides the following service. The network layer in the source
host accepts a segment of maximum size 1,200 bytes and a destination host address from the
transport layer. The network layer then guarantees to deliver the segment to the transport
layer at the destination host. Suppose many network application processes can be running at
the destination host.
a. Design the simplest possible transport-layer protocol that will get application data to the
desired process at the destination host. Assume the operating system in the destination host
has assigned a 4-byte port number to each running application process.
b. Modify this protocol so that it provides a "return address" to the destination process.
c. In your protocols, does the transport layer "have to do anything" in the core of the computer
network? - ANS a)
4 Byte Destination Address
4 Byte Destination Port Number 1196 Bytes of Data
The network layer in the source host accepts a segment of max size of 1200 bytes and a
destination host address (4 bytes) from the transport layer. So 1200 Bytes - 4 Byte Port Number
is 1196 Bytes of data per segment.
b)
4 Byte Destination Address
4 Byte Destination Port Number
4 Byte Source Address
4 Byte Source Port Number
1188 Bytes of Data
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,1200 Bytes - 4 Byte Port Number - 4 Byte Source Address - 4 Byte Source Port Number = 1188
Bytes of Data per segment.
c)
Not by itself, but routers and switches read the transport layer header in order to deliver the
data segment properly to the destination host and application.
R2. Consider a planet where everyone belongs to a family of six, every family lives in its own
house, each house has a unique address, and each person in a given house has a unique name.
Suppose this planet has a mail service that delivers letters from source house to destination
house. The mail service requires that (1) the letter be in an envelope, and that (2) the address
of the destination house (and nothing more) be clearly written on the envelope. Suppose each
family has a delegate family member who collects and distributes letters for the other family
members. The letters do not necessarily provide any indication of the recipients of the letters.
a. Using the solution to Problem R1 above as inspiration, describe a protocol that the delegates
can use to deliver letters from a sending family member to a receiving family member.
b. In your protocol, does the mail service ever have to open the envelope and examine -
ANS a)
This first strategy depends on a hierarchical structure. Divide the planet into regions, the
regions into sub-regions, and keep dividing until you get to the house level. One of the family
members delegates to the rest, and there is a person responsible for delivering mail to and
from each sub-region within their region.
The second strategy is a distributed structure. Give each delegate a radius to operate in. Make
the address on the envelope a GPS coordinate. Then have each delegate pass an envelope the
neighbor delegate that is closest to the destination address.
b)
No, the only required information for delivery is written in big letters on the envelope whereas
the letters contain no identifying information. In fact, opening the envelope to examine the
letters will most likely destroy this information.
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,R3. Consider a TCP connection between Host A and Host B. Suppose that the TCP segments
traveling from Host A to Host B have source port number x and destination port number y.
What are the source and destination port numbers for the segments traveling from Host B to
Host A? - ANS Source port number y and destination port number x.
[This is The Reverse As Above]
R4. Describe why an application developer might choose to run an application over UDP rather
than TCP. - ANS An application developer may not want its application to use TCP's
congestion control, which can throttle the application's sending rate at times of congestion.
Often, designers of IP telephony and IP videoconference applications choose to run their
applications over UDP because they want to avoid TCP's congestion control. Also, some
applications do not need the reliable data transfer provided by TCP.
R5. Why is it that voice and video traffic is often sent over TCP rather than UDP in today's
Internet? (Hint: The answer we are looking for has nothing to do with TCP's congestion-control
mechanism.) - ANS Since most firewalls are configured to block UDP traffic, using TCP for
video and voice traffic lets the traffic though the firewalls
R6. Is it possible for an application to enjoy reliable data transfer even when the application
runs over UDP? If so, how? - ANS Yes. The application developer can put reliable data
transfer into the application layer protocol. This would require a significant amount of work and
debugging, however.
R7. Suppose a process in Host C has a UDP socket with port number 6789. Suppose both Host A
and Host B each send a UDP segment to Host C with destination port number 6789. Will both of
these segments be directed to the same socket at Host C? If so, how will the process at Host C
know that these two segments originated from two different hosts? - ANS Yes, both
segments will be directed to the same socket. For each received segment, at the socket
interface, the operating system will provide the process with the IP addresses to determine the
origins of the individual segments.
R8. Suppose that a Web server runs in Host C on port 80. Suppose this Web server uses
persistent connections, and is currently receiving requests from two different Hosts, A and B.
Are all of the requests being sent through the same socket at Host C? If they are being passed
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, through different sockets, do both of the sockets have port 80? Discuss and explain. -
ANS For each persistent connection, Web server creates a separate connection socket. Each
connection socket is identified with a four-tuple: (source IP address, source port number,
destination IP address, destination port number). When host C receives an IP datagram, it
examines these four fields in the datagram/segment to determine to which socket it should
pass the payload of the TCP segment. Thus, the requests from A and B pass through different
sockets. The identifier for both of these sockets has 80 for the destination port; however, the
identifiers for these sockets have different values for source IP addresses. Unlike UDP, when
the transport layer passes a TCP segment's payload to the application process, it does not
specify the source IP address, as this is implicitly specified by the socket identifier.
R9. In our rdt protocols, why did we need to introduce sequence numbers? - ANS Sequence
numbers are required for a receiver to find out whether an arriving packet contains new data or
is a retransmission.
R10. In our rdt protocols, why did we need to introduce timers? - ANS To handle losses in the
channel. If the ACK for a transmitted packet is not received within the duration of the timer for
the packet, the packet (or its ACK or NACK) is assumed to have been lost. Hence, the packet is
retransmitted.
R11. Suppose that the roundtrip delay between sender and receiver is constant and known to
the sender. Would a timer still be necessary in protocol rdt 3.0, assuming that packets can be
lost? Explain. - ANS A timer would still be necessary in the protocol rdt 3.0. If the round trip
time is known then the only advantage will be that, the sender knows for sure that either the
packet or the ACK (or NACK) for the packet has been lost, as compared to the real scenario,
where the ACK (or NACK) might still be on the way to the sender, after the timer expires.
However, to detect the loss, for each packet, a timer of constant duration will still be necessary
at the sender.
R12. Visit the Go-Back-N Java applet at the companion Web site.
a. Have the source send five packets, and then pause the animation before any of the five
packets reach the destination. Then kill the first packet and resume the animation. Describe
what happens.
b. Repeat the experiment, but now let the first packet reach the destination and kill the first
acknowledgment. Describe again what happens.
Copyright ©2025 BRIGHTSTARS ALL RIGHTS RESERVED 4
QUESTIONS AND ANSWERS
R1. Suppose the network layer provides the following service. The network layer in the source
host accepts a segment of maximum size 1,200 bytes and a destination host address from the
transport layer. The network layer then guarantees to deliver the segment to the transport
layer at the destination host. Suppose many network application processes can be running at
the destination host.
a. Design the simplest possible transport-layer protocol that will get application data to the
desired process at the destination host. Assume the operating system in the destination host
has assigned a 4-byte port number to each running application process.
b. Modify this protocol so that it provides a "return address" to the destination process.
c. In your protocols, does the transport layer "have to do anything" in the core of the computer
network? - ANS a)
4 Byte Destination Address
4 Byte Destination Port Number 1196 Bytes of Data
The network layer in the source host accepts a segment of max size of 1200 bytes and a
destination host address (4 bytes) from the transport layer. So 1200 Bytes - 4 Byte Port Number
is 1196 Bytes of data per segment.
b)
4 Byte Destination Address
4 Byte Destination Port Number
4 Byte Source Address
4 Byte Source Port Number
1188 Bytes of Data
Copyright ©2025 BRIGHTSTARS ALL RIGHTS RESERVED 1
,1200 Bytes - 4 Byte Port Number - 4 Byte Source Address - 4 Byte Source Port Number = 1188
Bytes of Data per segment.
c)
Not by itself, but routers and switches read the transport layer header in order to deliver the
data segment properly to the destination host and application.
R2. Consider a planet where everyone belongs to a family of six, every family lives in its own
house, each house has a unique address, and each person in a given house has a unique name.
Suppose this planet has a mail service that delivers letters from source house to destination
house. The mail service requires that (1) the letter be in an envelope, and that (2) the address
of the destination house (and nothing more) be clearly written on the envelope. Suppose each
family has a delegate family member who collects and distributes letters for the other family
members. The letters do not necessarily provide any indication of the recipients of the letters.
a. Using the solution to Problem R1 above as inspiration, describe a protocol that the delegates
can use to deliver letters from a sending family member to a receiving family member.
b. In your protocol, does the mail service ever have to open the envelope and examine -
ANS a)
This first strategy depends on a hierarchical structure. Divide the planet into regions, the
regions into sub-regions, and keep dividing until you get to the house level. One of the family
members delegates to the rest, and there is a person responsible for delivering mail to and
from each sub-region within their region.
The second strategy is a distributed structure. Give each delegate a radius to operate in. Make
the address on the envelope a GPS coordinate. Then have each delegate pass an envelope the
neighbor delegate that is closest to the destination address.
b)
No, the only required information for delivery is written in big letters on the envelope whereas
the letters contain no identifying information. In fact, opening the envelope to examine the
letters will most likely destroy this information.
Copyright ©2025 BRIGHTSTARS ALL RIGHTS RESERVED 2
,R3. Consider a TCP connection between Host A and Host B. Suppose that the TCP segments
traveling from Host A to Host B have source port number x and destination port number y.
What are the source and destination port numbers for the segments traveling from Host B to
Host A? - ANS Source port number y and destination port number x.
[This is The Reverse As Above]
R4. Describe why an application developer might choose to run an application over UDP rather
than TCP. - ANS An application developer may not want its application to use TCP's
congestion control, which can throttle the application's sending rate at times of congestion.
Often, designers of IP telephony and IP videoconference applications choose to run their
applications over UDP because they want to avoid TCP's congestion control. Also, some
applications do not need the reliable data transfer provided by TCP.
R5. Why is it that voice and video traffic is often sent over TCP rather than UDP in today's
Internet? (Hint: The answer we are looking for has nothing to do with TCP's congestion-control
mechanism.) - ANS Since most firewalls are configured to block UDP traffic, using TCP for
video and voice traffic lets the traffic though the firewalls
R6. Is it possible for an application to enjoy reliable data transfer even when the application
runs over UDP? If so, how? - ANS Yes. The application developer can put reliable data
transfer into the application layer protocol. This would require a significant amount of work and
debugging, however.
R7. Suppose a process in Host C has a UDP socket with port number 6789. Suppose both Host A
and Host B each send a UDP segment to Host C with destination port number 6789. Will both of
these segments be directed to the same socket at Host C? If so, how will the process at Host C
know that these two segments originated from two different hosts? - ANS Yes, both
segments will be directed to the same socket. For each received segment, at the socket
interface, the operating system will provide the process with the IP addresses to determine the
origins of the individual segments.
R8. Suppose that a Web server runs in Host C on port 80. Suppose this Web server uses
persistent connections, and is currently receiving requests from two different Hosts, A and B.
Are all of the requests being sent through the same socket at Host C? If they are being passed
Copyright ©2025 BRIGHTSTARS ALL RIGHTS RESERVED 3
, through different sockets, do both of the sockets have port 80? Discuss and explain. -
ANS For each persistent connection, Web server creates a separate connection socket. Each
connection socket is identified with a four-tuple: (source IP address, source port number,
destination IP address, destination port number). When host C receives an IP datagram, it
examines these four fields in the datagram/segment to determine to which socket it should
pass the payload of the TCP segment. Thus, the requests from A and B pass through different
sockets. The identifier for both of these sockets has 80 for the destination port; however, the
identifiers for these sockets have different values for source IP addresses. Unlike UDP, when
the transport layer passes a TCP segment's payload to the application process, it does not
specify the source IP address, as this is implicitly specified by the socket identifier.
R9. In our rdt protocols, why did we need to introduce sequence numbers? - ANS Sequence
numbers are required for a receiver to find out whether an arriving packet contains new data or
is a retransmission.
R10. In our rdt protocols, why did we need to introduce timers? - ANS To handle losses in the
channel. If the ACK for a transmitted packet is not received within the duration of the timer for
the packet, the packet (or its ACK or NACK) is assumed to have been lost. Hence, the packet is
retransmitted.
R11. Suppose that the roundtrip delay between sender and receiver is constant and known to
the sender. Would a timer still be necessary in protocol rdt 3.0, assuming that packets can be
lost? Explain. - ANS A timer would still be necessary in the protocol rdt 3.0. If the round trip
time is known then the only advantage will be that, the sender knows for sure that either the
packet or the ACK (or NACK) for the packet has been lost, as compared to the real scenario,
where the ACK (or NACK) might still be on the way to the sender, after the timer expires.
However, to detect the loss, for each packet, a timer of constant duration will still be necessary
at the sender.
R12. Visit the Go-Back-N Java applet at the companion Web site.
a. Have the source send five packets, and then pause the animation before any of the five
packets reach the destination. Then kill the first packet and resume the animation. Describe
what happens.
b. Repeat the experiment, but now let the first packet reach the destination and kill the first
acknowledgment. Describe again what happens.
Copyright ©2025 BRIGHTSTARS ALL RIGHTS RESERVED 4
