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CS 6250 COMPUTER NETWORKS EXAM 1
2025 QUESTIONS AND ANSWERS.
What is encapsulation, and how is it used in a layered model? -
CORRECT ANSWER-Encapsulation is when data (called a header) is appended to the
packet through each layer to signify its on the correct path to the destination host.
What is the EvoArch model? - CORRECT ANSWER-An hourglass shaped model of the
Internet where the outer bands are more frequently modified or replaced and the further in
you go the harder it is for that layer to be altered or modified.
Provide examples of popular protocols at each layer of the fivelayered Internet model. -
CORRECT ANSWER-Application: NFS,
DNS, SNMP, ftp, rcp, telnet, HTTP
Transport: TCP, UDP
Internet: IP, ARP, ICMP
Data Link: PPP, IEEE 802.2, Ethernet
Physical Network: Token Ring, RS-232
What is the end-to-end (e2e) principle? - CORRECT ANSWER-A design choice that shaped
the current internet architecture. It states the network core should be simple and minimal,
while the end systems should carry the intelligence. Network functions should be simple and
essential commonly used functions so any host can utilize the service and higher form
functions should be built into the application itself. Lower level layers should be independent
and free to perform only their designed function and the higher-level layers deal with the
more intricate functions that deal with the specific application.
What are the differences between UDP and TCP? - CORRECT ANSWER-UDP is A.) an
unreliable protocol as it lacks the mechanism that TCP has in place. B.) a connectionless
protocol that does not require the establishment of a connection (example: three way
handshake) before sending packets. Some benefits to UDP are:
A.) no congestion control or similar mechanisms. B.) No connection management overhead.
The UDP packet structure is a 64 bits header consisting: 1.) Source and destination ports. 2.)
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Length of the UDP segment (header and data). 3.) Checksum (an error checking mechanism).
Since there is no guarantee for link-by-link reliability, we need a basis mechanism in place
for error checking. The UDP sender adds the src port, the dest port and the packet length.
Then it takes the sum and performs an 1s complement (all 0s are turned to 1 and all 1s are
turned to 0s). If during the sum there is an overflow, it's wrapped around. The receiver adds
all the four 16-bit words (including the checksum). The result should be all 1s unless an error
has occurred.
What is the purpose of the Spanning Tree Algorithm? - CORRECT ANSWER-The purpose of
the Spanning Tree Algorithm is to prevent broadcast storm through the network and cause
stalls or heavy congestion.
The spanning tree also prevents loops (cycles) from occurring in a network.
What are the examples of a violation of e2e principle? - CORRECT ANSWER-Violations
include firewalls and traffic filters. Firewalls violate because they are intermediate devices
that are operated between two end hosts and they can drop the end host communications.
Network Address Translation (NAT) boxes are also a violation because it uses the single
public IP address and distributes a new IP scheme to the hosts connected to it to route data
through rewriting the header info to route to the correct destination host. NAT boxes are a
violation because they are not globally addressable or routable.
What are advantages and disadvantages of a layered architecture? - CORRECT ANSWER-
Each protocol layer offers different services. Some advantages are scalability, flexibility, and
ease of adding / removing components making it easier for cost-effective implementations.
Disadvantages include: some layers functionality depends on the information from the other
layer and violates the goal of layer separation; one layer may duplicate lower layer
functionalities; overhead both in computation and in message headers caused by abstraction
barriers between layers.
What are the differences and similarities of the OSI model and fivelayered Internet model? -
CORRECT ANSWER-The OSI model and the 5-layered Internet Model have many of the
same layers, with the difference being three of the layers are combined in the 5-layered
model. Specifically the five-layer model combines the application, presentation, and session
layers from the OSI model into a single application layer.
What are sockets? - CORRECT ANSWER-A network socket is a software structure within a
network node of a computer network that serves as an endpoint for sending and receiving
data across the network. The structure and properties of a socket are defined by an
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application programming interface (API) for the networking architecture. Sockets are created
only during the lifetime of a process
of an application running in the node.https://en.wikipedia.org/wiki/Network_socket."A
process sends messages into, and receives messages from, the network through a software
interface called a socket. Let's consider an analogy to help us understand processes and
sockets. A process is analogous to a house and its socket is analogous to its door....a socket is
the interface between the application layer and the transport layer within a host." - Kurose
and Ross, 2.1
What is a distributed algorithm? - CORRECT ANSWER-A distributed algorithm is an
algorithm designed to run on computer hardware constructed from interconnected
processors. Distributed algorithms are used in many varied application areas of distributed
computing, such as telecommunications, scientific computing, distributed information
processing, and real-time process control.
Describe each layer of the OSI model. - CORRECT ANSWERApplication layer: Service,
Interface, Protocol. Ex: Turn on your smartphone and look at the list of apps. (HTTP, SMTP,
FTP, DNS) Presentation layer: Plays intermediate role of formatting the information received
from the layer below and delivering it to the application layer. Ex: converting big endian to
little endian. Session layer: Responsible for the mechanism that manages the different
transport streams that belong to the same session between end-user and application process.
Ex: teleconference app, it is responsible for tying together audio and video streaming.
Transport layer: Responsible for the end-to-end communication between end hosts. 2
transport protocols, TCP and UDP. TCP includes a connection-oriented service to the
applications that are running on the layer above, guaranteed delivery of the applicationlayer
messages, flow control, and congestion control mechanism. UDP provides a connectionless,
best-effort service to the applications that are running in the layer above without reliability,
flow, or congestion control. In this layer the packet is called a segment. Network layer: This
layer is responsible for moving the packet of information, called a datagram, from one host
to another. The network layer is responsible for delivering the datagram to the Transport
layer on the destination host. In this layer there are the IP Protocol and the routing tables.
Data Link layer: Packets are referred to as frames. Examples include: ethernet, ppp, wifi.
Responsible for moving the frames from one node (host or router) to the next node. Services
offered by the data link layer protocol include reliable delivery (transmission of the data from
one transmitting node, across one link, to the receiving node. Physical layer: This layer is the
actual hardware responsible to transfer bits within a frame between two nodes c
Explain a round in the EvoArch model. - CORRECT ANSWEREvoArch is a discrete-time
model that is executed over rounds. At each round, we perform the following steps: A) We
introduce new nodes, and we place them randomly at layers. B) We examine all layers, from
the top to the bottom, and we perform the following tasks: 1) We connect the new nodes that
CS 6250 COMPUTER NETWORKS EXAM 1
2025 QUESTIONS AND ANSWERS.
What is encapsulation, and how is it used in a layered model? -
CORRECT ANSWER-Encapsulation is when data (called a header) is appended to the
packet through each layer to signify its on the correct path to the destination host.
What is the EvoArch model? - CORRECT ANSWER-An hourglass shaped model of the
Internet where the outer bands are more frequently modified or replaced and the further in
you go the harder it is for that layer to be altered or modified.
Provide examples of popular protocols at each layer of the fivelayered Internet model. -
CORRECT ANSWER-Application: NFS,
DNS, SNMP, ftp, rcp, telnet, HTTP
Transport: TCP, UDP
Internet: IP, ARP, ICMP
Data Link: PPP, IEEE 802.2, Ethernet
Physical Network: Token Ring, RS-232
What is the end-to-end (e2e) principle? - CORRECT ANSWER-A design choice that shaped
the current internet architecture. It states the network core should be simple and minimal,
while the end systems should carry the intelligence. Network functions should be simple and
essential commonly used functions so any host can utilize the service and higher form
functions should be built into the application itself. Lower level layers should be independent
and free to perform only their designed function and the higher-level layers deal with the
more intricate functions that deal with the specific application.
What are the differences between UDP and TCP? - CORRECT ANSWER-UDP is A.) an
unreliable protocol as it lacks the mechanism that TCP has in place. B.) a connectionless
protocol that does not require the establishment of a connection (example: three way
handshake) before sending packets. Some benefits to UDP are:
A.) no congestion control or similar mechanisms. B.) No connection management overhead.
The UDP packet structure is a 64 bits header consisting: 1.) Source and destination ports. 2.)
, Page|2
Length of the UDP segment (header and data). 3.) Checksum (an error checking mechanism).
Since there is no guarantee for link-by-link reliability, we need a basis mechanism in place
for error checking. The UDP sender adds the src port, the dest port and the packet length.
Then it takes the sum and performs an 1s complement (all 0s are turned to 1 and all 1s are
turned to 0s). If during the sum there is an overflow, it's wrapped around. The receiver adds
all the four 16-bit words (including the checksum). The result should be all 1s unless an error
has occurred.
What is the purpose of the Spanning Tree Algorithm? - CORRECT ANSWER-The purpose of
the Spanning Tree Algorithm is to prevent broadcast storm through the network and cause
stalls or heavy congestion.
The spanning tree also prevents loops (cycles) from occurring in a network.
What are the examples of a violation of e2e principle? - CORRECT ANSWER-Violations
include firewalls and traffic filters. Firewalls violate because they are intermediate devices
that are operated between two end hosts and they can drop the end host communications.
Network Address Translation (NAT) boxes are also a violation because it uses the single
public IP address and distributes a new IP scheme to the hosts connected to it to route data
through rewriting the header info to route to the correct destination host. NAT boxes are a
violation because they are not globally addressable or routable.
What are advantages and disadvantages of a layered architecture? - CORRECT ANSWER-
Each protocol layer offers different services. Some advantages are scalability, flexibility, and
ease of adding / removing components making it easier for cost-effective implementations.
Disadvantages include: some layers functionality depends on the information from the other
layer and violates the goal of layer separation; one layer may duplicate lower layer
functionalities; overhead both in computation and in message headers caused by abstraction
barriers between layers.
What are the differences and similarities of the OSI model and fivelayered Internet model? -
CORRECT ANSWER-The OSI model and the 5-layered Internet Model have many of the
same layers, with the difference being three of the layers are combined in the 5-layered
model. Specifically the five-layer model combines the application, presentation, and session
layers from the OSI model into a single application layer.
What are sockets? - CORRECT ANSWER-A network socket is a software structure within a
network node of a computer network that serves as an endpoint for sending and receiving
data across the network. The structure and properties of a socket are defined by an
, Page|3
application programming interface (API) for the networking architecture. Sockets are created
only during the lifetime of a process
of an application running in the node.https://en.wikipedia.org/wiki/Network_socket."A
process sends messages into, and receives messages from, the network through a software
interface called a socket. Let's consider an analogy to help us understand processes and
sockets. A process is analogous to a house and its socket is analogous to its door....a socket is
the interface between the application layer and the transport layer within a host." - Kurose
and Ross, 2.1
What is a distributed algorithm? - CORRECT ANSWER-A distributed algorithm is an
algorithm designed to run on computer hardware constructed from interconnected
processors. Distributed algorithms are used in many varied application areas of distributed
computing, such as telecommunications, scientific computing, distributed information
processing, and real-time process control.
Describe each layer of the OSI model. - CORRECT ANSWERApplication layer: Service,
Interface, Protocol. Ex: Turn on your smartphone and look at the list of apps. (HTTP, SMTP,
FTP, DNS) Presentation layer: Plays intermediate role of formatting the information received
from the layer below and delivering it to the application layer. Ex: converting big endian to
little endian. Session layer: Responsible for the mechanism that manages the different
transport streams that belong to the same session between end-user and application process.
Ex: teleconference app, it is responsible for tying together audio and video streaming.
Transport layer: Responsible for the end-to-end communication between end hosts. 2
transport protocols, TCP and UDP. TCP includes a connection-oriented service to the
applications that are running on the layer above, guaranteed delivery of the applicationlayer
messages, flow control, and congestion control mechanism. UDP provides a connectionless,
best-effort service to the applications that are running in the layer above without reliability,
flow, or congestion control. In this layer the packet is called a segment. Network layer: This
layer is responsible for moving the packet of information, called a datagram, from one host
to another. The network layer is responsible for delivering the datagram to the Transport
layer on the destination host. In this layer there are the IP Protocol and the routing tables.
Data Link layer: Packets are referred to as frames. Examples include: ethernet, ppp, wifi.
Responsible for moving the frames from one node (host or router) to the next node. Services
offered by the data link layer protocol include reliable delivery (transmission of the data from
one transmitting node, across one link, to the receiving node. Physical layer: This layer is the
actual hardware responsible to transfer bits within a frame between two nodes c
Explain a round in the EvoArch model. - CORRECT ANSWEREvoArch is a discrete-time
model that is executed over rounds. At each round, we perform the following steps: A) We
introduce new nodes, and we place them randomly at layers. B) We examine all layers, from
the top to the bottom, and we perform the following tasks: 1) We connect the new nodes that
