CS6250 Computer Networks Exam 1 Questions Fully Solved Latest Update 2025
What are advantages and disadvantages of a layered architecture? - Answers 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 five-layered Internet model? - Answers
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? - Answers 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 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
Describe each layer of the OSI model. - Answers Application 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 application-layer 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
Provide examples of popular protocols at each layer of the five-layered Internet model. - Answers
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 encapsulation, and how is it used in a layered model? - Answers 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 end-to-end (e2e) principle? - Answers 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 examples of a violation of e2e principle? - Answers 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 re-writing the header info to route to the correct destination host.
NAT boxes are a violation because they are not globally addressable or routable.
What is the EvoArch model? - Answers 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.
, Explain a round in the EvoArch model. - Answers EvoArch 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 we may have just introduced to that layer, by choosing
substrates based on the generality probabilities of the layer below s(l−1), and by choosing products for
them based on the generality probability of the current layer s(l). 2) We update the value of each node
at each layer l, given that we may have new nodes added to the same layer l. 3) We examine all nodes,
in order of decreasing value in that layer, and remove the nodes that should die. C) Finally, we stop the
execution of the model when the network reaches a given number of nodes.
What are the ramifications of the hourglass shape of the internet? - Answers A. Many technologies that
were not originally designed for the internet have been modified so that they have versions that can
communicate over the internet (such as Radio over IP).
B. It has been a difficult and slow process to transition to IPv6, despite the shortage of public IPv4
addresses.
Repeaters, hubs, bridges, routers operate on which layers? - Answers Repeaters and Hubs work over L1
(Physical Layer)
Bridges and Layer 2-Switches work over L2 (Data link layer)
Routers and Layer 3-Switches work over L3 (Network layer)
What is a bridge, and how does it "learn"? - Answers A bridge is a device with multiple inputs/outputs. A
bridge transfers frames from an input to one (or multiple) outputs. Though it doesn't need to forward all
the frames it receives.
A learning bridge learns, populates and maintains a forwarding table. The bridge consults that table so
that it only forwards frames on specific ports, rather than over all ports. So how does the bridge learn?
When the bridge receives any frame this is a "learning opportunity" to know which hosts are reachable
through which ports. This is because the bridge can view the port over which a frame arrives and the
source host.
What is a distributed algorithm? - Answers 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.
https://en.wikipedia.org/wiki/Distributed_algorithm
Explain the Spanning Tree Algorithm. - Answers The algorithm runs in "rounds" and at every round each
node sends to each neighbor node a configuration message with three fields: a) the sending node's ID,
What are advantages and disadvantages of a layered architecture? - Answers 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 five-layered Internet model? - Answers
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? - Answers 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 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
Describe each layer of the OSI model. - Answers Application 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 application-layer 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
Provide examples of popular protocols at each layer of the five-layered Internet model. - Answers
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 encapsulation, and how is it used in a layered model? - Answers 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 end-to-end (e2e) principle? - Answers 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 examples of a violation of e2e principle? - Answers 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 re-writing the header info to route to the correct destination host.
NAT boxes are a violation because they are not globally addressable or routable.
What is the EvoArch model? - Answers 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.
, Explain a round in the EvoArch model. - Answers EvoArch 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 we may have just introduced to that layer, by choosing
substrates based on the generality probabilities of the layer below s(l−1), and by choosing products for
them based on the generality probability of the current layer s(l). 2) We update the value of each node
at each layer l, given that we may have new nodes added to the same layer l. 3) We examine all nodes,
in order of decreasing value in that layer, and remove the nodes that should die. C) Finally, we stop the
execution of the model when the network reaches a given number of nodes.
What are the ramifications of the hourglass shape of the internet? - Answers A. Many technologies that
were not originally designed for the internet have been modified so that they have versions that can
communicate over the internet (such as Radio over IP).
B. It has been a difficult and slow process to transition to IPv6, despite the shortage of public IPv4
addresses.
Repeaters, hubs, bridges, routers operate on which layers? - Answers Repeaters and Hubs work over L1
(Physical Layer)
Bridges and Layer 2-Switches work over L2 (Data link layer)
Routers and Layer 3-Switches work over L3 (Network layer)
What is a bridge, and how does it "learn"? - Answers A bridge is a device with multiple inputs/outputs. A
bridge transfers frames from an input to one (or multiple) outputs. Though it doesn't need to forward all
the frames it receives.
A learning bridge learns, populates and maintains a forwarding table. The bridge consults that table so
that it only forwards frames on specific ports, rather than over all ports. So how does the bridge learn?
When the bridge receives any frame this is a "learning opportunity" to know which hosts are reachable
through which ports. This is because the bridge can view the port over which a frame arrives and the
source host.
What is a distributed algorithm? - Answers 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.
https://en.wikipedia.org/wiki/Distributed_algorithm
Explain the Spanning Tree Algorithm. - Answers The algorithm runs in "rounds" and at every round each
node sends to each neighbor node a configuration message with three fields: a) the sending node's ID,
