CS6250 Computer Networks Exam 1 || Latest Updated 2025/2026 (Graded A+)
What are advantages and disadvantages of a layered architecture? ✔️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? ✔️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? ✔️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. ✔️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. ✔️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? ✔️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? ✔️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? ✔️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? ✔️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. ✔️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? ✔️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? ✔️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"? ✔️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? ✔️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. ✔️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, b) the ID of the roots as perceived by the sending node, and c) the number of
hops between that (perceived) root and the sending node.
At every round, each node keeps track of the best configuration message that it has received so far, and it compares that against the
configuration messages it receives from neighboring nodes at that round.
At the very first round of the algorithm, every node thinks that it is the root.
What is the purpose of the Spanning Tree Algorithm? ✔️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 does the transport layer provide? ✔️The transport layer is the logical connection. Transport layer consists of TCP and UDP protocols. The
transport layer provides an end-to-end connection between two applications that are running on different hosts. Of course the transport layer
provides this logical connection regardless if the hosts are in the same network. User datagram protocol (UDP) and the Transmission Control
Protocol (TCP).
-Some applications that use UDP include video streaming, DNS, and Online Multiplayer Games.
What are advantages and disadvantages of a layered architecture? ✔️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? ✔️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? ✔️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. ✔️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. ✔️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? ✔️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? ✔️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? ✔️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? ✔️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. ✔️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? ✔️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? ✔️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"? ✔️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? ✔️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. ✔️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, b) the ID of the roots as perceived by the sending node, and c) the number of
hops between that (perceived) root and the sending node.
At every round, each node keeps track of the best configuration message that it has received so far, and it compares that against the
configuration messages it receives from neighboring nodes at that round.
At the very first round of the algorithm, every node thinks that it is the root.
What is the purpose of the Spanning Tree Algorithm? ✔️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 does the transport layer provide? ✔️The transport layer is the logical connection. Transport layer consists of TCP and UDP protocols. The
transport layer provides an end-to-end connection between two applications that are running on different hosts. Of course the transport layer
provides this logical connection regardless if the hosts are in the same network. User datagram protocol (UDP) and the Transmission Control
Protocol (TCP).
-Some applications that use UDP include video streaming, DNS, and Online Multiplayer Games.
