CS 6250 Quiz 7 Exam guide (All Quizzes) | Georgia Institute of
Technology | Latest 2026/2027 Verified Questions and
Answers
2026/2027 | GRADED A+ | 1
...
00 out of 100
Question:
A few of the main reasons that SDN arose are: a diversity of different network equipment (eg routers, switches,
firewalls, etc.) using different protocols that made managing the network difficult, and second a lack of a central
platform to control network equipment.
Answer
true
Question:
The main idea behind SDNs is to divide tasks into smaller functions so the code is more modular and easy to manage.
Answer
true
Question:
With SDNs the control plane and data plane have independent evolution and development.
Answer
true
Question:
In the SDN approach, the SDN controller is physically located at each router that is present in a network.
Answer
false
Question:
By separating the control plane and the data plane, controlling the router's behavior became easier using higher order
programs. For example, it is easier to update the router's state or control the path selection.
Answer
true
Question:
In the SDN approach, ISPs or other third parties can take up the responsibility for computing and distributing the
router's forwarding tables.
Answer
true
Question:
Having the software implementations for SDNs controllers increasingly open and publicly available makes it hard to
control, since any person could modify the software easily.
Answer
false
Question:
In SDN networks, the SDN controller is responsible for the forwarding of traffic.
Answer
false
Question:
The network-control applications are programs that manage the underlying network with the help of the SDN
controller.
Answer
true
Question:
In SDN networks forwarding rules of traffic still have to be based on IP destination and cannot be based on other
metrics, packet header info etc.
Answer
false
Question:
SDN-controlled switches operate on the:
Answer
Data plane
Question:
In an SDN Architecture, the northbound interface keeps track of information about the state of the hosts, links,
switches and other controlled elements in the network, as well as copies of the flow tables of the switches.
Answer
false
Question:
In SDN networks, the southbound interface is responsible for the communication between SDN controller and the
controlled devices.
Answer
true
In SDN networks, the controller needs to be implemented over a centralized server.
Question:
What spurred the development of Software Defined Networking (SDN)?
Answer
SDN was developed in order to make computer networks more programmable. With so many different technologies
and vendors for network devices, SDN helps redesign networks to make them more manageable.
Question:
3 phases in the history of SDN
Answer
1. Active Networks
2. Control and Data Plane Separation
3. OpenFlow API and network operating systems
Question:
1. Active Networks
Answer
1990s-2000s
Active networking envisioned a programming interface that exposed network nodes and supported customization of
functionalities for subsets of packets passing through the network nodes.
Question:
Two types of programming models in active networking
Answer
1. Capsule Model - code to execute was carried in-band in data packets
2. Programmable router/switch model - code to execute was established by out-of-band mechanisms.
Question:
Pushes that encouraged active networking
Answer
1. Reduction in computation cost
2. Advancement in programming languages and VM technology to protect active code.
3. Advancement in code compilation
Question:
3 Major Contributions of Active Networking towards SDN
Answer
1. Programmable functions in the network to lower the barrier to innovation.
2. Network virtualization
3. The vision of a unified architecture for middlebox orchestration
Question:
Biggest Downfall of Active Networking
Answer
Too Ambitious
Since active networking was more involved in redesigning the architecture of networks, not as much emphasis was
given to performance and security - which users were more concerned about
Question:
2. Control and Data Plane Separation
Answer
2001 to 2007
Packet forwarding was implemented directly in the hardware. This era tailored to administrators being able to control
/ manage routing decisions. Emphasized programmability in the control domain rather than the data domain.
3. OpenFlow API and network operating systems
Answer
2007 to 2010
The vision of fully programmable networks for research is realized, and then actually used by businesses / ISPs.
Abstracted away the specifics of vendor software / proprietary products and allowed the entire network to be treated
as one programmable entity.
Lead to the rise of network operating systems and distributed state management.
How does an OpenFlow switch work?
Answer
Each switch contains a table of packet-handling rules. Each rule has a pattern, list of actions, set of counters and a
priority. When an OpenFlow switch receives a packet, it determines the highest priority matching rule, performs the
action associated with it and increments the counter.
Function of the control pane and data plane
Answer
The control plane contains the logic that implements the forwarding behavior of routers such as routing protocols and
network middlebox configurations.
The data plane performs the actual forwarding as dictated by the control plane
Why separate the control from the data plane?
Answer
Independent evolution and development - prior to their separation, any changes to the control plane had to be met
with upgrades to the hardware.
Control from high-level software programs - software computes the forwarding tables in SDN, allowing us to use high
order or more abstract programming languages to implement routing algorithms. This also makes debugging and
checking the behavior of the network easier.
4 areas of opportunities with control data plane separation
1. Data centers. Consider large data centers with thousands of servers and VMs. Management of such large network is
not easy. SDN helps to make network management easier.
2. Routing. With SDN, it is easier to update the router's state, and SDN can provide more control over path selection.
3. Enterprise networks. SDN can improve the security applications for enterprise networks (such as for DDOS)
4. Research networks. SDN allows research networks to coexist with production networks.
Forwarding
the router inspects the packet coming in from a port, consults the routing table, and sends the packet to the output
port dictated by the forwarding table. Forwarding is a function of the data plane.
Routing
routers use routing algorithms to generate the forwarding table, implementing policies and using these algorithms to
determine the best path for traffic. Routing is a function of the control plane.
What is the difference between a traditional and SDN approach in terms of coupling of control and data plane?
In the traditional approach, the control and data planes are closely coupled. The router runs routing algorithms and
generates the forwarding table.
In the SDN approach, the remote controller computers and distributes forwarding tables. The controller is physically
separated from the router.
3 Main Components of SDN Network
1. SND-controlled network elements - infrastructure layer, responsible for forwarding traffic using rules computed by
the SDN control plane.
2. SDN controller - logically centralized entity that behaves as an interface between the network elements and the
network-control applications.
3. Network-control applications - programs that manage the underlying network by collecting information about the
network elements with the help of the SDN controller
4 defining features in an SDN network
1. Flow-base forwarding - rules for packets can be computed based on any number of header field values in various
layers (transport, network, link).
2. Separation of data and control planes
3. Network control functions - controller maintains up-to-date network state information and provides it to the
network-control applications. This enables network-control applications to monitor and control network devices.
4. Programmable network - enables the implementation of sophisticated routing applications to include network
management, traffic engineering, security, automation, analytics, etc.
3 layers of SDN controller
1. Communication layer - comms between the controller and the network elements
2. Network-wide state-management layer - stores information about the network state
3. Interface to the network-control application layer - communication between controller and network-control
applications
false
What spurred the development of Software Defined Networking (SDN)?
Software Defined Networking (SDN) arose as part of the process to make computer networks more programmable.
Computer networks are very complex and especially difficult to manage for two main reasons: - Diversity of equipment on the network - Proprietary technologies for the equipment
What are the three phases in the history of SDN?
1. Active networks
2. Control and data plane separation
3. OpenFlow API and network operating systems
Summarize each phase in the history of SDN.
Active networking envisioned a programming interface (a network API) that exposed resources/network nodes and
supported customization of functionalities for subsets of packets passing through the network nodes.
Control and data plane separation - This phase lasted from around 2001 to 2007. During this time, there was a steady
increase in traffic volumes and thus network reliability, predictability and performance became more important.
Network operators were looking for better network-management functions such as control over paths to deliver traffic
(traffic engineering). Researchers started exploring short-term approaches that were deployable using existing
protocols. They identified the challenge in network management lay in the way existing routers and switches tightly
integrated the control and data planes.
OpenFlow API and network operating systems - OpenFlow was born out of the interest in the idea of network
experimentation at a scale (by researchers and funding agencies). It was able to balance the vision of fully
programmable networks and the practicality of ensuring real world deployment. OpenFlow built on the existing
hardware and enabled more functions than earlier route controllers. Although this dependency on hardware limited its
flexibility, it enabled immediate deployment.
What is the function of the control and data planes?
The control plane contains the logic that controls the forwarding behavior of routers such as routing protocols and
network middlebox configurations.
The data plane performs the actual forwarding as dictated by the control plane. For example, IP forwarding and Layer
2 switching are functions of the data plane.
Why separate the control from the data plane?
1. Independent evolution and development. In the traditional approach, routers are responsible for both routing and
forwarding functionalities. This meant that a change to either of the functions would require an upgrade of hardware.
In this new approach, routers only focus on forwarding. Thus, innovation in this design can proceed independently of
other routing considerations. Similarly, improvement in routing algorithms can take place without affecting any of the
existing routers. By limiting the interplay between these two functions, we can develop them more easily.
2. Control from high-level software program. In SDN, we use software to compute the forwarding tables. Thus, we can
easily use higher-order programs to control the routers' behavior. The decoupling of functions makes debugging and
checking the behavior of the network easier.
Why did the SDN lead to opportunities in various areas, such as data centers, routing, enterprise networks, and
research networks?
1. Data centers. Consider large data centers with thousands of servers and VMs. Management of such large networks is
not easy. SDN helps to make network management easier.
2. Routing. The interdomain routing protocol used today, BGP, constrains routes. There are limited controls over
inbound and outbound traffic. There is a set procedure that needs to be followed for route selection. Additionally, it is
hard to make routing decisions using multiple criteria. With SDN, it is easier to update the router's state, and SDN can
provide more control over path selection.
3. Enterprise networks. SDN can improve the security applications for enterprise networks. For example, using SDN it
is easier to protect a network from volumetric attacks such as DDoS, if we drop the attack traffic at strategic locations
of the network.
4. Research networks. SDN allows research networks to coexist with production networks.
What is the relationship between forwarding and routing?
Routing involves determining the path from the sender to the receiver across the network. It constructs the forwarding
table for routers, which consult the table in making forwarding decisions (which output link to send a packet through).
What is the difference between a traditional and SDN approach in terms of coupling of control and data plane?
In the traditional approach, the routing algorithms (control plane) and forwarding function (data plane) are closely
coupled. The router runs and participates in the routing algorithms. From there it is able to construct the forwarding
table which consults it for the forwarding function.
In the SDN approach, on the other hand, there is a remote controller that computes and distributes the forwarding
tables to be used by every router. This controller is physically separate from the router. It could be located in some
remote data center, managed by the ISP or some other third party.
What are the main components of an SDN network and their responsibilities? - Network-control applications (control plane) -
The network-control applications are programs that manage the underlying network by collecting information about
the network elements with the help of the SDN controller.
...Northbound API ...
SDN controller (control plane) -
The SDN controller is a logically centralized entity that acts as an interface between the network elements and the
network-control applications.
...Southbound API...
SDN-controlled network elements (data plane) -
The SDN-controlled network elements, sometimes called the infrastructure layer, is responsible for the forwarding of
traffic in a network based on the rules computed by the SDN control plane.
What are the four defining features of an SDN architecture?
1) Flow-based forwarding: The rules for forwarding packets in the SDN-controlled switches can be computed based on
any number of header field values in various layers such as the transport-layer, network-layer and link-layer. This
differs from the traditional approach where only the destination IP address determines the forwarding of a packet. For
example, OpenFlow allows up to 11 header field values to be considered.
2) Separation of data plane and control plane: The SDN-controlled switches operate on the data plane and they only
execute the rules in the flow tables. Those rules are computed, installed, and managed by software that runs on
separate servers.
3) Network control functions: The SDN control plane, (running on multiple servers for increased performance and
availability) consists of two components: the controller and the network applications. The controller maintains up-to
date network state information about the network devices and elements (for example, hosts, switches, links) and
provides it to the network-control applications. This information, in turn, is used by the applications to monitor and
control the network devices.
4) A programmable network: The network-control applications act as the "brain" of the SDN control plane by
managing the network. Example applications can include network management, traffic engineering, security,
automation, analytics, etc. For example, we can have an application that determines the end-to-end path between
sources and destinations in the network using Dijkstra's algorithm.
What are the three layers of SDN controllers?
Communication layer: communicating between the controller and the network elements
Network-wide state-management layer: stores information of network-state
Interface to the network-control application layer: communicating between controller and applications
CS 6250 Quiz 6 Exam Prep (All Quizzes) Latest 2026/2027
Update | 100% Verified Questions & Answers | Georgia
Institute of Technology
2026/2027 | GRAD
...
ED A+ | 100 out of 100
Question:
What are the basic components of a router?
Answer
Main job of a router is to implement the "forwarding plane" and "control plane" functions.
Forwarding plane + control pane
Input/Output ports
Switching fabric moves packets from input to output ports. 3 types of fabric: memory, bus, and crossbar.
Question:
Explain the forwarding (or switching) function of a router.
Answer
Forwarding (or switching) function: Transfer a packet from input link to appropriate output link. Very short
timescales (nanoseconds),
Question:
The switching fabric moves the packets from input to output ports. What are the functionalities performed by the
input and output ports?
Answer
Input ports: Physically terminates incoming links, decapsulates packets (data processing unit) and performs lookup
function.
Output ports receive and queue the packets to send to outgoing link.
Question:
What is the purpose of the router's control plane?
Answer
Implements routing protocols, maintains routing tables, computes forwarding table. Implemented by software in the
routing processor (or SDN), but could also be done by remote controller.
Question:
What tasks occur in a router?
Answer
Lookup: Router looks up destination IP and determines output link using forwarding table. Forwarding Information
Base (FIB) provides mapping between destination prefixes and output links
Switching: Transfer from input to output link.
Queueing: queues packets if link is congested
Header validation + checksum: Checks packet version number, decrements TTL, recalculates header checksum
Route processing: Builds forwarding table using protocol like RIP, OSPF, and BGP.
Protocol processing: Routers need to implement the following protocol to implement their functions: SNMP (simple
network management protocol) for counters for remote inspection, TCP and UDP for remote comms, and ICMP
(internet control message protocol) for sending messages when TTL is exceeded
Question:
List and briefly describe each type of switching. Which, if any, can send multiple packets across the fabric in parallel?
Answer
Switching via memory: Input/Output ports operate as I/O devices in an OS, controlled by the router. Input port sends
an interrupt to routing processor, and packet is copied into memory. Processor uses forwarding table and copies into
output port.
Switching via bus:Routing processor does NOT intervene like with memory.Input port puts an internal header onto
the packet that designates output port, and sends to shared bus. All output ports receive it, but only the designated one
keeps it and removes the header. Only one packet can cross the bus at a time, so the speed of the bus is the bottleneck.
Switching via interconnected network (crossbar). Basically 2d bus?
Question:
What are two fundamental problems involving routers, and what causes these problems?
Answer
Bandwidth and Internet population scaling: These scaling issues are caused by:
An increasing number of devices that connect to the Internet,
Increasing volumes of network traffic due to new applications, and
New technologies such as optical links that can accommodate higher volumes of traffic.
Services at high speeds: New applications require services such as protection against delays in the presence of
congestion and protection during attacks or failures. But offering these services at very high speeds is a challenge for
routers.
Question:
What are the bottlenecks that routers face, and why do they occur?
Answer
Longest prefix matching: Routers need to look up a packet's destination address to forward it. Increasing number of
Internet hosts and networks has made it impossible for routers to have explicit entries for all possible destinations. So
instead, routers group destinations into prefixes. But then, routers run into the problem of more complex algorithms
for efficient longest prefix matching.
Service differentiation: Routers can also offer service differentiation which means different quality-of-service (or
security guarantees) to different packets. In turn, this requires the routers to classify packets based on more complex
criteria beyond destination. For example, they can include source or applications/services associated with the packet.
Switching limitations: As we have seen, a fundamental operation of routers is to switch packets from input ports to
output ports. A way to deal with high-speed traffic is to use parallelism by crossbar switching. But at high speeds, this
comes with its problems and limitations (e.g., head of line blocking).
Bottlenecks about services: Providing performance guarantees (quality of service) at high speeds is nontrivial, as is
providing support for new services such as measurements or security guarantees.
Question:
What is CIDR, and why was it introduced?
Answer
Classless internet Domain Routing. It assigns IP addresses with arbitrary length prefixes. It helped decrease router
table size as Ip addresses were being exhausted, but it also introduced the "longest matching prefix lookup" problem.
Question:
Name 4 takeaway observations around network traffic characteristics. Explain their consequences.
Answer
Measurement studies on network traffic had shown a large number (in the order of hundreds of thousands - 250,000
according to a measurement study in the earlier days of the Internet) of concurrent flows of short duration. This
already large number has only been increasing, and as a consequence, caching solutions will not work efficiently.
The important element of any lookup operation is how fast it is done (lookup speed). A large part of the cost of
computation for lookup is accessing memory.
An unstable routing protocol may adversely impact the update time in the table: add, delete or replace a prefix.
Inefficient routing protocols increase this value up to additional milliseconds.
A vital trade-off is memory usage. We can use expensive fast memory (cache in software, SRAM in hardware) or
cheaper but slower memory (e.g., DRAM, SDRAM).
Question:
Why do we need multibit tries?
Answer
Unibit tries most significant problem is the number of memory accesses required to perform a lookup. For 32 bit
addresses, we can see that looking up the address in a unibit trie might require 32 memory accesses, in the worst case.
Assuming a 60 nsec latency, the worst-case search time is 1.92 microseconds. This could be very inefficient in high
speed links.
Instead, we can implement lookups using a stride. The stride is the number of bits that we check at each step.
So an alternative to unibit tries are the multibit tries. A multibit trie is a trie where each node has 2k children, where k
is the stride. Next, we will see that we can have two flavors of multibit tries: fixed-length stride tries and variable
length stride tries.
Question:
What is prefix expansion, and why is it needed?
Answer
If we have a prefix of length 3 (101) and stride in 2 bits at a time, we'll miss prefixes like 101. So we do prefix expansion
(ensure the expanded prefix is a multiple of the stride length), and remove all that are not multiples of stride length.
Question:
How would you perform a prefix lookup given a list of pointers for unibit tries, fixed-length multibit ties, and variable
length multibit tries.
Answer
Just keep traversing the tree with the bits until you either hit the end, have no where to go, or hit an empty (if you hit
an empty, go back until the last non-empty you found)
Question:
How woudl you perform a prefix expansion? How many prefix lengths do old prefixes have? What about new prefixes?
Answer
Just create all possibilities of the existing prefix that is a multiple of the stride length. I.e if stride is 3 and you have 01*,
you'd do 010 011, and then throw out any collisions.
Question:
What are the benefits of variable-stride versus fixed-stride multibit tries?
Answer
Variable length can make our prefix database smaller and optimize for memory
Question:
Why is packet classification needed?
Answer
Networks need quality of service + security as the internet gets bigger. Packet forwarding based on longest prefix
match is insufficient, so we need to handle packets based on other things like TCP flags, source addresses, etc.
Question:
What are three established variants of packet classification?
Answer
Firewalls: routers have entry/exit points that filter out unwanted traffic and enforce some custom policies
Resource Reservation Protocols: Things like DiffServ are used to reserve bandwidth between source and destination
Routing based on traffic type: Routing based on the traffic type helps avoid delays for time-sensitive applications
Question:
What are the simple solutions to the packet classification problem?
Answer
Linear Search: Things like firewalls just do a linear search through all the rules to see which rule matches best, but this
can be slow.
Caching: Cache results from a search so it's faster in the future. Even with a 90% hit rate in the cache, it's still too slow
to search the rule space linearly from there.
Passing Labels: MPLS (Multiprotocol Label Switching) and Diffserv use label passing. When leaving the source, a
traffic label goes into a special MPLS header. Then intermediate routers just re-use this label without needing to redo
classification.
Question:
How does fast searching using set-pruning tries work?
Answer
Create a tree (or trie) using destination prefixes. At the leaves of the tree, put the source tries.
Problems: Big memory, as some leaves can have multiple source tries (some source prefixes can appear in multiple
destination tries).
Question:
What is the difference between the pruning approach and the backtracking approach for packet classification with a
trie?
Answer
Pruning approach uses lots of memory and hangs the source tries at destination leaves. Backtracking approach goes
through the destination trie to find the longest destination prefix match. Then it goes back up the destination trie and
searches each source trie (associated with each destination) until it finds the longest source match. Backtracking pays
more time in exchange for saving on memory.
Question:
What's the benefit of a grid of tries approach?
Answer
Avoids backtracking using a 'switch pointer' precomputed. When you finish traversing the destination trie and it fails,
you follow switch pointers until you reach a match for the source trie.
Question:
Describe the "Take the Ticket" algorithm.
Answer
In an NxN grid, each input line requests a ticket from an output line. Then the tickets are just served in order.
Question:
What is the head-of-line problem?
Answer
When the first input line in "take a ticket" is waiting, all other lines are also waiting because the head of the line has yet
to be served, even though there are empty output lines.
Question:
How is the head-of-line problem avoided using the knockout scheme?
Answer
(avoid queueing) Break up packets into fixed sizes (called cells). Assume the switching fabric is running N times faster
than the input links. Also assume that the same output rarely gets N cells at once, and it's usually k < N. If this is the
case, then we can run the fabric at k times faster instead of N.
If this is violated i.e k = 1 and N = 2, randomly pick the output (in this case, the switching element is called the
'concentrator').
Drawback is that it's hard to implement.
Question:
How is the head-of-line problem avoided using parallel iterative matching?
Answer
Still allows queueing. Assume N inputs. In round 1, each input sends a request to N outputs. If an output gets more
than 1 request, it chooses someone at random. If an input gets multiple acceptances, it chooses an output at random.
In round 2, they send N-1 requests and so on until only 1 request is sent.
Question:
Describe FIFO with tail drop.
Answer
The switching system places packets at appropriate output ports. The ports have a FIFO queue. Once the buffer is full,
all other incoming packets up to the tail of the queue are dropped.
Fast but potential packet loss.
Question:
What are the reasons for making scheduling decisions more complex than FIFO?
Answer
Router support for congestion: congestion is pretty common, routers need to be able to handle it.
Fair sharing of links among competing flows: If packets flood the buffers at an output link, then with FIFO + tail drop,
tons of packets are dropped and other flows get blocked. This is bad.
Providing QoS guarantees to flow: Need to give guarantees on the flow rate of packets. Otherwise things like video
streaming would be awful.
Question:
Describe Bit-by-bit Round Robin scheduling.
Answer
Send a bit from each active flow one at a time in a round-robin manner (very fair). The bit-by-bit part is just a
conceptual idea, since you can't actually break up a packet into bits, so you use bit-by-bit to instead compute the
"finish time" of a packet and send it all at once when the 'finish time' of a packet == the current time of the scheduler.
Basically the "most-starved stream" each round goes next.
Question:
Bit-by-bit Round Robin provides fairness; what's the problem with this method?
Answer
Requires a priority queue to keep track of finish time at which the head of the packet of each queue would have
departed. Priority queues have logarithmic time complexity.
If a new queue becomes active, then all other timestamps need to change, which has linear complexity.
Question:
Describe Deficit Round Robin (DRR).
Answer
Each queue has packets with sizes. Each queue also has its own "Deficit counter', initialized to 0. We also have a
quantum size Q. Each round, we add Q in a round robin fashion to each queue's deficit counter. If the deficit counter of
a queue Di is >= the size of the next packet in that queue, send the packet and decrease Di by the size sent.
Question:
What is a token bucket shaping?
Answer
You might want to limit a type of traffic without putting it into a separate queue. Token bucket shaping can limit the
burstiness of a flow by limiting the average rate or the max burst size. Bucket fills up with tokens at a rate of R. If
bucket overflows, excess tokens are discarded.
When a packet arrives, if num tokens B > size of packet, send the packet and consume tokens from the bucket.
Problem is: one queue per flow, and one flow may have a full bucket while others may be empty and waiting
unnecessarily.
In traffic scheduling, what is the difference between policing and shaping?
Answer
Token bucket policing says that if a packet arrives and there are no tokens in the bucket, it gets dropped. Shaping
means just wait for the bucket to be full enough to send.
Policing = when traffic reaches the max rate, excess traffic is dropped. Graph looks like a sawtooth with flattened
peaks (cut off)
Shaping = Retains excess packets in a queue/buffer, waits till bucket is full enough to send. Graph looks smooth.
Can use shaping and policing together.
How is a leaky bucket used for traffic policing and shaping?
Answer
Hole in the bucket makes it so that traffic enters the network at a constant rate regardless of how fast the bucket is
filled up. If the bucket is not full and a packet arrives, the packet is 'conforming' and added to the bucket.
If a packet arrives and the bucket is full, it's 'nonconforming' and dropped (policing).
Constant output rate (shaping).
T/F: Using packet classification techniques, we can perform packet forwarding based on multiple criteria and not just
based on destination IP address.
Answer
True
T/F: The backtracking approach has a higher cost in terms of time, whereas the set pruning technique approach has
higher cost in terms of memory?
Answer
True
T/F: The grid of tries offers a "middle ground" approach, merging the backtracking and the set pruning techniques.
Answer
True
T/F: The head of line blocking refers to the problem where an entire queue remains blocked because the head of the
queue is blocked.
True
T/F: One technique to avoid head of line blocking is with parallel iterative matching.
True
T/F · With parallel iterative matching the input links are "matched" with output links in a fixed manner that stays the
same as the rounds are progressing.
False
T/F · With the token bucket traffic approach, we can still have bursts of traffic entering the network, but these bursts
are capped.
True
T/F · With the leaky bucket approach, we only allow the traffic to enter the network in a configured rate.
True
T/F · Traffic policers target to limit traffic bursts to a configured max, whereas traffic shapers target to smooth out the
overall rate.
True
T/F · With the leaky bucket we can still have discarded packets.
True
CS 6250 Quiz 5 Exam Prep (All Quizzes) Latest 2026/2027
Update | 100% Verified Questions & Answers | Georgia
Institute of Technology
2026/2027 | GRAD
...
ED A+ | 100 out of 100
Question:
What are the basic components of a router?
Answer
the input/output ports,
the switching fabric,
the routing processor.
Question:
Explain the forwarding (or switching) function of a router
Answer
This is the router's action to transfer a packet from an input link interface to the appropriate output link interface.
Forwarding takes place at very short timescales (typically a few nanoseconds), and is typically implemented in
hardware.
Question:
What are the functionalities of the input ports?
Answer
i. Physically terminate the incoming links to the router
ii. The data link processing unit decapsulates the packets
iii. Perform the lookup function. At this point the input ports consult the forwarding table to ensure that each packet is
forwarded to the appropriate output port through the switching fabric.
Question:
What are the functionalities of the output ports?
Answer
receive and queue the packets which come from the switching fabric and then send them over to the outgoing link
Question:
What is the purpose of the routers control plane?
Answer
implementing the routing protocols,
maintaining the routing tables,
computing the forwarding table.
All these functions are implemented in software in the routing processor, or by a remote controller.
Question:
What tasks occur in a router?
Answer
1. Lookup
2. Switching
3. Queueing
4. Header validation and checksum
5. Route processing
6. Protocol Processing.
Question:
Explain the lookup task of a router
Answer
Time Sensitive, when a packet arrives at the destination IP and determines the output link by looking at the
forwarding table or forwarding information base (FIB). The
Question:
Explain the switching task of a router
Answer
Time Sensitive, after lookup, the switching system takes over to transfer the packet from the input link to the output
link. Modern fast routers use crossbar for this task. Scheduling can be difficult because multiple inputs may want to
send to the same output.
Question:
Explain the Queuing task of the router
Answer
Time Sensitive, After the switch has been switched to a specific output, it will need to be queued (if the link is
congested). Queue may be FIFO or other.
Question:
Explain the Header validation and checksum task of the router
Answer
Time insensitive, the router checks the packets version number, it decrements the time to live field and recalculates
the header checksum. Data Plane
Question:
Explain the route processing task of the router
Answer
Time insensitive, the router builds their forwarding table using routing protocols such as RIP, OSPF and BGP. These
protocols are implemented in routing processors. Control Plane
Question:
Explain the protocol processing task of the router
Answer
Time insensitive, the routers, in order to implement their functions, need to implement the following protocols
(Control Plane):
i. SNMP - Remote inspection
ii. TCP and UPD - remote communication with the router
iii. ICMP - for sending error messages when time to live time is exceeded.
Question:
Explain switching via memory
Answer
a. Input/Output ports operate as I/O devices in an operating system, and they are controlled by the routing processor.
When an input port receives a packet, it sends an interrupt to the routing processor and the packet is copied to the
processor's memory. Then the processor extracts the destination address and looks into the forward table to find the
output port, and finally the packet is copied into that output's port buffer.
Question:
Explain switching via bus
Answer
a. the routing processor does not intervene as we saw the switching via memory. When an input port receives a new
packet, it puts an internal header that designates the output port, and it sends the packet to the shared bus. Then all
the output ports will receive the packet, but only the designated one will keep it. When the packet arrives at the
designated output port, then the internal header is removed from the packet. Only one packet can cross the bus at a
given time, and so the speed of the bus limits the speed of the router.
Question:
Explain switching via interconnection network (Crossbar)
Answer
Connects N input ports to N output ports using 2N buses. Horizontal buses meet the vertical buses at crosspoints
which are controlled by the switching fabric. For example, let's suppose that a packet arrives at port A that will need to
be forwarded to output port Y, the switching fabric closes the crosspoint where the two buses intersect, so that port A
can send the packets onto the bus and then the packet can only be picked up by output port Y. Can carry multiple
packets at the same time, as long as they are using different input and output ports. For example, packets can go from
A-to-Y and B-to-X at the same time.
Question:
Explain Bandwidth and internet population scaling (1 of 2 fundamental problems of routers)
Answer
These scaling issues are caused by: a) An increasing number of devices that connect to the Internet, 2) Increasing
volumes of network traffic due to new applications, and 3) New technologies such as optical links that can
accommodate higher volumes of traffic.
Question:
Explain services at high speeds (1 of 2 fundamental problems of routers)
Answer
New applications require services such as protection against delays in presence of congestion, and protection during
attacks or failures. But offering these services at very high speeds is a challenge for routers.
Question:
Explain longest prefix matching (Bottleneck of routers)
Answer
The increasing number of the Internet hosts and networks, has made it impossible for routers to have explicit entries
for all possible destinations. Instead, routers group destinations into prefixes. But then, routers run into the problem
of more complex algorithms for efficient longest prefix matching.
Question:
Explain Service Differentiation (Bottleneck of routers)
Answer
Routers are also able to offer service differentiation which means different quality of service (or security guarantees) to
different packets.a. In turn, this requires the routers to classify packets based on more complex criteria that go beyond
destination and they can include source or applications/services that the packet is associated with.
Question:
Explain Switching Limitations (Bottleneck of routers)
Answer
A way to deal with high-speed traffic, is to use parallelism by using crossbar switching. But in high speeds, this comes
with its own problems and limitations (e.g., head of line blocking).
Question:
Explain bottlenecks about services (Bottleneck of routers)
Answer
Providing the best possible service at the highest possible speed.
Question:
What is Classless Internet Domain Routing (CIDR) and why was it introduced?
Answer
essentially assigns IP addresses using arbitrary-length prefixes. Has helped to decrease the router table size but at the
same time it introduced us to a new problem: longest-matching-prefix lookup.
Question:
Convert between different prefix notations (dot-decimal, slash, and masking).
Answer
Dot decimal: e.g. of 16-bit prefix: 132.234
Slash notation: Standard notation: A/L (where A=Address, L=Length) e.g.: 132.238.0.0/16
Masking: We can use a mask instead of the prefix length. e.g.: The Prefix 123.234.0.0/16 is written as 123.234.0.0 with
a mask 255.255.0.0
Question:
Name 4 takeaway observations around network traffic characteristics. Explain their consequences.
Answer
These challenges revolve around lookup speed, memory, and update time:
a caching solution would not work efficiently.
The important element while performing any lookup operation is how fast it is done (lookup speed). A large part of the
cost of computation for lookup is accessing memory.
An unstable routing protocol may adversely impact the update time in the table: add, delete or replace a prefix.
Inefficient routing protocols increase this value up to additional milliseconds.
An important trade-off is memory usage. We have the option to use expensive fast memory (cache in software, SRAM
in hardware) or cheaper but slower memory (e.g., DRAM, SDRAM).
Question:
Why we we need multibit tries
Answer
Unibit trie requires a large number of memory accesses for lookup
Question:
What is prefix expansion and why is it needed?
Answer
We expand a given prefix to more prefixes. We ensure that the expanded prefix is a multiple of the chosen stride
length. At the same time we remove all lengths that are not multiples of the chosen stride length. We end up with a
new database of prefixes, which may be larger (in terms of actual number of prefixes) but with fewer lengths. So, the
expansion gives us more speed with an increased cost of the database size.
Question:
When a prefix is expanded, can a collision occur (overlap of prefixes)? what happens?
Answer
when an expanded prefix collides with an existing prefix. In that case the expanded prefix gets dropped.
What is a multibit trie?
Answer
Purpose is to examine several bits of a prefix at a time, called a stride/stride length in order to improve performance.
What are the benefits of variable stride versus fixed multibit tries?
Answer
i. Every node can have a different number of bits to be explored.
ii. The optimizations to the stride length for each node are all done in pursuit of saving trie memory and the least
memory access.
iii. An optimum variable stride is selected by using dynamic programming.
The data plane functions of a traditional router are implemented in
Answer
hardware
The control plane functions of a traditional router are implemented in
Answer
software
What plane operates on a shorter timescale
Answer
dataplane
multibit tri is ___ than a unibit tri representing the same prefix database and requires ___ memory accesses to
perform a lookup
shorter, fewer
Answer
Multibit tries (can/cannot) support an arbitrary number of prefix lenghts?
cannot
The forwarding plane operations take place in a (shorter/longer) timescale than the control planes?
shorter
In a router, there is no interplay between the control plane and ___ plane to compute the forwarding table
forwarding plane
in traditional routers, traffic forwarding is performed based on
destination IP address only.
What are basic components of router?
Input ports (receive packets lookup FIB) Switching fabric (moves packets input to output) Output ports (queue and
transmit) Routing processor (control plane functions)
Explain forwarding/switching function of router
Transfer packet from input link to appropriate output link. Occurs in nanoseconds implemented in hardware consults
forwarding table for output port determination.
What functionalities do input/output ports perform?
Input: 1) Physical termination 2) Data link processing (decapsulation) 3) Lookup function (consult FIB). Output:
receive and queue packets from fabric transmit over outgoing link buffer management.
What is purpose of router control plane?
Implements routing protocols (OSPF BGP RIP) maintains routing tables computes forwarding table. Software-based
in routing processor or remote SDN controller.
What tasks occur in router?
Time-sensitive: lookup switching queuing. Less time-sensitive: header validation (TTL checksum) route processing
protocol processing (SNMP TCP/UDP ICMP)
List and describe types of switching
Memory: CPU copies packets via memory (limited by bandwidth). Bus: shared bus with headers (one packet at a time).
Crossbar: N×N with controllable crosspoints (parallel transfers). Only crossbar supports parallel packets.
What are 2 fundamental router problems?
1) Bandwidth and population scaling (more devices traffic optical speeds) 2) Services at high speeds (QoS security low
latency). Caused by Internet growth exceeding technology improvement.
What bottlenecks do routers face?
Longest prefix matching (complex algorithms) Service differentiation (packet classification) Switching limitations
(head-of-line blocking) Service bottlenecks (QoS at high speeds)
Convert between prefix notations
Dot decimal: 132.234. Slash: 132.238.0.0/16. Masking: 123.234.0.0 mask 255.255.0.0. /24 = 255.255.255.0 /16 =
255.255.0.0 /8 = 255.0.0.0
What is CIDR and why introduced?
Classless Inter-Domain Routing: arbitrary-length prefixes introduced 1993. Replaced fixed classes (A B C) to address
IP exhaustion reduce routing tables but introduced longest prefix matching complexity.
Name 4 network traffic characteristics
1) Many concurrent flows (hundreds of thousands short duration) → caching ineffective 2) Memory access cost major
component → minimize accesses 3) Update time impact from routing → efficient mechanisms critical 4) Memory
tradeoff fast expensive vs slow cheap → balance cost/performance
Why do we need multibit tries?
Unibit tries require too many memory accesses (up to 32 for 32-bit address = 1.92 microseconds). Multibit checks
multiple bits per step (stride k → 2^k children) reducing memory accesses.
What is prefix expansion and why needed?
Fixed-stride tries require prefixes at multiples of stride length. Expand shorter prefixes to next multiple generate all bit
combinations for additional bits remove collisions. Trade increased memory for reduced lookup time.
Perform prefix lookup process
Unibit: follow path bit by bit remember last match return longest prefix found. Multibit: check preset bits per level
remember last match faster but requires expansion.
How many prefix lengths before/after expansion?
Example: Original 5 different lengths (1 3 4 5 6 bits) After expansion: more prefixes but fewer distinct lengths (3 6 bits
only)
Benefits of variable-stride vs fixed-stride?
Variable: different stride lengths per node memory optimization using dynamic programming better memory usage
while maintaining speed. Fixed: same stride per level simpler but may waste memory.
What are the basic components of a router?
Forwarding/switching
Input ports
Switch fabric
Outport ports
Switching fabric
Moves packets from input to output ports, makes connections between input and output
Control plane f(x)
Implements routing protocols , maintains routing tables, and computes forwarding table
Lookup
When a packet arrives at input link, look at the destination IP address and determine output link by looking at
forwarding table/FIB
Switching
Transfers the packet from the input to output link
Route processing
Building forwarding tables, using routing protocols such as RIP, OSPF, and BGP
Different types of switching
Via memory, bus, and interconnection network
Switching via memory
Input/output ports operate on system controlled by a routing processor
Switching via bus
Routing processor doesn't intervene when input receives new packet, it instead adds internal header that designates
output port and sends packet to shared bus
Switching via interconnection network
crossbar switch is an interconnection network that connects N input ports to N output ports via 2N bus
Fundamental problems that a router faces
Bandwidth/internet population scaling and services at high speeds
Service differentiation
Routers can offer service differentiation which means different quality of service and service guarantees to different
packets
Prefix match lookup
Way to help with scalability is to group multiple IPs by same prefix
Controlled prefix expansion
Where we expand a given prefix to more prefixes
Multibit tries
Fixed stride
Multibit tries with variable # of strides
We encode stride using a pointer node and the root stays as is
Classless internet domain routing
Assigns IP addresses using arbitrary-length prefixes. Decreased router table size but introduced longest prefix
matching lookup
CS 6250 Quiz 4 Exam Prep (All Quizzes) Latest 2026/2027
Update | 100% Verified Questions & Answers | Georgia
Institute of Technology
2026/2027 | GRAD
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Question:
Describe the relationships between ISPs, IXPs, and CDNs.
Answer
ISPs are organizations that provide a myriad of services related to accessing, using, managing, or participating in the
Internet.
IXPs are interconnection infrastructures that provide the physical infrastructure where multiple networks (e.g., ISPs
and CDNs) can interconnect and exchange traffic locally.
CDNs are networks that content providers create with the goal of having greater control of how the content is delivered
to the end-users while reducing connectivity costs.
Question:
What is an AS?
Answer
An AS is a group of routers (including the links among them) that operate under the same administrative authority. An
ISP, for example, may operate as a single AS, or it may operate through multiple ASes. Each AS implements its own
policies, makes its own traffic engineering decisions and interconnection strategies, and determines how the traffic
leaves and enters its network.
Question:
What kind of relationship does AS have with other parties?
Answer
1. Provider-Customer relationship (or transit): This relationship is based on a financial settlement that determines
how much the customer will pay the provider. The provider forwards the customer's traffic to destinations found in the
provider's routing table (including the opposite direction of the traffic).
2. Peering relationship: In a peering relationship, two ASes share access to a subset of each other's routing tables. The
routes shared between two peers are often restricted to the respective customers of each one. The agreement holds as
long as the traffic exchanged between the two peers is not highly asymmetric.
Question:
What is BGP?
Answer
A protocol that the border routers of the ASes use to exchange routing information with each other.
Question:
How does an AS determine what rules to import/export?
Answer
AS business relationships drive an AS's routing policies and influence which routes an AS needs to import or export.
Exporting - Advertising a route for a destination, to a neighboring AS, means that this route may be selected by that
AS, and traffic will start to flow through. An AS X will advertise routes learned from customers, and routes learned
from providers (but only to X's customers) because those are the only cases where X will likely make money because
more traffic will likely flow through X.
Importing - When an AS receives multiple route advertisements towards the same destination from multiple ASes, it
needs to rank the routes before selecting which one to import. In order of preferred source:
Question:
Customer > Peer > Provider. Why?
Answer
1. An AS wants to ensure that routes toward its customers do not traverse other ASes, unnecessarily generating costs.
2. An AS uses routes learned from peers since these are usually "free" (under the peering agreement).
3. An AS resorts to importing routes learned from providers only when necessary for connectivity since these will add
to costs.
Question:
What were the original design goals of BGP? What was considered later?
Answer
Original goals: - Scalability: As the size of the Internet grows, the same is true for the number of ASes, the number of prefixes in the
routing tables, the network churn, and the BGP traffic exchanged between routers. One of the design goals of BGP is to
manage the complications of this growth while achieving convergence in reasonable timescales and providing loop
free paths. - Express routing policies: BGP has defined route attributes that allow ASes to implement policies (which routes to
import and export) through route filtering and route ranking. Each ASes routing decisions can be kept confidential,
and each AS can implement them independently. - Allow cooperation among ASes: Each AS can still make local decisions (which routes to import and export) while
keeping these decisions confidential from other ASes.
Later consideration: - Security: due to the increase in size and complexity of the Internet, security measures are increasingly desirable.
Question:
What are the basics of BGP?
Answer
A pair of routers, known as BGP peers, exchange routing information over a semi-permanent TCP port connection
called a BGP session. In order to begin a BGP session, a router will send an OPEN message to another router. Then the
sending and receiving routers will send each other announcements from their routing tables.
BGP messages can be 1) UPDATES (advertising new routes or updates to existing routes, or indications that previous
routes are no longer valid) or 2) KEEPALIVE messages to preserve a session.
Question:
What is the difference between iBGP and eBGP?
Answer
iBGP - involves a pair of routers that belong to the same AS
eBGP - invovles a pair of routers in two different ASes
Once a router hears about a route via eBGP, it disseminates that route to other internal (same AS) routers via iBGP.
Question:
What is the difference between iBGP and IGP-like protocols (RIP or OSPF)?
Answer
IGP-like protocols are used to establish paths between the internal routers of an AS based on specific costs within the
AS. In contrast, iBGP is only used to disseminate external routes within the AS.
Question:
How does a router use the BGP decision process to choose which routes to import?
Answer
In the simplest scenario, where there is no policy in place (meaning it does not matter which route will be imported),
the router uses the attribute of the path length to select the route with the fewest number of hops. This simple scenario
rarely occurs in practice.
A router compares a pair of routes by going through the list of attributes, as shown in the figure below. For each
attribute, it selects the route with the attribute value that will help apply the policy. If for a specific attribute, the values
are the same, then it goes to the next attribute.
Example attribute: LocalPref attribute is used to prefer routes learned through a specific AS over other ASes, for
outbound traffic. Assigned by the local AS; higher = more preferred.
Example attribute: MED attribute is used by ASes connected by multiple links to designate which links are preferred
for inbound traffic. Assigned by the neighbor AS; lower = more preferred.
Question:
What are the 2 main challenges with BGP? Why?
Answer
1. Scalability. A large number of updates, overly specific prefixes, etc. can overload router processor/memory capacity.
2. Misconfigurations. Related to the above, misconfigurations can result in excessive route updates, which can lead to
stability and scalability issues.
Question:
What is an IXP?
Answer
IXPs are physical infrastructures that provide the means for ASes to interconnect and directly exchange traffic with
one another.
Question:
What are four reasons for IXP's increased popularity?
Answer
1. IXPs are interconnection hubs handling large traffic volumes
2. An important role in mitigating DDoS attacks
3. "Real-world" infrastructures with a plethora of research opportunities
4. IXPs are active marketplaces and technology innovation hubs
Question:
Which services do IXPs provide?
Answer
1. Public peering
2. Private peering
3. Route servers and Service level agreements
4. Remote peering through resellers
5. Mobile peering
6. DDoS blackholing
7. Free value-added services, like Internet Routing Registry, consumer broadband speed tests, DNS root name servers
Question:
How does a route server work?
Answer
An RS is an IXP entity that makes peering more manageable in the face of a rising number of BGP sessions.
An RS - It collects and shares routing information from its peers or participants of the IXP that connect to the RS. - It executes its own BGP decision process and re-advertises the resulting information (e.g., best route selection) to all
RS's peer routers.
Question:
The Internet topology has been evolving to an increasingly prominent hierarchical structure.
True
False
Answer
False
Increasingly flat, with the rise of IXPs and CDNs
Question:
An Autonomous System operates across multiple administrative domains.
True
False
Answer
False
For two ASes to form a peering agreement, they need to find common ground regarding the internal policies and
traffic engineering approaches that each AS implements.
True
False
Answer
False
A Content Distribution Network (CDN) or an ISP can operate over multiple Autonomous Systems.
True
False
Answer
True
ISP X has the incentive to advertise a route for a customer to a peer AS
True
False
Answer
True
ISPs want to advertise customer routes to as many neighbors as possible so that more traffic flows through the ISP to
the customer
An ISP X has the incentive to advertise the route for its provider's customers to X's peers
True
False
Answer
False
X has no incentive to carry traffic for its provider's routes
ISP X learns multiple routes for the same external destination W. These are advertised from customer C3, peer Y, and
provider P. How would X rank these routes before deciding which one to import?
A. C3 > Y > P
B. Y > C3 > P
C. P > Y > C3
Answer
A.
How does a border router advertise an external route to other routers in the same AS?
A. Using iBGP
B. Using eBGP
C. Using IGP
A.
How does a border router advertise an internal (within the same AS) route to other routers in the same AS?
A. Using iBGP
B. Using eBGP
C. Using IGP
C.
How does a border router advertise an external route to another router in a different AS?
A. Using iBGP
B. Using eBGP
C. Using IGP
B.
Since the BGP path selection process is fixed, an AS has no control over which routes are selected.
True
False
False
e.g. through LocalPref or MED values
An AS can show preference to an external route advertised by one AS over another by assigning a higher LocalPrefer
value to the preferred route.
True
False
True
An AS can show preference for one router for inbound traffic (from neighbors) by assigning that router a lower MED
value.
True
False
True
An AS can use LocalPref to control which routers are used as exit points (for the outgoing traffic), and it can use the
MED attribute to control which routers are used as entry points (for the incoming traffic).
True
False
True
One of the services provided by IXPs is additional security protections such as mitigation of DDoS (Distributed Denial
of Service) attacks.
True
False
True
There are no costs involved for an AS to participate at an IXP.
True
False
False
Since local traffic stays local at IXPs, the IXP infrastructures deal with limited volumes of traffic.
True
False
False
When a large provider or Content Delivery Network joins an IXP, this can act as an incentive for other networks to join
as well.
True
False
True
At an IXP, the members have the choice to peer privately or publicly.
True
False
True
IXPs leading incentive to establish route servers was to charge the participants for using it.
True
False
False
The main incentive for establishing an RS is to make peering more manageable in light of a rising number of BGP
sessions
An IXP route server does not need to run the BGP protocol to facilitate the establishment of multi-lateral peering
sessions.
True
False
False
BGP is core to these peering relationships
For multi-lateral BGP peering sessions at an IXP, the participants have the choice to advertise routes, either directly to
other participants, or to the route server.
True
False
False
I think the idea is that _all_ advertisements pass through the RS server, which applies both import and export filters
to the advertisements.
The Internet topology has been evolving to an increasingly prominent hierarchical structure.
F
An Autonomous System operates across multiple administrative domains.
F
For two ASes to form a peering agreement, they need to find common ground regarding the internal policies and
traffic engineering approaches that each AS implements.
F
A Content Distribution Network (CDN) or an ISP can operate over multiple Autonomous Systems.
T
Consider the figure below that denotes ASes and their relationships.
Note: C1, C2, C3 are customers of ISP-X. ISP-P is a provider of ISP-X.
ISP-X has the incentive to advertise routes for C3 to Y.
T
ISP-X has the incentive to advertise the routes for P's customers to Y and Z.
F
Assume that AS-X learns multiple routes for the same external destination W. These multiple routes are advertised
from C3, from Y and from P. How would AS-X rank these routes before deciding which one to import? - 1) route learned from C3, 2) route learned from Y, 3) route learned from P.
- 1) route learned from Y, 2) route learned from C3, 3) route learned from P. - 1) route learned from P, 2) route learned from Y, 3) route learned from C3.
1) route learned from C3, 2) route learned from Y, 3) route learned from P.
Consider the topology below and the types of BGP sessions the routers form. Select the true statements.
R-A1 <-> R-A3 : iBGP
R-B1 <-> R-A3 : eBGP
R-B3 <-> R-D2 : iBGP
R-D3 <-> R-D2 : eBGP
R-A1 <-> R-A3 : iBGP
R-B1 <-> R-A3 : eBGP
Assume that router R-D2 learns about a route to a destination in AS-A. How would router R-D2 disseminate this route
to R-D3 and R-D1?
Using iBGP.
Using eBGP.
Using IGP.
Using iBGP.
Assume that router R-D2 learns about a route to an internal destination in AS-D. How would router R-D2 disseminate
this route to R-D3 and R-D1?
Using iBGP.
Using eBGP.
Using IGP.
Using IGP.
Assume that router R-B1 learns about a route to AS-C. How would router R-B1 disseminate this route to R-A3?
Using iBGP.
Using eBGP.
Using IGP.
Using eBGP.
Since the BGP path selection process is fixed, an AS has no control over which routes are selected.
True
False
False
Assume that AS-B learns about an external destination both from AS-C and from AS-A. AS-B can show preference to
use the route heard from AS-C by assigning higher LocalPref value to that route.
T
Assume that AS-B advertises the routes to its internal destination to AS-A using the routers R-B1 and R-B4. AS-B can
communicate to AS-A that it prefers R-B1 as an entry point to the network, by assigning lower MED values to these
routes.
T
An AS can use LocalPref to control which routers are used as exit points (for the outgoing traffic), and it can use the
MED attribute to control which routers are used as entry points (for the incoming traffic).
T
One of the services provided by IXPs is additional security protections such as mitigation of DDoS (Distributed Denial
of Service) attacks.
T
There are no costs involved for an AS to participate at an IXP.
T
Since local traffic stays local at IXPs, the IXP infrastructures deal with limited volumes of traffic.
F
When a large provider or Content Delivery Network joins an IXP, this can act as an incentive for other networks to join
as well.
T
At an IXP, the members have the choice to peer privately or publicly.
T
IXPs leading incentive to establish route servers was to charge the participants for using it.
F
An IXP route server does not need to run the BGP protocol to facilitate the establishment of multi-lateral peering
sessions.
F
For multi-lateral BGP peering sessions at an IXP, the participants have the choice to advertise routes, either directly to
other participants, or to the route server.
F
IXP
Interconnection infrastructures which provide physical infrastructure where multiple networks interconnect and
exchange traffic locally
CDNs
Networks created by content providers with the goal of greater control of how it's delivered to end users, reducing
connectivity costs
POPs
1+ routers in a provider network which customer networks can use to connect to provider
Autonomous system
Group of routers including link that operates with same administrative authority
Peering relationship
2 autonomous systems share access to subset of each other's routing tables
What are the design goals of border routing protocol (BGP)?
Scalability, express routing policies, allow cooperation among AS, and security
Express routing policies
BGP has defined route attributes that allow ASes to implement policies such as export/import through route
filtering/ranking
BGP Peers
Exchange routing information over a semi-permanent TCP port called BGP session
2 types of BGP messages
Update and keep alive
BGP update
Contains info about routes that have changed since previous update
BGP announcement
Msgs that advertise new routes, updates existing routes
BGP withdrawal
Msgs that that inform about a previously announced route that is no longer available
BGP Keepalive
Keeps the session going
2 types of BGP Updates
Withdrawal and announcement
BGP Prefix Reachability
Destinations are represented by IP prefixes
eBGP
Sessions b/w border routers of neighboring ASes
iBGP
Sessions between internal routers of the same AS
LocalPref
Attributed used to prefer routes learned through a specific AS over other ASes
MED (multi-exit discirminator)
Used by ASes connected by multiple links to designate which links are preferred for inbound traffic
BGP protocol's 2 limitations are:
Misconfiguration and faults
Peering IXPs
ASes can either peer with one another directly or peer at IXPs which are infrastructures that facilitate peering and
provide more services
Why are IXPs important?
They are interconnection hubs handling high traffic volumes
Important role in DDoS attacks
Real world infrastructures with a plethora of research ops
Active market/tech for innovation hubs
Public peering
2 networks use IXP network infrastructure to establish a connection to exchange traffic based on bilateral relations
and traffic requirements
Private peering
Allow direct traffic exchange between 2 parties, doesn't use IXP public peering infrastructure
Route servers/service level agreement
Many IXPs use service level agreements and free use of IXPs route servers for participants
Remote peering through resellers
Sell IXP ports when they have infrastructure connected to the IXP
DDoS Blackholing
Alleviate effects of DDoS attacks
Multilateral BGP peering session
Route server that facilitates/manages how multiple ASes can talk on control planes simultaneously
Route servers have 2 important route filters:
Import and export filters
Import filters
Ensure that each member of AS only advertises routes that it should advertise
Export filters
Triggered by IXP members themselves to restrict the other set of IXP member ASes that receive their routes
Provider customer/transit relationship
Financial settlement that determines how much a customer will pay provider
ASPath
Attribute for utonomous system number, as an announcement passes through various ASes, identifiers are included in
ASPath
Next Hop
Attribute for router's IP address with interface along the path towards a destination
Flap damping
AS can limit the # of routing changes by explicitly limiting the propagation of unstable routes
CS 6250 Quiz 3 (All Quizzes) Latest Update | Verified
Questions and Correct Answers | Complete Study Guide –
Georgia Institute of Technology
2026/202
...
7 | GRADED A+ | 100 out of 100
Question:
What is the drawback to using the traditional approach of having a single, publicly accessible web server?
Answer:
Geographical distance to all users
Wasteful for a single data center to send the same data over and over
Single point of failure
Question:
What is a CDN?
Answer:
Multiple geographically distributed data centers with copies of content that direct users to server to best serve their
request.
Question:
What are the six major challenges that Internet applications face?
Answer:
Peering point congestion
Inefficient routing protocols
Unreliable networks
Inefficient communication protocols
Scalability
Application limitations
Question:
What are the major shifts that have impacted the evolution of the Internet ecosystem?
Answer:
Increased demand for online content, especially videos
"Topological Flattening"
Question:
Compare the "enter deep" and "bring home" approach of CDN server placement.
Answer:
Enter Deep: Deploy many small networks around the world. Goal is to make distance between user and closest server
as small as possible
Bring Home: Deploy fewer, larger servers in critical areas. Fewer clusters to maintain, but users will experience higher
delay and slower throughput
Question:
What is the role of DNS in the way CDN operates?
Answer:
DNS intercepts requests so that CDNs can choose where to direct users based on location/other conditions
Question:
What are the two main steps in CDN server selection?
Answer:
Map a client to a cluster
Select a server from the cluster
Question:
What is the simplest approach to select a cluster? What are the limitations of this approach?
Answer:
"As the crow flies": Pick the closest geographic cluster
Limitations: actually picking the location closest to the Local Domain Name Server, not the user. The closest
geographical server may also not be the best in end-to-end performance
Question:
What metrics could be considered when using measurements to select a cluster?
Answer:
Network-layer metrics: delay, bandwidth
Application-layer metrics: re-buffering ratio, average bitrate
Question:
How are the metrics for cluster selection obtained?
Answer:
Active measurements: LDNS can probe multiple clusters. Most cannot do this though and it would create a lot of
traffic
Passive measurements: server in CDN can track performance metrics based on current traffic conditions. This would
require a real-time controller with real-time view at all cluster-client pairs
Question:
Explain the distributed system that uses a 2-layered system. What are the challenges of this system?
Answer:
Coarse-grained global layer: Operates at large time scales. Has global view of client quality metrics. Builds prediction
model of video quality
Fine-grained per-client layer: Operates at millisecond timescale. Makes decisions upon client request
Challenges: Hard to design a centralized system with scale of today's networks. Needs data for different subnet pairs,
so some clients need to be routed to sub-optimal clusters
Question:
What are the strategies for server selection? What are the limitations of these strategies?
Answer:
Assign server randomly: might end up assigning server with high workload while lower workload was available
Load-Balancing to route to least-loaded server: not all servers have same content so they'd need to fetch all content for
every server
Question:
What is consistent hashing? How does it work?
Answer:
Tends to balance load by assigning roughly the same number of content IDs, and little movement is needed as nodes
join and leave the system. (uniform circle example)
Question:
Why would a centralized design with a single DNS server not work?
Answer:
Single point of failure
Difficult to handle all volume of querying traffic
Cannot be close to all clients
Updating a huge db for every host on Internet
Question:
What are the main steps that a host takes to use DNS?
Answer:
User host runs client side of DNS application
Browser extracts hostname and passes it to client side of DNS application
DNS client sends a query containing the hostname of DNS
DNS client receives a reply including IP address for the hostname
Host can initiate TCP connection to HTTP server at that port IP
Question:
What are the services offered by DNS, apart from hostname resolution?
Answer:
Mail server/Host aliasing
Load distribution
Question:
What is the structure of DNS hierarchy?
Answer:
Root DNS servers
Top level domain (TLD) servers
Authoritative servers
Local DNS servers
Question:
Why does DNS use a hierarchical scheme?
Answer:
Hierarchy scheme solves the scaling issue
Question:
What is the difference between iterative and recursive DNS queries?
Answer:
Iterative: querying host is referred to a different DNS server in the chain until it can fully resolve the request
Recursive: querying host and each DNS server in chain queries next server and delegates query to it
Question:
What is DNS caching?
Answer:
Once a server receives the DNS reply of mapping any host to IP address, it stores that info in the Cache memory before
sending it to the client
Question:
What is a DNS resource record?
Answer:
Mapping between hostnames and IP addresses
Question:
What are the most common types of resource records?
Answer:
TYPE=A: domain name - IP address
TYPE=NS: domain name - DNS server that can get IP addresses for hosts in that domain
TYPE=CNAME: alias hostname - canonical name
TYPE=MX: alias hostname of mail server - canonical name of email server
Question:
Describe the DNS message format.
Answer:
ID: So client can match queries with responses
Flags: Specifications about DNS message, query, etc
Question: Info about the query like the hostname
Answer: Resource record for hostname that was queried
Authority: Resource records for more authoritative servers
Additional Info
Question:
What is IP Anycast?
Answer:
Route a client to the "closest" server as determined by BGP.
Question:
What is HTTP Redirection?
Answer:
If a client sends a GET request to a server A, it can redirect the client to another server B by sending a HTTP response
with code 3xx and name of new server. Can slow things down but useful for load balancing. Does not require any
central coordination
What is the difference between forwarding and routing?
Answer:
Forwarding refers to transferring a packet from an incoming link to an outgoing link within a single router.
Routing refers to how routers work together using routing protocols to determine the good paths (or good routes as we
call them) over which the packets travel from the source to the destination node
What is the main idea behind a link-state routing algorithm?
Answer:
Link state routing protocols are underpinned by Dijkstra's algorithm. The link costs and network topology are known
to all nodes.
Key equation
D(v) = min(D(v), D(w) + c(w, v))
What is an example of a link-state routing algorithm?
Answer:
OSPF - Open Shortest Path First protocol
Walk through an example of the link-state routing algorithm.
Answer:
Lecture slides.
What is the computational complexity of the link-state routing algorithm?
Answer:
O(N^2)
This is derived from the fact that each iteration, we need to search through the nodes not in N' for the node with
smallest D. This group starts off as size N and decreases by 1 each iteration
N + (N - 1) + .. + 1 = O(N^2)
What is the main idea behind the distance vector routing algorithm?
Answer:
The distance vector routing algorithm is underpinned by the Bellman Ford algorithm. Unlike link state, DV is a
distributed algorithm. Nodes exchange information to update their view of the network.
Key equation:
For node x,
Dx(y) = minv{c(x,v) + Dv(y)} for each destination node y in the network
where minv in the equation is taken over all of x's neighbors v
Walk through an example of the distance vector algorithm.
Lecture slides.
When does the count-to-infinity problem occur in the distance vector algorithm?
A count-to-infinity problem occurs when we have a routing loop e.g. in order to get to x, y routes through z, and z
routes through y.
We saw in the lecture where this happened with a large increase in link cost, that took a long time to propagate among
nodes in the network.
How does poison reverse solve the count-to-infinity problem?
The idea of poison reverse is to lie about a node z's ability to reach a particular other node x, so that other nodes y don't
try to reach x via z. In this manner, we prevent a routing loop.
This technique will solve the problem with 2 nodes, however poison reverse will not solve a general count to infinity
problem involving 3 or more nodes that are not directly connected.
What is the Routing Information Protocol (RIP)?
RIP is a distance vector routing protocol. It uses hop count as a metric (link cost = 1), and messages exchanged are
known as RIP advertisements.
RIP uses UDP to send request and response messages, and is actually implemented as an application-level process.
Some of the challenges with RIP include updating routes, reducing convergence time, and avoiding loops/count-to
infinity problems.
What is the Open Shortest Path First (OSPF) protocol?
OPSF is a link-state routing protocol. OSPF was introduced as an advancement of the RIP Protocol, operating in
upper-tier ISPs. It is a link-state protocol that uses flooding of link-state information and a Dijkstra least-cost path
algorithm.
OPSF involves configuring an AS into areas. One area in an AS is the backbone area.
Routing topologies are communicated between routers in an area with link state advertisements (LSAs).
How does a router process advertisements?
In the context of OSPF: Updating the link-state database, calculating the shortest path using the shortest path first
SPF algorithm., feeding information to the Forwarding Information Base...
What is hot potato routing?
A technique/practice of choosing a path within the network, by choosing the closest egress point based on intradomain
path cost (Interior Gateway Protocol/IGP cost).
This is useful when packets are intended for destinations outside the current network, and we want to get the traffic
out ASAP.
Hot potato routing simplifies computations for the routers as they are already aware of the IGP path costs. It makes
sure that the path remains consistent, since the next router in the path will also choose to send the packet to the same
egress point.
"Routing" and "forwarding" are interchangeable terms.
True
False
False
Consider a source and destination host. Before packets leave the source host, the host needs to define the path over
which the packets will travel to reach the destination host.
True
False
False
Intradomain routing refers to routing that takes place among routers that belong to the same administrative domain.
In contrast when routers belong to different administrative domains, we refer to routing as interdomain routing.
True
False
True
Consider the link-state routing protocol. The link costs are known to all nodes.
True
False
True
As is the network topology
Consider the link-state routing protocol. The network topology is known to all nodes.
True
False
True
As are all link costs.
Consider the link-state routing protocol and a node u as our source node. The goal of the algorithm is to compute the
least-cost paths from the source node u to every other node in the network.
True
False
True
Consider the link-state routing protocol with a node u as our source node. Consider the initialization of the algorithm.
We initialize the least-cost path from node u to directly attached neighbors to be the cost of the direct links, and for
non-directly attached neighbors we initialize the least-cost path to be infinity.
True
False
True
The distance vector (DV) routing algorithm continues iterating as long as neighbors send new updates to each other.
True
False
True
The distance vector (DV) routing algorithm is an example of a ______________ algorithm.
A. decentralized
B. centralized
A.
The distance vector (DV) routing algorithm requires synchronization between routers.
True
False
False
In the distance vector (DV) routing algorithm, each node maintains and updates its own view of the network.
True
False
True
Consider the distance (DV) routing algorithm. Which is used by each node to update the node's distance vector?
A. Dijkstra's algorithm
B. None of the above
C. The spanning tree protocol
D. The Bellman Ford equation
D.
The count-to-infinity problem states that good news (e.g., a decrease in a link cost) propagates slowly among nodes in
the network.
True
False
False
Bad news (increased link cost) propagates slowly.
The poison reverse technique solves the count-to-infinity problem for all network topologies.
True
False
False
Not a general solution.
The Routing Information Protocol (RIP) is based on the Distance Vector protocol.
True
False
True
Open Shortest Path First (OSPF) is based on the link state routing algorithm.
True
False
True
The number of egress points that a network has is upper bounded, e.g., two or three egress points per network.
True
False
False
Consider a network with multiple egress points. Further consider that these egress points offer different paths to the
same external destinations. Then these paths must have different costs.
True
False
False
According to the hot potato routing technique, it is in a network's best interest to route the traffic so that it exits the
network at the router geographically closest to the one from which it entered the network.
True
False
False
The one with the best IGP cost
Assume a source and a destination host. As packets travel over a path from the source host to the destination host the
packets are handled by multiple routers over that path. If these routers belong to different administrative domains,
they need to run the same intradomain routing algorithm, since they are on the same path for that pair of hosts.
True
False
False
What is the difference between forwarding and routing.
We refer to forwarding as the action of transferring a packet from an incoming link to an outgoing link within a single
router.
By routing we refer to how routers work together using routing protocols to determine the good paths over which the
packets travel from the source to the destination node.
Dijkstra's algorithm is an example of what?
A Global, Link State Algorithm
What is the main idea behind link state algorithm
The link state routing algorithm looks to determine the shortest paths (determined by the link costs) between a source
node and all other nodes in the network. The link costs and the network topology are known to all nodes
In the initialization step, we initialize the least cost path to all directly attached neighbors and set all others to infinity.
What is the computational complexity of the link state routing algorithm?
The complexity of the algorithm is in the order of n squared O(n^2). The algorithm searches through n (n+1) / 2
nodes.
What is the main idea behind distance vector routing algorithm?
Each node maintains its own distance vector, with the costs to reach every other node in the network. The neighboring
nodes exchange their distance vectors to update their own view of the network.
Algorithm is Iterative, Asynchronous, distributed, and based on BFA.
When does the count-to-infinity problem occur in the distance vector algorithm?
___ occurs in the distance vector algorithm when the cost of a link increases and two nodes think they can get to a
third node through each other based on their previous outdated costs. This link cost change took a long time to
propagate among the nodes of the network
How does poison reverse solve the count-to-infinity problem?
The way it works is that a node a, that uses node b to get a node c, will tell node b that the cost for its path to node c is
infinity (Da(c)=infinity). Node b assumes that node a has no path to node c except through node b, so it will never send
packets to node c via node a.
Poison reverse helps prevent the count-to-infinity problem only for 2 nodes, it does not solve a general count to
infinity involving 3 or more nodes that are not directly connected.
What is the Routing Information Protocol (RIP)?
is based on the Distance Vector protocol.
The metric for choosing a path could be shortest distance, lowest cost or a load-balanced path.
Routing updates between neighbors are done periodically, using ___ advertisements which contain information about
sender's distances to destination subnets.
Each router maintains a routing table, which contains its own distance vector as well as the router's forwarding table.
Routers send request and response messages over UDP, using port number 520, which is layered on top of network
layer IP protocol.
Some of the challenges with RIP include updating routes, reducing convergence time, and avoiding loops/count-to
infinity problems.
What is Open Shortest Path First (OSPF)?
is a routing protocol which uses a link state routing algorithm to find the best path between the source and the
destination router. intradomain.
How does a router process advertisements?
Process begins when an LS update is received.
a. Every Link State Advertisements (LSA) is unpacked, the OSPF protocol checks whether it is a new or a duplicate
LSA, compared to the link-state (LS) DB.
i.If it is a duplicate, it sends an LS ACK packet back immediately.
ii.If it is new, it updates the LS DB, schedules a Shortest Path First (SPF) algorithm calculation and it determines
which interface the LSA needs to be flooded out of.
Once the LS update packet has been processed, it prepares new LSAs updates into a new LS update packet and sends it
to the next router.
After this, the SPF calculations are computed.
And finally the Forwarding Information Base (FIB) is updated. The information in the FIB is used to decide which
outgoing interface card is the incoming packet forwarded to.
What is hot potato routing?
is a technique/practice of choosing a path within the network, by choosing the closest egress point based on intra
domain path cost (Interior Gateway Protocol/IGP cost).
effectively reduces the network's resource consumption by getting the traffic out as soon as possible.
simplifies computations for the routers as they are already aware of the IGP path costs. It makes sure that the path
remains consistent, since the next router in the path will also choose to send the packet to the same egress point.
What could weights on the graph edges be represented by when we are seeking the least cost path between two nodes?
Cable Length, Time Delay, Monetary Cost, Link Capacity, Current load on the link.
Forwarded
A packet is ___ when it is moved from the routers input link to the appropriate output link.
What is true about the link state algorith?
The number of immediate neighbors a node has will have no impact on the number of iterations required to complete
the algorithm.
Routing loops
Can cause the count to infinity problem.
The Routing Information Protocol (RIP) is an example of
a DV algorithm and intradomain algorithm.
what is Interior gateway protocol (IGP)
a type of protocol used for exchanging routing information between gateways (commonly routers) within an
autonomous system. ___ can be divided into two categories: distance-vector routing protocols and link-state routing
protocols.
Additional details of the Distance Vector Algorithm
Each node maintains and updates its own view of the network (doesn't have the complete picture).
Keeps iterating as long as neighbors send new updates
CS 6250 Quiz 2 (All Quizzes) Latest Update | Verified
Questions and Correct Answers | Complete Study Guide –
Georgia Institute of Technology
2026/202
...
7 | GRADED A+ | 100 out of 100
Questions:
What are 2 reasons computer networks are complex and difficult to manage that led to the development of SDN?
Answer
1. Diversity of network equipment
2. Proprietary technology required for equipment
Questions:
Why would equipment diversity cause complexity when managing a network?
Answer
Different software must be used for different protocols (7)
Questions:
Why do proprietary technologies cause complexity when managing networks?
Answer
Some equipment (router, switch, etc.) run on closed/proprietary software
Configuration interfaces vary as a result.
This causes issues for managing devices centrally. (7)
Questions:
What is the simple idea of SDN that helps make networks more manageable?
Answer
Separate tasks into control plane and data plane. (7)
Questions:
What are the 3 phases of history of SDN?
Answer
1. Active networks
2. control plane and data plane separation
3. OpenFlow API and network operating systems (7)
Questions:
What led to the growth of active networks, and what is their goal?
Answer
1. the tediousness of standardizing new protocols
2. goal is to open up network control with an API that exposes resources and supports customization of functionalities
for subsets of packets passing through network nodes
(7)
Questions:
Why do active networks conflict with idea of E2E?
Answer
They require customization at network nodes (not simplicity at network core)
(7)
Questions:
What were the two types of programming models in active networking? How are thy different?
Answer
1. Capsule model - carries in band data packets - most used in relation to active networking - code distribution ACROSS networks - data plane
2. Programmable router/switch model - out of band mechanisms - decision-making a job for the NETWORK OPERATOR - Difference is where the code to execute at the nodes was carried (7)
Questions:
What are 4 tech developments that encouraged active networking?
Answer
1. reduction in computation cost (more processing can take place in the network)
2. advancement in programming languages
3. advancement in rapid code compilation and formal methods
4. funding agencies promoted interoperability among projects - necessary because there were no short-term use cases
(7)
Questions:
What are 4 problems that encouraged active networking?
Answer
1. developing network services took a long time
2. third party interests to dynamically meeting needs of app and network conditions
3. researches needed a network for large-scale experiments
4. disadvantage of diverse models and vendors meant a desire for unified control over middleboxes (7)
Questions:
What are 3 contributions of SDN?
Answer
1. programmable functions in network = lower barrier to innovation
2. network virtualization and ability to demultiplex based on packet headers
3. vision of unified architecture for middlebox orchestration
(7)
Questions:
Which plane are active networks associated with?
Answer
Data plane (7)
Questions:
What plane do programmable functions in active networks focus on increasing programmability, and why?
Answer - Data plane - isolate experimental traffic from normal traffic (7)
Questions:
What were some downfalls of active networking?
Answer - it was too ambitions - didn't emphasize performance and security - it didn't solve any short term problems - end users need to be ale to write in java (7)
Questions:
Did active network do anything for performance? Security?
Answer
No (7)
Questions:
What challenge did control and data plane separation tackle?
Answer
Challenging how existing routers and switches tightly integrated the data and control plane (7)
Questions:
What were 4 tech developments that paved the way for control and data plane separation?
Answer
1. packet forwarding (data plane) was implemented in hardware (separate from control plane)
2. ISPs couldn't meet increasing demands for greater reliability and new services b/c of increasing scope and size
3. servers have much more memory and processing resources - routing decisions could be done on one server for one
ISP network
4. open-source routing lowers barrier to creating prototypes of centralized routing controllers
(7)
Questions:
How did higher link speeds in backbone networks encourage control and data plane separation?
Answer
packet forwarding implemented in hardware, separate from control plane (7)
Questions:
What 2 innovations led to the control and data plane separation?
Answer
1. open interface between control and data plane
2. logically centralized control of network(7)
Was the network centrally controlled logically, physically, or both under control and data plane separation?
Answer
LOGICALLY centrally controlled(7)
What are 3 ways control plane and data plane separation differed form active networks?
Answer
1. Spurred innovation for network ADMINS instead of users
2. emphasized programmability in CONTROL domain instead of data domain
3. worked towards NETWORK-WIDE visibility and control rather than device level configurations(7)
Was visibility and control at network level or device level for active networks? Control/data plane separation?
Device level
Network level
(7)
Was programmability in control domain or data domain for active networks? Control/data plane separation?
Data domain
control domain
(7)
Was innovation for network admins or users for active networks? Control/data plane separation?
Users
Admins
(7)
What are 5 use cases for control plane/data plane?
1. path selection based on network traffic load
2. minimized disruptions during planned routing changes
3. redirecting or dropping suspected attack traffic
4. allow customer networks more control over network flow
5. offer value added services for virtual private network customers
(7)
Were control plane and data plane separation efforts focused on managing routing WITHIN ONE ISP?
No
some wanted to enable flexible route control across many ASes
(7)
What 2 concepts resulted from control and data plane separation?
1. LOGICALLY centralized control w/ open interface to data plane
2. DISTRiBUTED state management
(7)
Was control plane/data plane separation logically distributed?
No logically CENTRALIZED (7)
Was control plane/data plane state management distributed or centralized?
Distributed (7)
Why was OpenFlow built on existing hardware? What was the weakness of this?
Enabled immediate deployment
Less flexibility
(7)
How does an openflow switch work?
Switch has TABLE OF PACKET HANDLING RULES
each rule has a pattern, list of actions, set of counters, and priority
when packet is received, the switch: - determines highest priority matching rule - performs action - increments counter
(7)
What happens when an openflow switch receives a packet? - determines highest priority matching rule - performs action - increments counter
(7)
Which SDN tech was adopted in the industry: active networks, control/data plane separation, Openflow?
Openflow (7)
What are 3 tech developments that encouraged openflow?
1. programmers could control some forwarding behavior (b/c of switch chipset vendors)
2. companies could build switches without designing their own DATA PLANE
3. early versions of openflow were built on tech that switches ALREADY SUPPORTED - just a firm upgrade
(7)
Were early versions of openflow built from the ground up?
No, built on tech that switches already supported (7)
Was it difficult to integrate openflow? Why?
No, built on already supported tech (7)
What were 3 use cases for openflow?
1. large scale experiments on network
2. managing traffic at large scales in data-center networks
3. investing in programmers to write control programs, and less in proprietary switches that couldn't support new
features
(7)
Did openflow prevent small players from being competitve? Why
No, became more competitive b/c they could just focus on writing control programs instead of proprietary switches
(7)
What are 3 key effects of openflow?
1. GENERALIZATION of network devices and functions
2. Vision of NETWORK OPERATING SYSTEM
3. DISTRIBUTE STATE MANAGEMENT techniques
(7)
What does the control plane do?
Logic that controls forwarding behavior of routers such as ROUTING PROTOCOLS and NETWORK MIDDLEBOX
configurations
DICTATES forwarding behavior
(7)
What does the data plane do?
PERFORMS actual forwarding as dictated by control plane
IP FORWARDING and LAYER 2 SWITCHING
(7)
What are 2 reasons to separate control and data plane?
1. independent evolution and development of both planes - ex. routing algorithms can improve WITHOUT AFFECTING EXISTING ROUTERS
2. Control from HIGH LEVEL software program - SDN uses software to COMPUTE forwarding tables - software to CONTROL router's behavior - DEBUGGING IS EASIER
(7)
What are 4 areas the separation of control/data plane helps?
1. Data centers - network management is easier
2. routing - SDN makes it easier to update state - more control over path selection
3. enterprise networks - SDN improves security applications - drop traffic at strategic locations
4. research networks - research can coexist w/ production networks
(7)
what are 2 important functions of network layer?
1. forwarding - determine which output link a packet should be sent to using forwarding table - LOCAL function implemented in HARDWARE - data plane - nanoseconds
2. determines path from sender to receiver across the network (usually with routing algorithms)
CS 6250 Quiz 1 (All Quizzes) Latest Update | Verified
Questions and Correct Answers | Complete Study Guide –
Georgia Institute of Technology
2026/202
...
7 | GRADED A+ | 100 out of 100
Question:
How did LIcklider and his team in the early 1960s experiment with a precursor to the Internet?
Answer
Connecting two computers over a dial-up telephone line
Question:
What is the DNS designed to do primarily
Answer
Translate domain names into IP addresses
Question:
What is the architecture design of the internet protol stack based on?
Answer
Layers
Question:
Both the data link and transport layer provide error correction
Answer
True
Question:
What allows for communication between the application layer and transport layer
Answer
Socket
Question:
Whis protocol belongs to application layer?
Answer
DNS(Domain Name Service)
Question:
Which two protocols belong to the transport layer?
Answer
TCP, UDP
Question:
When an application sends a packet of information across the network, this packt travels down the IP stack and
undergoes what process?
Answer
Encapsulation
Question:
According to the e2e principle, where should most of the Internet's functionality/Intelligence be implemented
Answer
At the edges of a network
Question:
The difference between hubs, bridges, and routers?
Answer
Hubs: physical layer; bridge: data-link layer; routers:network layer
Question:
What are advantages and disadvantages of a layered architecture?
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.
Question:
What are the differences and similarities of the OSI model and five-layered Internet model?
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.
Question:
What are sockets?
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 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
Question:
Describe each layer of the OSI model.
Answer
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
connected through a physical link. Ex: Ethernet (twisted-pair copper, coax, fiber-optics).
Question:
Provide examples of popular protocols at each layer of the five-layered Internet model.
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
Question:
What is encapsulation, and how is it used in a layered model?
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.
Question:
What is the end-to-end (e2e) principle?
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.
Question:
What are the examples of a violation of e2e principle?
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 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.
Question:
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.
Question:
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.
Question:
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.
Question:
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)
Question:
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.
Question:
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 proces
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