CS 6250 MIDTERM EXAM QUESTIONS WITH VERIFIED SOLUTIONS LATEST UPDATE 2026

CS 6250 MIDTERM EXAM QUESTIONS WITH VERIFIED SOLUTIONS LATEST UPDATE 2026 What are the advantages and disadvantages of a layered architecture? - Answers Advantages: - scalability - modularity - flexibility Disadvantages - Some layers functionality depends on the info from other layers which violates the goal of layer separation - One layer may duplicate lower layer functionality - Some additional overhead that is caused by the abstraction between layers What are the differences and similarities between the OSI model and the five-layered Internet model? - Answers Application layer in IP model combines the session, presentation, and application layers in the OSI model What are sockets? - Answers a software endpoint that enables communication between two or more processes running on different devices over a network Describe each layer of the OSI model. - Answers Physical Layer: Deals with the physical transmission of data over the network, including the electrical, mechanical, and physical aspects. Data Link Layer: Provides error-free transmission of data frames between nodes on a network, ensuring reliable communication over the physical layer. Network Layer: Focuses on addressing, routing, and forwarding data packets across multiple networks, enabling internetwork communication. Transport Layer: Manages end-to-end communication, ensuring reliable and error-free data delivery between hosts. It may also provide mechanisms for flow control and congestion control. Session Layer: Establishes, manages, and terminates connections between applications on different devices, allowing for synchronized communication. Presentation Layer: Handles data formatting, encryption, compression, and other transformations to ensure that information from the application layer is properly understood by the receiving application. Application Layer: Provides a user interface and services for applications to access network resources, allowing users to interact with network services such as email, file transfer, and browsing. Provide examples of popular protocols at each layer of the five-layered Internet model. - Answers Physical Layer: - Point-to-Point Protocol (PPP) - Ethernet - Asynchronous Transfer Mode (ATM) - Frame Relay Data Link Layer: - Ethernet - Wi-Fi (IEEE 802.11) - Bluetooth Internet Layer: - Internet Protocol (IP) - Internet Control Message Protocol (ICMP) - Address Resolution Protocol (ARP) Transport Layer: - Transmission Control Protocol (TCP) - User Datagram Protocol (UDP) Application Layer: - Hypertext Transfer Protocol (HTTP) - File Transfer Protocol (FTP) - Simple Mail Transfer Protocol (SMTP) - Domain Name System (DNS) - Secure Shell (SSH) - Simple Network Management Protocol (SNMP) - Post Office Protocol (POP) - Internet Message Access Protocol (IMAP) What is encapsulation, and how is it used in a layered model? - Answers Refers to the process of adding protocol-specific headers and trailers to the data as it moves down the protocol stack and preparing it for transmission over the network. Overall, encapsulation in a layered model ensures that data is properly formatted, organized, and prepared for transmission over the network, enabling effective communication between devices across different layers and protocols. What is the end-to-end (e2e) principle? - Answers design choice that characterized and shaped the current architecture on the internet. Suggests that specific application level functions usually cannot, and preferably should not be built into the lower levels of the system at the core of the network. - the network core should be simple and minimal, while the end systems should carry the intelligence - Original goals were to move functions and services closer to the apps that use them, increases the flexibility and the autonomy of the app designer to offer these services to the needs of the specific application What are the examples of a violation of e2e principle? - Answers - Firewalls and Traffic filters - FWs operate on the periphery of the network and monitor traffic going through. They are intermediate devices operated between two end hosts and can drop end hosts' communication - NAT Boxes - Help us as a bandaid measure to deal with the shortage of internet addresses. It translates the source and destination addresses and ports as packets come into it and go out. The devices behind the NAT box are not globally addressable or routable so it violates the e2e principle What is the EvoArch model? - Answers - helps study layered architectures and their evolution in a quantative manner. - Able to use it to explain how the hierarchical structure of the layer architecture eventually led to the hourglass shape with IPv4, TCP, UDP in the middle Explain a round in the EvoArch model. - Answers Discrete time model that is executed over several rounds. At each round we introduce new nodes and place them at random layers, and examine all layers from the top to the bottom performing the following tasks: 1. Connect the new nodes that we may have just introduced to that layer 2. Update the value of each node at each layer I, given that we may have new nodes added to the same layer I 3. Examine all nodes, in order of decreasing value in that layer and remove nodes that should die 4. Stop execution when the network reaches a given number of nodes What are the ramifications of the hourglass shape of the internet? - Answers the hourglass shape of the internet enables interoperability, flexibility, innovation, and decentralization. However, it also introduces challenges related to diversity, security, and privacy that need to be addressed for the sustainable and secure growth of the network. Repeaters, hubs, bridges, and routers operate on which layers? - Answers Repeaters and Hubs - operate at L1, receive and forward digital signals to connect different Ethernet segments, provide connectivity for hsots on the same network - Advantage - simple and inexpensive devices - Disadvantage - hosts that connect through these belong to the same collision domain meaning they compete for access to the same link Bridges and L2 Switches Operate at layer 2 and are based on MAC addresses. Enable communication between hosts that are not directly connected. - Have finite bandwidth so it will forward packets to a buffer but if it gets overfilled then it can cause packet loss Routers and L3 Switches - operate on L3 What is a bridge, and how does it "learn"? - Answers - a device with multiple inputs and outputs. Transfers frames from an input to one or more outputs - Bridge learns, populates and maintains, a forwarding table. Only forwards frames on specific ports rather than all ports. Bridge learns anytime it receives a frame to know which hosts are reachable through which ports What is a distributed algorithm? - Answers A distributed algorithm is a computational algorithm designed to be executed on a network of multiple interconnected computers or nodes. Unlike traditional algorithms that are executed on a single machine, distributed algorithms are specifically developed to solve problems in distributed computing environments. In distributed systems, multiple nodes work together to achieve a common goal, with each node having its own local memory and computational capabilities. The nodes communicate and coordinate with each other by passing messages over a network. Distributed algorithms leverage this distributed nature to solve complex problems that cannot be easily tackled by a single machine. Explain the Spanning Tree Algorithm. - Answers At each round, each node sends each neighbor a message with a. the sending nodes ID, b. the ID fo the root, and c. the number of hops between the perceived root and the sending node At each round, the node keeps track of the best configuration message. First round, every node thinks it's the root Comparison of messages: --A message is better if: ----The root of the configuration message has a smaller ID ----The roots have equal IDs, but one configuration indicates a smaller distance from the root ----Both root IDs are the same and the distances are the same then the tie is broken by selecting the node with the smaller ID What is the purpose of the Spanning Tree Algorithm? - Answers Purpose: an algo that will exclude links that result in loops What is decapsulation? - Answers Refers to the process of removing protocol-specific headers and trailers from the received data as it moves up the protocol stack What does the transport layer provide? - Answers Provides the logical end to end connection between processes that are running on two hosts What is a packet for the transport layer called? - Answers Segment What are the two main protocols within the transport layer? - Answers TCP/UDP What is multiplexing, and why is it necessary? - Answers the sending host will need to gather data form different sockets and encapsulate each data chunk with header info to create segments and then forward the segments to the network layer -- allows a host to run multiple applications to use the network simultaneously -- Transport layer also uses ports and each application will bind itself to a port by opening sockets and listening for any data from a remote app Describe the two types of multiplexing/demultiplexing. - Answers Connectionless - UDP, contains a two tuple: source port, destination port, if the packet is delivered successfully then destination host will forward the segment to the specific port Connection oriented - TCP, four tuple: source IP, source port, destination IP, destination port. What are the differences between UDP and TCP? - Answers TCP - connection oriented, in order delivery, reliable, flow and congestion control UDP - connectionless, best effort delivery When would an application layer protocol choose UDP over TCP? - Answers Advantages of UDP - Is connectionless that does not require an established connection before sending packets so it has fewer delays and better control over sending data - No congestion control - No connection management overhead - Real time applications that are sensitive to delays might choose UDP - DNS, streaming services, routing protocol, etc typically uses UDP Explain the TCP Three-way Handshake. - Answers -- Client sends special segment with no data with the SYN bit set to 1 and generates an initial sequence number -- Server, upon receiving packet, allocates resources and sends back "connection-granted" segment called SYNACK. Packet has SYN bit set to 1 and header set to initial sequence number +1 and the server also generates a random initial sequence number -- When the client receives the SYNACK, it allocates buffer and resources and then sends an ACK back with SYN bit set to 0, client_isn + 1, server_isn + 1 Explain the TCP connection tear down. - Answers -- Client sends segment with FIN bit = 1 -- Server acknowledges that it receives the clsoing request -- Server sends a segment with FIN bit = 1 indicating the connection is closed -- Client sends ACK for it to the server and waits for some time to resend the ACK incase the first is lost. After some time the connection is closed What is Automatic Repeat Request or ARQ? - Answers if the sender does not receive an ACK within a given period of time, the sender can assume the packet was lost and resend it. Uses acknowledgments and timeouts What is Stop and Wait ARQ? - Answers the sender sends a packet and waits for its ack from the receiver -- Needs to figure out the wait time before resending which can be tricky. Small = unnecessary retransmissions, large = delays -- Low performance What is Go-back-N? - Answers Receiver sends an ACK for the most recently received in order packet then the sender would send all packets from the most recently received in order packet even if some of them have been sent before. Receiver will discard any packets already successfully received What is selective ACKing? - Answers used by TCP. The send retransmits only those packets that it suspects were received in error. Out of order packets are buffered until in order packets have been received. Requires the use of a timeout. What is fast retransmit? - Answers TCP also uses duplicate ACKs as a means to detect packet loss. When the sender received 3 dup ACKs for a packet, it considers the packet to be lost and will retransmit it instead of waiting for a timeout What is transmission control, and why do we need to control it? - Answers Controls the transmission rate to deal with fairness in using the network What is flow control, and why do we need to control it? - Answers Flow Control - controlling the transmission rate to protect the receiver buffer - Protect the receiver buffer from overflowing - LastByteRcvd - LastByteRead = RcvBuffer What is congestion control? - Answers mechanism to control the transmission rate at the sender to avoid congestion in the network Determines how fast each sender should transmit data What are the goals of congestion control? - Answers Efficiency Fairness Low Delay Fast Convergence What is network-assisted congestion control? - Answers rely on the network layer to provide explicit feedback to the sender about congestion in the network What is end-to-end congestion control? - Answers E2E does not provide any explicit feedback about congestion to the end hosts. Instead, the hosts infer congestion from the network behavior and adapt the transmission rate How does a host infer congestion? - Answers The host infer congestion from the network behavior mainly through 2 signals: First is the packet delay. As the network gets congested, the queues in the router buffers build up. This leads to increased packet delays. Thus, an increase in the round-trip time, which can be estimated based on ACKs, can be an indicator of congestion in the network. However, it turns out that packet delay in a network tends to be variable, making delay-based congestion inference quite tricky. Another signal for congestion is packet loss. As the network gets congested, routers start dropping packets. Note that packets can also be lost due to other reasons such as routing errors, hardware failure, TTL expiry, error in the links, or flow control problems, although it is rare. How does a TCP sender limit the sending rate? - Answers TCP uses a probe-and-adapt approach in adapting the congestion window. Under regular conditions, TCP increases the congestion window trying to achieve the available throughput. Once it detects congestion then the congestion window is decreased. Explain Additive Increase/Multiplicative Decrease (AIMD) in the context of TCP. - Answers AI -- Connection starts with a constant initial window and increases it additively one by one every Round Trip Time -- Increment = MSS × (MSS / CongestionWindow) MD -- Once TCP detects congestion it reduces the rate by dividing the number of packets sent by 2 Creates a sawtooth pattern What is a slow start in TCP? - Answers Slow start is called "slow" start despite using an exponential increase because in the beginning it sends only one packet and starts doubling it after each RTT. Is TCP fair in the case where connections have the same RTT? Explain. - Answers If RTT is the same then it is fair - AIMD is fair Is TCP fair in the case where two connections have different RTTs? Explain. - Answers If RTT is not the same then not fair since smaller RTTs would increase their congestion window quicker Explain how TCP CUBIC works. - Answers CUBIC uses a cubed polynomial as its growth function. To maintain the TCP fairness it uses a multiplicative decrease and reduces the window to half. TCP CUBIC is fair regardless of RTT because the calculation that is used depends on the elapsed time from two congestion events, rather than being dependent on the RTT of a connection. Explain TCP throughput calculation. - Answers The calculation is: P = Probability of packet loss MSS = Maximum Segment Size RTT = Round-Trip Times BW = data per cycle / time per cycle BW = MSS/RTT * C / sqrt(P) What is the difference between forwarding and routing? - Answers Forwarding - transferring a paket from an incoming link to an outgoing link within a single router Routing - how routers work together using routing protocols to determine the good paths over which packets travel from the source to destination What is the main idea behind a link-state routing algorithm? - Answers Link costs and the network topology are known to all nodes by broadcasting these values. Goal is to compute the least cost path to every other node from a source node. What is an example of a link-state routing algorithm? - Answers Dijkstra's algorithm What is the computational complexity of the link-state routing algorithm? - Answers O(n^2) complexity What is the main idea behind the distance vector routing algorithm? - Answers an iterative algo, asynchronous, and distributed. Bellman Ford Algorithm, each node maintains its own distance vector with the costs to reach every other node. Dx(y) = minv{c(x,v) + Dv(y)} Walk through an example of the distance vector algorithm. - Answers Dx(y) = minv{c(x,v) + Dv(y)} - calculated for each message received to update the node When does the count-to-infinity problem occur in the distance vector algorithm? - Answers In certain cases, when routers have not yet received the updated routing information, they might incorrectly assume that they can still reach the failed network via a longer path. This can lead to a situation where routers keep increasing the distance/cost value in their routing tables, approaching infinity, as they exchange updates with each other How does poison reverse solve the count-to-infinity problem? - Answers sets a path to infinity to break the loop causing the count to infinity problem and then when the correct line is choosen it will fix the path cost. This will solve the issue with two nodes, but will not solve a general count to infinity problem for 3 or more nodes that are not directly connected What is the Routing Information Protocol (RIP)? - Answers The Routing Information Protocol (RIP) is based on the Distance Vector protocol. The first version, released as a part of the BSD version of Unix, uses hop count as a metric (i.e. assumes link cost as 1). The metric for choosing a path could be shortest distance, lowest cost or a load-balanced path. In RIP, routing updates are exchanged between neighbors periodically, using a RIP response message, as opposed to distance vectors in the DV Protocols. These messages, called RIP advertisements, contain information about sender's distances to destination subnets. What is the Open Shortest Path First (OSPF) protocol? - Answers routing protocol that uses link state routing algo to find the best path between source and destination router. Uses Dijkstra least cost algo. OSPF AS can be configured hierachically and each area can run its own OSPF link state algo in its broadcast area. Exactly one OSPF area in the AS is the backbone to route traffic between other areas in the AS How does a router process advertisements? - Answers The router consists of a route processor (which is the main processing unit) and interface cards that receive data packets which are forwarded via a switching fabric. Let us break down router processing in a few steps: 1. Initially, the LS update packets which contain LSAs from a neighboring router reaches the current router's OSPF (which is the route processor). This is the first trigger for the route processor. As the LS Updates reach the router, a consistent view of the topology is being formed and this information is stored in the link-state database. Entries of LSAs correspond to the topology which is actually visible from the current router. 2. Using this information from the link-state database, the current router calculates the shortest path using the shortest path first (SPF) algorithm. The result of this step is fed to the Forwarding Information Base (FIB) 3. The information in the FIB is used when a data packet arrives at an interface card of the router, where the next hop for the packet is decided and its forwarded to the outgoing interface card. What is hot potato routing? - Answers Hot potato routing is a technique/practice of choosing a path within the network, by choosing the closest egress point(network exit) based on intra domain path cost (Interior Gateway Protocol/IGP cost). 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. Hot potato routing also effectively reduces the network's resource consumption by getting the traffic out as soon as possible. Describe the relationships between ISPs, IXPs, and CDNs. - Answers ISP - provide the backbone of the internet -- Large Global Scale (Tier 1) -- Regional (Tier 2) -- Access (Tier 3) IXP - provide the physical infra where multiple networks can interconnect and exchange traffic locally CDN - netwroks that contenet providers create with the goal of having greater control of how the content is delivered to the end users (Google, netflix) What is an AS? - Answers Autonomous System: a group of routers that oeprate under the same administrative authority Each AS implements its own policies, makes its own traffic decisions, and determines how the traffic leaves its network