CS 4310 Computer Networks Fall 2024 Midterm Exam PDF

**CS 4310 Computer Networks** **Fall 2024** **Midterm Exam** **- answers** **Name:** **Note there are 50 multiple-choice questions with 2 points for each question. Good luck!** 1. Which of the following descriptions below correspond to a \"services\" view of the Internet? a. A \"netw...

**CS 4310 Computer Networks** **Fall 2024** **Midterm Exam** **- answers** **Name:** **Note there are 50 multiple-choice questions with 2 points for each question. Good luck!** 1. Which of the following descriptions below correspond to a \"services\" view of the Internet? a. A \"network of networks\". b. A collection of hardware and software components executing protocols that define the format and the order of messages exchanged between two or more communicating entities, as well as the actions taken on the transmission and/or receipt of a message or other event. c. A collection of billions of computing devices, and packet switches interconnected by links. d. An infrastructure for building network applications. 2. Which of the following physical media technologies has the highest transmission rate and lowest bit error rate in practice? e. Coaxial cable f. 802.11 WiFi Channel g. 4G/5G cellular h. Fiber optic cable 3. Which of the characteristics below is associated with the packet-switching technique? i. All needed resources are reserved for a call from source to destination. j. Frequency Division Multiplexing (FDM) and Time Division Multiplexing (TDM) are two approaches for implementing this technique. k. Congestion loss and variable end-end delays are possible with this technique. l. It is commonly used in traditional telephone networks. **The following circuit-switching network applies to the next three questions. There are 4 routers, A, B, C, and D. The number of circuits in each link are indicated in the diagram**. 1. 2. 3. 4. 1. 2. 3. 4. What is the maximum number of connections that can exist in the network at any one time?? a. 23 b. 29 c. 31 d. 60 5. Suppose that every connection requires 2 consecutive circuits, and calls are connected clockwise only. For example, a connection can be A-\>B-\>C or D-\>A-\>B. With these constraints, what is the is the maximum number of connections that can exist in the network at any time? a. 23 b. 29 c. 32 d. 37 1. 2. 3. 4. 5. 6. Suppose that every connection requires 3 consecutive circuits, and calls are connected clockwise only. For example, a connection can be A-\>B-\>C-\>D or C-\>D-\>A-\>B. With these constraints, what is the is the maximum number of connections that can exist in the network at any time? a. 11 b. 12 c. 17 d. 23 7. Which of the following descriptions is correct? a. Propagation delay is the time needed for bits to physically propagate through the transmission medium from end one of a link to the other. b. Queuing delay is the time needed to perform an integrity check, lookup packet information in a local table and move the packet from an input link to an output link in a router c. Transmission delay is the time spent waiting in the packet buffers for link transmission. d. None of the above. 8. Suppose a packet's length is 1,200 bytes (one byte = 8 bits), and link transmits at R = 100 Mbps. What is the transmission delay for this packet? a. 0.0012 secs b. 0.00012 secs c. 0.000096 secs d. 0.00096 secs 9. Consider a packet queuing situation: if the packet arrival rate is 100 packets per second, the packet length is 20k bits, and the link transmission rate is 1 Mbps, what will the queue look like after some time? a. The queue is empty. b. The queue size is very small (1-2 packets in average). c. The queue size varies between 10 and 20 packets on a constant basis. d. The queue becomes extremely large. **Consider the following network scenario for the next two questions. R~S~ = 50 Mbps, R = 300 Mbps, R~C~ = 90 Mbps. Assume that the middle link's capacity R is equally shared among all connections and all servers send packets to clients at a maximum rate.**  10. What is the maximum achievable end-end throughput for each of the four client-to-server pairs? a. 50 Mbps b. 75 Mbps c. 90 Mbps d. 300 Mbps 11. What is the link utilization of the shared middle link? a. 0.50 b. 0.67 c. 0.80 d. 1.0 12. Which of the following defense techniques can be used to effectively detect tampering of message contents? a. digital signature b. two-factor authentication c. firewall d. encryption 1. 13. Which of the following Internet protocol layers provides services to route datagrams from a source host to a destination host? a. application b. transport c. network d. link 14. Which of the definitions below describes what is meant by the term \"encapsulation\"? a. Receiving a "packet" from the layer below, extracting the payload field, and after some internal actions possibly delivering that payload to an upper layer protocol. b. Determining the name of the destination host, translating that name to an IP address and then placing that value in a packet header field. c. Computing the sum of all the bytes within a packet and placing that value in the packet header field. d. Taking data from the layer above, adding header fields appropriate for this layer, and then placing the data in the payload field of the "packet" for that layer. 15. Which of the characteristics below is usually NOT associated with a Client-Server approach to structuring network applications (as opposed to a P2P approach)?? a. A process requests service from those it contacts and will provide service to processes that contact it. b. There is a server with a well-known server IP address. c. HTTP uses this network application structure. d. There is not a server that is always on. 16. Which of the following network applications can tolerate some data loss but require a very low delay like \< 10's msec? a. e-mail b. Web c. interactive games d. streaming audio/video (e.g., Netflix) 17. Which of the following transport layer services is NOT provided by TCP? a. reliable data transfer b. flow control c. congestion control d. throughput guarantee 18. Which of the following is NOT a correct statement for comparing non-persistent HTTP and persistent HTTP? a. With non-persistent HTTP, only one object can be sent over one TCP connection, but with persistent HTTP, multiple objects can be sent over a single TCP connection. b. With non-persistent HTTP, the server closes a TCP connection right after the requested object is sent, while with persistent HTTP, the server keeps a TCP connection open after sending a response. c. Non-persistent HTTP is 'stateless' while persistent HTTP is 'stateful'. d. Non-persistent HTTP uses 2 RTT for fetching each object while persistent HTTP uses 1 RTT. 19. What is an HTTP cookie used for? a. A cookie is a code used by a client to authenticate the client's identity to an HTTP server. b. A cookie is a code used by a server, carried on a client's HTTP request, to access information the server had stored about an earlier interaction with the client. c. A cookie is used to encode a default set of preferences that a client has previously specified for a web site. d. A cookie is used to encode the format of the reply preferred by a client in response to a GET request. 20. What is the purpose of the 'If-Modified-Since' field in a HTTP GET request message? a. To inform the web cache that it should retrieve the full object from the server, and then cache it until the specified time. b. To indicate to the server that the client has cached this object from a previous GET and the time it was cached, and it should not return the object if it has not been modified since the specified time. c. To indicate to the server that the client wishes to receive this object, and the time until which it will cache the returned object in the browser\'s cache. d. To indicate to the server that the server should replace this named object with the new version of the object attached to the GET, if the object has not been modified since the specified time. 21. Which of the following is NOT an advantage of using a web cache? a. Web caching uses less bandwidth coming into an institutional network where the client is located. b. Web caching allows an origin server to more carefully track which clients are requesting and receiving which web objects. c. Web caching generally provides a faster page load time at the clients in average. d. Web caching can help reduce the overall workload of origin servers. 22. Which of the following pieces of information will NOT appear in an HTTP response message? a. status code b. status phrase c. the web server's IP address d. content type 23. Which of the following is NOT a correct statement for the enhancement of HTTP/2 over HTTP 1.1? a. HTTP/2 has many more status codes and header fields than HTTP 1.1. b. HTTP/2 allows the transmission order of requested objects based on a client-specified priority. c. HTTP/2 allows dividing objects into frames and frame transmission interleaved to mitigate HOL blocking. d. HTTP/2 can push an object to a client before the client asks for it. 24. Comparing HTTP and SMTP, which of the following statements is NOT correct? a. HTTP is a 'client pull' protocol, while SMTP is a 'client push' protocol. b. Both protocols have ASCII command/response interaction, status codes. c. HTTP only uses non-persistent TCP connections, while SMTP uses persistent TCP connections. d. HTTP uses port 80, while SMTP uses port 25. 25. Which of the following is NOT a correct statement about the E-mail components and protocols? a. SMTP is the protocol used for two mail servers to send and receive email messages. b. User Agent is the program used to compose, edit, and read messages from its mail server. c. All the emails to be sent are put in the message queue of the 'client' SMTP server first. d. All the emails received by the 'server' SMTP server are forwarded to User Agents immediately. 26. What is the role of an authoritative name server in DNS? e. It provides a list of TLD servers that can be queried to find the IP address of the DNS server that can provide the definitive answer to this query. f. It provides a definitive answer to the query with respect to a name in the authoritative name server\'s domain. g. It provides the IP address of the DNS server that can provide the definitive answer to the query. h. It is a local server that caches name-to-IP address translation pairs, so it can answer authoritatively and quickly. 27. Which of the following is NOT a correct statement about DNS name resolution? i. DNS contains billions of resource records distributed among millions of name servers. j. A hierarchical name resolution is initiated only if the name requested is not found in a local DNS server. k. A DNS query always contains one question and gets one resource record in the reply. l. Recursive query puts more burden on contacted name servers, compared to iterative query. 28. We have learned HTTP browser caching, HTTP web cache, and local DNS server caching. All these caching forms can help a user reduce the time it takes to find an object based on a locally stored copy. Which of the following caching forms can benefit a user not only from its own recent requests but also from recent requests made by other users? m. HTTP browser caching and HTTP web cache n. HTTP web cache and local DNS server caching o. Local DNS server caching and HTTP browser caching p. All three caching forms 29. What information does the type "NS" resource record hold in the DNS database?? q. A domain name and the name of the authoritative name server for that domain r. A hostname and an IP address s. A name and the name of the SMTP server associated with that name t. An alias name and a real name for a server 30. Which of the following is NOT a correct statement for the BitTorrent protocol used for P2P file sharing? a. Peers in a torrent download equal-size chunks of the file from one another, with a typical chunk size of 256 Kbytes. b. Once a peer has acquired the entire file, it must leave the torrent and stop sharing its chunks with other peers. c. 'Rarest first' is a technique used by a peer to prioritize the missing chunks that it requests from its neighbors. d. Every 30 seconds, a peer randomly picks one neighbor not in its top 4 list and sends it chunks to 'optimistically unchoke' this neighbor. 31. In DASH (Dynamic, Adaptive Streaming over HTTP), a server divides a video file into chunks that \... (pick the best completion from below)? u.... are stored, each encoded at multiple rates (video quality). The client receives multiple video chunks (encoded at different rates) and plays out the chunks that best fit the screen size. v.... are download smallest-chunk-first in order to maximize the number of chunks received by the client. w.... are downloaded just before their playout time. Chunking is used primarily because a viewer may jump around (e.g., fast forward) in a video. x.... are stored, each encoded at multiple rates (video quality). The client plays the video chunk-by-chunk, with each chunk requested at encoding rate that fits the available bandwidth at the time. 32. What is the purpose of a manifest file in streaming multimedia? y. To let a client know where it can retrieve different video segments, encoded at different rates. z. To let a CDN server know the video that the client wants to download. a. To allow a client to reserve bandwidth along a path from a server to that client, so the client can view a stream video without experiencing any delay. b. To allow a client to buffer the downloaded video segments to compensate for network delay. 33. What is transport-layer demultiplexing? c. Taking data from multiple sockets, all associated with the same destination IP address, adding destination port numbers to each piece of data, and then concatenating these to form a transport-layer segment, and passing this segment to the network layer. d. Receiving a transport-layer segment from the network layer, extracting the payload, determining the destination IP address for the data, and then passing the segment and the IP address back down to the network layer. e. Receiving a transport-layer segment from the network layer, extracting the payload (data) and delivering the data to the correct socket. f. Taking data from a socket, encapsulating the data with header information -- thereby creating a transport layer segment -- and passing this segment to the network layer. 34. Which of the following is NOT a correct statement for multiplexing / demultiplexing using TCP or UDP? g. It is possible for two TCP segments with source port 80 to be sent by a server to different clients. h. It is possible for two UDP segments with source port 5723 to be sent from a server to different clients. i. It is possible for two TCP segments with destination port 8374 to be sent by two clients to different processes at a server. j. It is possible for two UDP segments with destination port 6481 to be sent by different clients to different processes at a server. 35. Which of the data fields below is NOT in a UDP segment header? k. Destination IP address l. Destination port \# m. Length of the UDP segment n. Checksum 36. What is a cumulative acknowledgment, ACK(n)? o. A cumulative ACK(n) allows the receiver to let the sender know that it has not yet received an ACK for packet with sequence number n. p. A cumulative ACK(n) acknowledges to the sender that all packets with a sequence number up to and including n have been received. q. A cumulative ACK(n) allows the receiver to let the sender know that it has not received any packets with a new sequence number since the last cumulative ACK(n) was sent. r. A cumulative ACK(n) allows the receiver to let the sender know that it has received all packets with sequence number up to n-1 and expects the next packet to have a sequence number n. 37. Which of the following statements about pipelining is NOT true? s. Pipelining can effectively reduce the network delay of sending packets. t. Pipelining can effectively increase network utilization. u. With a pipelined sender, there may be transmitted packets "in flight" -- propagating through the channel -- packets that the sender has sent but that the receiver has not yet received. v. Pipelining may require the sender and the receiver to buffer packets that have not been acknowledged or received in order. 38. What is the real reason for discarding received but out-of-sequence packets at the receiver in Go-Back-N? w. Discarding out-of-sequence packets will really force the sender to retransmit. x. Keeping out-of-sequence packets may simplify the packet consolidation after the in-order packet is received later. y. The sender will resend the packet that was not received and other higher sequenced packets later anyway. z. If some packets are in error, then it's likely that other packets are in error as well. 39. Which of the following statements about reliable data transfer mechanisms is NOT true? a. Sequence number allows for duplicate detection at the receiver. b. Timer allows for retransmitting a packet, possibly because the packet (or its ACK) was lost within the channel. c. Negative acknowledge lets the sender know that a packet was not received correctly at the receiver. d. Checksum allows detection and recovery of bit errors in a transmitted packet. 40. Which of the following statements about TCP is NOT true? e. TCP provides a full-duplex service, which means bi-directional data between client and server can flow in a single connection. f. TCP views data as an unstructured, but ordered, stream of bytes. g. TCP supports "multicasting" which means the transfer of data from one sender to many receivers in a single send operation. h. TCP runs only in the end systems and not in the intermediate network elements (routers and link-layer switches). 41. Consider the TCP Telnet scenario below. Which of the following statements about the Seq and ACK fields of segment 2 or segment 3 is correct? i. segment 2: Seq = 80, ACK = 45 j. segment 2: Seq = 82, ACK = 47 k. segment 3: Seq = 47, ACK = 80 l. segment 3: Seq = 47, ACK = 82 42. Consider the TCP scenario below. What timer-related action does the TCP sender take on the receipt of ACK 120?  m. Cancels any running timers. n. Restarts a timer for the segment with sequence number 92. o. Leaves any currently running timers running. p. Starts a timer for the next segment before it is available. 43. Which of the following statements about 'TCP fast retransmit' is true? q. It is an approach used by TCP to increase the number of segments sent but unacknowledged. r. It is an approach used by TCP to retransmit a segment after receiving 3 duplicate ACKs before that segment's timer expires. s. It is an approach used by TCP to retransmit a segment after receiving 3 duplicate ACKs after that segment's timer expires. t. It is an approach used by TCP to ignore the ACKs for the segments received not in order. 44. Which of the following statements about TCP flow control is NOT true? u. It is a mechanism used to eliminate the possibility of the TCP sender overflowing the TCP receiver's buffer. v. The receive window, rwnd, is set to the amount of spare room in the receiver's buffer. w. The receiver places its current value of rwnd in the receive window field of every segment it sends to the sender. x. The value of rwnd never changes throughout the duration of a connection. 45. Consider the figure below, which shows the application-to-application throughput achieved when two senders are competing at a shared bottleneck link. Suppose that when the overall arrival rate  λ~in~' (for each sender) is close to R/2, the throughput to the application layer λ~out~ (at each receiver) is equal to 0.8 \* λ~in~', what fraction of the packets transmitted at the sender are retransmissions? y. 0 z. 20% a. 25% b. 80% 46. Which of the following actions is used in network-assisted congestion control (versus end-end congestion control) to signal congestion? c. A router drops a packet at a congested router. d. The transport-layer receiver informs the sender of the size of its receive window. e. A router sends an explicit message to a host telling it to slow down its sending rate. f. A sender decreases its sending rate in response to packet loss detected via timeout. 47. Which of the following statements about TCP Additive-Increase-Multiplicative-Decrease (AIMD) algorithm is NOT true? g. AIMD is an end-end approach to congestion control. h. AIMD uses the measured RTT delay to detect congestion. i. AIMD cuts the congestion window size, cwnd, to 1 whenever a timeout occurs. j. AIMD cuts the congestion window size, cwnd, in half whenever loss is detected by a triple duplicate ACK. 48. Which of the following statements about TCP Slow Start and Congestion Avoidance is NOT true? k. TCP Slow Start increases cwnd at an exponential rate every RTT. l. TCP Slow Start transitions to Congestion Avoidance when cwnd reaches ssthresh which is set to 1/2 of cwnd just before the last loss event. m. TCP Congestion Avoidance never falls back to Slow Start. n. In the absence of loss, TCP Slow Start increases the sending rate faster than TCP AIMD. 49. Which of the following statements about Delay-based TCP congestion control is NOT true? o. It proactively detects congestion onset before packet loss occurs. p. It attempts to maximize the throughout while keeping the delay low. q. It stops updating cwnd when the currently measured throughput is very close to the uncongested throughput (cwnd/RTT~min~). r. It decreases cwnd linearly when the currently measured throughput is much less than the uncongested throughput (cwnd/RTT~min~). 50. Which of the following statements about QUIC (Quick UDP Internet Connections) is NOT true? s. QUIC performs congestion control and error recovery differently from TCP, and therefore has better performance than TCP. t. QUIC can establish all connection parameters (security, reliability, flow and congestion control) in just one handshake rather than two handshakes. u. QUIC allows multiple application-level "streams" multiplexed over a single QUIC connection and each stream has separate reliable data transfer and security. v. As an application-layer protocol, QUIC can be updated at "app frequency" rather than at the frequency of operating system updates like TCP or UDP.