辅导案例-CS 132/EECS
1Midterm - CS 132/EECS 148 Computer Networks - Winter 2020 Instructor Georgios Bouloukakis Full Name: ID: Seat no.: Date: February 13th, 2020 • This is a closed book test. Keep your desk clear (no handouts, phones, notes, books). • Indicate name and UCI ID number in ALL the solution sheets. • You should write all your answer clearly on these pages. Make sure that your final answers and reasons are easily recognizable and are on the provided lines/areas. You must show work for each answer to receive full credit. If you need extra space, use the backs of these pages for any scratch work (indicate on the main page if any/all of your answer is on a back page). Try to solve each problem easily and directly. • In fairness to all students, when time is called, you have to stop writing immediately. • Read each question fully, before writing its answer. When answering, don’t restate the problem; get right to the answer. • Time allowed: 75 minutes. Full Name: PROBLEM 1 (18 POINTS) In the following 4 questions, we are sending a 10 Mbit MP3 file from a source host to a destination host. All links in the path between source and destination have a transmission rate of 5 Mbps. Assume that the propagation speed is 2 * 108 meters/sec, and the distance between source and destination is 5,000 km. (1) Initially suppose there is only one link between source and destination. Also suppose that the entire MP3 file is sent as one packet. The transmission delay is: 50 milliseconds 2 seconds 5 seconds none of the above Solution: The transmission delay is: dtr = LD = 10∗106 5∗106 = 2 sec. (2) Referring to the above question, the end-to-end delay (transmission delay plus propagation delay) is: 2.025 seconds 5.025 seconds 50 milliseconds 2 seconds Solution: The propagation delay is: τ = d s = 5,000Km 2∗108 = 0.025 sec. The end-to-end delay is: dtr + τ = 2 + 0.025 = 2.025 sec. (3) Referring to the above question, how many bits will the source have transmitted when the first bit arrives at the destination. 5,000,000 bits 15,000,000 bits 25,000 bits 125,000 bits Solution: The first bit arrives after τ = 0.025 sec. During that time the source is sending at rate 5Mbps. Therefore, in 0.025 sec it sends 125,000 bits. (4) Now suppose there are two links between source and destination, with one router connecting the two links. Each link is 2,500 km long. Again suppose the MP3 file is sent as one packet. Suppose there is no congestion, so that the packet is transmitted onto the second link as soon as the router receives the entire packet. The end-to-end delay is: 4.025 seconds 10.025 seconds 100 milliseconds 4 seconds Solution: The propagation over one of the links is: τ ′ = 0.0125 sec. The transmission delay is the same dtr = 2 sec. The end-to-end-delay of the packet is now 2(dtr+τ ′ ) = 2∗2.0125 = 4.025 sec. 2 Full Name: PROBLEM 2 (12 POINTS) (1) Cable Internet access is a shared broadcast medium. True False (2) The Internet provides its applications two types of services, a TDM service and a FDM service. True False (3) Consider a queue preceding a transmission link of rate R. Suppose a packet arrives to the queue periodically every 1/a seconds. Also suppose all packets are of length L. The queue will increase without bound and the queueing delay will approach infinity if: La/R < 1 La/R > 1 La/R ≤ 1 La/R ∼ 0 (4) Before sending a packet into a datagram network, the source must determine all of the links that packet will traverse between source and destination. True False (5) With SMTP, it is possible to send multiple mail messages over the same TCP connection. True False (6) Suppose a DNS resource record has Type=MX. Then Value is the hostname of the mail server that has the alias hostname Name Value is the IP address of the mail server that has the alias hostname Name Value is the hostname of the DNS server that is authoritative for Name none of the above (7) With non-persistent connections between browser and origin server, it is possible for a single TCP segment to carry two distinct HTTP request messages. True False (8) A browser will generate header lines as a function of browser type and version user configuration of browser whether the browser has a cached version of the requested object all of the above 3 Full Name: (9) Suppose that the last SampleRTT in a TCP connection is equal to 1 sec. Then Timeout for the connection will necessarily be set to a value >= 1 sec. True False (10) Suppose that host A wants to send data over TCP to host B, and host B wants to send data to host A over TCP. Two separate TCP connections - one for each direction - are needed. True False (11) When a TCP segment arrives to a host, the socket to which the segment is directed depends on: the destination port number the source port number the source IP address of the datagram/packet that encapsulated the segment all of the above (12) The stop-and-wait protocol is highly inefficient when when there is a short distance between source and destination and the transmission rate is low when there is a short distance between source and destination and the transmission rate is large when there is a large distance between source and destination and the transmission rate is high 4 Full Name: PROBLEM 3 – RTT (20 POINTS) 1) HTTP Non-persistent Connection: strictly one web object per TCP connection; HTTP Persistent connection: allowing multiple web objects per TCP connection. 2) HTTP Non-persistent connection: The total transmission time for these 4 objects would be T1 + 2 ∗ T2 + T3. For each object, the round-trip time would be the 3.5RTT (1 RTT for the initialization of TCP connection; 1 RTT for sending the request and receiving the first bit of the response; and 1.5 RTT for closing the TCP connection). However, when the sender sends the ACK bit for closing a connection, the SYN bit for requesting a new TCP connection is also sent at the same time. In addition, we don’t have to consider the last TCP closure time because we must stop counting the time once we receive all 4 objects. Therefore, the total time is 11RTT+T1+2T2+T3. The detailed process can be illustrated by the figure below. In case you have considered the last TCP closure time, the total time would be 11RTT + T1+2T2+T3+1.5RTT = 12.5RTT+T1+2T2+T3 – we will accept this answer as well. 5 Full Name: 3) HTTP Persistent Connection: As already pointed out, an HTTP persistent connection allows to send multiple objects using the same TCP connection. In addition, because Pipelining is used as the default mode in HTTP, the total time is 2RTT+T1+2T2+T3 (without the TCP closure time) or 3.5RTT+T1+2T2+T3 (with the TCP closure time). The detailed process can be illustrated by the figure below. 6 Full Name: PROBLEM 4 (20 POINTS) The flowcharts together with their respective sequence numbers and protocol types are shown below. 7 Full Name: PROBLEM 5 (30 POINTS) Number of packets N = L D = 10000 1000 = 10. Each packets has size P = D + H ' 1000 bits (since the header size is negligible). (a) Transmission of a single packet at source A or at the router: dtr = PW = 1000bits 1.106bps = 10−3sec = 1ms. (b) Please see the corresponding figure. For one packet to be received at the destination, it takes time: 2τ+2dtr = 12ms. For one packet to make it to the destination and for the ACK to be received back at the source it takes time: 2τ+2dtr+2τ = 4τ+2dtr = 4∗5ms+2∗1ms = 22ms. We are now ready to answer the questions. Midterm-16 question1 part(b) R Source A Destination B A Router 1 B *me *me L transmission = P/W Propagation Delay dprop Packet1 … Packet1 ACK 1 τ Packet10 Packet10 ACK 1 0 *me T1:message received First, the message delay requires all first N − 1 packets to be received and acknowledged, and the 10th packet to be received: Message delay = (N − 1) ∗ (4τ +2dtr) + 2τ +2dtr = 38τ +20dtr = 190ms+20ms = 210ms Second, the throughput calculation can be made for each packet or for all 10 packets in a message. For example, it takes time L D (4τ + 2dtr) to receive and acknowledge all N packets. During this time, only the transmission of the message (L bits in total) is useful. Therefore: Throughput = L L D ∗ (4τ + 2dtr) = D 4τ + 2dtr = 1000 22ms = 45.454kbps (c) See figure for the messages exchanged from the time that the first packet starts being transmitted at A until it is correctly received by B. The total time (not required in the answer) is: Timout+ 2dtr + 2τ = 42ms. Note that timer starts at the beginning of transmission. 8 Full Name: (d) The timeout value should be at least as large as the time from starting transmitting a packet until the ACK is received; otherwise the timeout will expire prematurely before the ACK can reach the source.: T ≥ 4τ + 2dtr = 22ms Midterm 16, Question 1, Part (c) R Source A Destination B A Router 1 B *me *me L Dtr = (P)/W Propagation Delay dprop Packet1 … Packet1 ACK 1 τ *me Packet1 9