CS代写 Midterm – Total 80 Points, 42 questions

Midterm – Total 80 Points, 42 questions

1. A stop-and-wait protocol cannot reach 100% channel utilization

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2. With SMTP, it is possible to send multiple mail messages over the same TCP connection

3. Suppose host A is sending a large file to host B over a TCP connection. If the sequence number for one data segment of this connection is m, then the sequence number for the subsequent segment to host B will necessarily be m+1

4. In datagram, packet switching, the order of packets is always maintained during transmission

5. Consider a queue preceding a transmission link of rate R. let a denote the average rate at which packets arrive at the queue and L denote the average length of the arriving packets. Then the queuing delay is limited as long as La < R. 6. The acronym API in our textbook stands for “Application Programming Internet” 7. Consider a queue preceding a transmission link of rate R. Support packets arrive to the queue at one packet for every b seconds. Let L denote the average length of the arriving packets. Then the queuing delay is limited as long as L/b > R.

8. When an email is sent through the Internet, the intermediate IP routers should check the SMTP message header to deliver email to the recipient.

9. Suppose 10 client-server connections traverse a shared core link of rate 5 Mbps. Also suppose that each client’s access link has a maximum rate of 10 Mbps. Then the maximum throughput for each connection is also 10 Mbps.

10. UDP uses header checksum to identify corrupted segments

11. For each web page opened by your computer, there is at least one TCP connection established to the web server.

12. UDP uses header checksums to identity corrupted segments and further request data retransmission.

13. Suppose that host A wants to send some application data over TCP to host B, and host B wants to reply back to host A. Then they must use Two TCP connections – each for data transfer in one direction.

14. A pipelined protocol with a fixed window size of ONE packet CANNOT reach 100% channel utilization

15. 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.

16. It is possible for an application between two hosts to have reliable data transfer even when the application uses UDP transport.

Calculation questions:

New caravan questions:
· Consider sending a 6 Mbit MP3 file form a source host to a destination host. All links in the path between source and destination have a transmission rate of 10 Mbps. Assume that the propagation speed is 2 * 10^7 meters/secs, and the distance between source and destination is 20,000 km.

Select correct answer for each of the following problems.

17. Initially suppose there is only ONE link between source and destination. Also suppose that the entire MP3 file is sent as one packet. Then the transmission delay at the source is
0.6 seconds

18. Referring to the above with same assumptions, the end-to-end delay (transmission delay plus propagation delay) is
1.6 seconds

19. Referring to the above with ONE link connecting source and destination, how many bits will the source have transmitted when the first bit arrives at the destination?
6,000,000 bits

20. Now suppose there are TWO links between source and destination, with one router connecting the two links. Each link is 10,000 km long. Again, assume that 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 from the first link. The end-to-end delay is
2.2 seconds

21. Now suppose that the MP3 file is broken into 3 packets, each of 2 Mbits, transmitted through two links. Ignore headers that may be added to these packets. Also ignore router processing delays. Assuming store and forward packet switching at the router, the total end-to-end delay is
1.8 seconds

C-S vs. P2P
· Shown below are Application Layer slides for Chapter 2 on file distribution time with client-server or P2P approaches. Suppose the upload rate of the server is 10 Mbps and the file is 100 million bits. There are a number of clients to download this file with the same download rate of 10 Mbps in each client, and the same individual upload rate of 1 Mbps if P2P is in use. Find the minimum file distribution time in following questions.

22. With client-server approach, the minimum file distribution time for 100 clients is
Approximately 1000 seconds

23. With P2P approach, the minimum distribution time for 100 peers is
Approximately 100 seconds

24. Again, with client-server approach, the minimum distribution time for 1000 clients is
Approximately 10,000 seconds

25. Now with P2P approach for 1,000 peers, the minimum distribution time is
Approximately 100 seconds

PoolB questions
Suppose within your Web browser you click on a link to obtain a Web page which initially gets a small base HTML file from the server with negligible(zero) transmission time. The IP address for the associated URL is not cached in your local host, so a DNS lookup is necessary to obtain the IP address. Suppose that only onw DNS server, the local DNS cache, is visited with an RTT delay of RTT_0, and the RTT between the local host and the web server containing the object is RTT_HTTP.

26. Given RTT_0 = 2 msecs, RTT_HTTP = 61 msecs, and the base HTML file references 4 very small objects on the same server. Assuming that the client is configured to support a maximum of 5 parallel TCP connections, with non-persistent HTTP, then the respective elapsing time (in msec) that
1) from when the client clicks on the link until the client receives the base HTML file, and
2) from when the client clicks on the link until the client receives all additional objects from the web server
Are calculated as
1) 124 msecs, 2) 246 seconds.

27. Host A sends two segments to host B over a TCP connection, host B sends back two ACKs one when each segment received. Suppose the first ACK is lost, and the second ACK arrives before the timeout timer for the first segment expires. Then,
Host A can/will retransmit neither segments.

28. Data transfer across each TCP connection is
Full duplex

29. SMTP is used
To transfer messages from one mail server to another mail server

30. To build a fully-connected network with N nodes, how many direct links do we need?

31. Suppose a client sends an HTTP request message with the If-modified-since: header. Also suppose the object in server hasn’t changed since the last time the client retrieved the object. Then the server will send a message with the status code
304 Not Modified

32. Suppose a Client needs to download a 5 MB file from a server through a communication link with bandwidth 100 Mbps. Then the file transmission delay is
0.4 seconds

33. Which of the following services does UCP provide?
Application process to application process data delivery

34. When a UDP segment arrives to a host, in order to direct the segment to the appropriate socket, the OS uses
The destination port number

35. A pipelining protocol requires which of the following?
All of the above

36. Suppose there are two links connecting a source host, a router, and a destination host. The first link between the source and the router has transmission rate 100 Mbps and the second link between the router and the destination has transmission rate 10 Mbps. Assuming that the only traffic in the network comes from the sourc, what is the maximum throughput achievable to transfer a file of 100 Mbytes?
10 questions (open the provided wireshark file)

37. Which one below correctly describes the program run at the computer created this trace file?
A client process running at host 192.168.1.102

38. How many HTTP GET requests did the program send out in the trace file?

39. To which Internet address was the (first) HTTP GET request message sent to?
128.119.245.12

40. How many TCP connections in total established to support the HTTP GET request(s) that you can identify from this trace file?

41. Referring to any TCP connection(s) identifiable in this trace file, please further point out its establishment in TCP 3-way handshaking by selecting a set of packet No.’s below.

42. Referring to packet No.10 shown in the Wireshark file, how many bytes of data this HTTP message carries?

43. Referring to packet No.15 shown in the file, how many bytes of data this HTTP message carries?

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