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ECE438 Homework 2 Solution
• This assignment has a total of 100 points.
• Please write your answer in the white space to the right of the corresponding problem.
1 Choose all that Apply - 3 × 4 points
1. Two distinct Web pages (for example, www.intl.zju.edu.cn/students.html and www.intl.zju.edu.cn/research.html) can be sent over the same persistent connection.
(a) True
(b) False
2. Is it possible for an organization’s Web server and mail server to have exactly the samealias for the hostname(for example, foo.com?)
(a) Yes
(b) No
3. Knowing the alias for the mail server, What type of RR should a DNS client to queryto get the cannonical name for the mail server?
(a) A
(b) NS
(c) CNAME
(d) MX
4. What protocol might be used if a user want to get email from user’s mail server to hislocal PC?
(a) SMTP
(b) POP3
(c) IMAP
(d) HTTP
2 Short Answer Questions - 5 × 2 points
1. Briefly explain the advantages and disadvantages of the use of cookie.
2. Describe how Web caching can reduce the delay in receiving a requested object. WillWeb caching reduce the delay for all objects requested by a user or for only some of the objects? Explain why.
3 Web Caching - 7 x 3 points
Assume a group of students in an institution want to access a private server A outside of the institution. The bottleneck link from the institution to this server supports a bitrate of 2MB/S. Assume the average request rate from the institution is 80 times/s and each request is 0.02MB. Assuming there is no other traffic within or outside of the institution, answer the following questions. Assume that queueing delay dominates so you can neglect the much smaller propagation delays, transmit times, and processing delays
1. What is the average access time for a user in the institution to access this server? Assume the queuing delay is 1/(1-L) milliseconds, where L is the fraction of link usage. (Your answer should be in milliseconds).
2. To improve network performance, we now increase the bitrate of this bottleneck linkto 6MB/s. Calculate the average access time again. Your unit should be milliseconds and must be computed up to 2 decimal places.
3. Another way to improve network performance is to add a cache server within the institution without increasing the bandwidth of bottleneck link. The bitrate to the cache server is 10MB/s. Assume there is a 60% cache hit rate. The queuing delay for both cache server and server A follows the formula in Q1. Calculate the average access time in this case. (Assume the network knows cache server so no additional delays are needed to find that cache server; also, your unit should be milliseconds, computed to 2 decimal places).
4 Traceroute - 4 × 3 points
In the next 2 figures, you will see a series of results from running traceroute (with the -q 1 option to send one probe per hop). For each of the results, please answer the following questions:
1. Which hop(s) (if any) is transoceanic
2. Based on the RTT to the last hop, what’s the furthest away the corresponding servercould possibly be located? (Note: use speed of packet propagation: (2 × 108) m/s.)
3. Sometimes the RTT of a subsequent hop is lower than the RTT of a previous one. Giveone reason for this.
5 HTTP - 7 × 3 points
Suppose a webpage has nothing but 10 large images each of size 10 MB. A client wants to access the webpage and load the images in his browser. The RTT between the client and the server is 40 ms and the transmission rate at the server is 500 MB/s. How long will it take to load the webpage in each of the following cases? (Note: the size of the object for indexing is negligible.) For all answers, please answer in milliseconds, and include the detail
1. Using Non-Persistent HTTP?
2. Using Persistent HTTP?
3. Using Pipelined Persistent HTTP?
6 Client-Server - 7 × 2 points
Think about spreading an F-bit file among N peers using a client-server structure. Let the server have a maximum upload capacity µs, and each client c has a download capacity dc. Assume that the server can serve multiple clients simultaneously and fluidly set the rate for each client rc.
1. Suppose that µs/N ≤ dmin, where dmin = mincdc be the minimum download rate. How would you set the rates rc for each client so that the file is fully distributed to all clients in a minimum time? (i.e., you are minimizing the time that the slowest client receives the file.) What would the distribution time be?
2. Suppose now that µs/N>dmin. How would you set the rates rc now to fully distribute the file to the clients in a minimum time? And what would this time be?
7 DNS - 7 + 3 points
The task requires using the dig command to provide answers. To ensure accurate results, it is recommended to perform these steps from a computer located on a campus network. The user can refer to the dig documentation to understand how to utilize it.
1. Starting from one of the root servers a–m.root-servers.net, perform an iterative lookup for the host www.eecs.mit.edu to get the ip address. For instance, you can initiate the search by using the following command:
dig @h.root-servers.net www.eecs.mit.edu
(1) the domain name of the name server being visited
(2) the IP address of the name server that is currently being used
(3) For how long can you store the results in cache.
2. Can you explain why the DNS protocol tends to utilize UDP rather than TCP?
• Please write your answer in the white space to the right of the corresponding problem.
1 Choose all that Apply - 3 × 4 points
1. Two distinct Web pages (for example, www.intl.zju.edu.cn/students.html and www.intl.zju.edu.cn/research.html) can be sent over the same persistent connection.
(a) True
(b) False
2. Is it possible for an organization’s Web server and mail server to have exactly the samealias for the hostname(for example, foo.com?)
(a) Yes
(b) No
3. Knowing the alias for the mail server, What type of RR should a DNS client to queryto get the cannonical name for the mail server?
(a) A
(b) NS
(c) CNAME
(d) MX
4. What protocol might be used if a user want to get email from user’s mail server to hislocal PC?
(a) SMTP
(b) POP3
(c) IMAP
(d) HTTP
2 Short Answer Questions - 5 × 2 points
1. Briefly explain the advantages and disadvantages of the use of cookie.
2. Describe how Web caching can reduce the delay in receiving a requested object. WillWeb caching reduce the delay for all objects requested by a user or for only some of the objects? Explain why.
3 Web Caching - 7 x 3 points
Assume a group of students in an institution want to access a private server A outside of the institution. The bottleneck link from the institution to this server supports a bitrate of 2MB/S. Assume the average request rate from the institution is 80 times/s and each request is 0.02MB. Assuming there is no other traffic within or outside of the institution, answer the following questions. Assume that queueing delay dominates so you can neglect the much smaller propagation delays, transmit times, and processing delays
1. What is the average access time for a user in the institution to access this server? Assume the queuing delay is 1/(1-L) milliseconds, where L is the fraction of link usage. (Your answer should be in milliseconds).
2. To improve network performance, we now increase the bitrate of this bottleneck linkto 6MB/s. Calculate the average access time again. Your unit should be milliseconds and must be computed up to 2 decimal places.
3. Another way to improve network performance is to add a cache server within the institution without increasing the bandwidth of bottleneck link. The bitrate to the cache server is 10MB/s. Assume there is a 60% cache hit rate. The queuing delay for both cache server and server A follows the formula in Q1. Calculate the average access time in this case. (Assume the network knows cache server so no additional delays are needed to find that cache server; also, your unit should be milliseconds, computed to 2 decimal places).
4 Traceroute - 4 × 3 points
In the next 2 figures, you will see a series of results from running traceroute (with the -q 1 option to send one probe per hop). For each of the results, please answer the following questions:
1. Which hop(s) (if any) is transoceanic
2. Based on the RTT to the last hop, what’s the furthest away the corresponding servercould possibly be located? (Note: use speed of packet propagation: (2 × 108) m/s.)
3. Sometimes the RTT of a subsequent hop is lower than the RTT of a previous one. Giveone reason for this.
5 HTTP - 7 × 3 points
Suppose a webpage has nothing but 10 large images each of size 10 MB. A client wants to access the webpage and load the images in his browser. The RTT between the client and the server is 40 ms and the transmission rate at the server is 500 MB/s. How long will it take to load the webpage in each of the following cases? (Note: the size of the object for indexing is negligible.) For all answers, please answer in milliseconds, and include the detail
1. Using Non-Persistent HTTP?
2. Using Persistent HTTP?
3. Using Pipelined Persistent HTTP?
6 Client-Server - 7 × 2 points
Think about spreading an F-bit file among N peers using a client-server structure. Let the server have a maximum upload capacity µs, and each client c has a download capacity dc. Assume that the server can serve multiple clients simultaneously and fluidly set the rate for each client rc.
1. Suppose that µs/N ≤ dmin, where dmin = mincdc be the minimum download rate. How would you set the rates rc for each client so that the file is fully distributed to all clients in a minimum time? (i.e., you are minimizing the time that the slowest client receives the file.) What would the distribution time be?
2. Suppose now that µs/N>dmin. How would you set the rates rc now to fully distribute the file to the clients in a minimum time? And what would this time be?
7 DNS - 7 + 3 points
The task requires using the dig command to provide answers. To ensure accurate results, it is recommended to perform these steps from a computer located on a campus network. The user can refer to the dig documentation to understand how to utilize it.
1. Starting from one of the root servers a–m.root-servers.net, perform an iterative lookup for the host www.eecs.mit.edu to get the ip address. For instance, you can initiate the search by using the following command:
dig @h.root-servers.net www.eecs.mit.edu
(1) the domain name of the name server being visited
(2) the IP address of the name server that is currently being used
(3) For how long can you store the results in cache.
2. Can you explain why the DNS protocol tends to utilize UDP rather than TCP?
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