CNT 4704: Computer Communication Networks

Fall 2009

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Homework 1

(assigned 09/24; due: 10/01, submitted via webcourse)

1.   The advantage of packet switching vs. circuit switching.  The following figure shows that many users are sharing a 1Mbps access link to the Internet. Suppose each user is either in active status that required data access rate of 100kbps, or in silence status that the user requires no data.  Each user is active only 10% of the time , and users are independent with each other in their activities.




a). If circuit switching is used, how many user can this access link support?
b). If packet switching is used and there are 10 users. How often is this access link idle? How often does this link serve exactly two users?
b). If packet switching is used and there are 35 users, how often does the access link be congested?  (Hint: the link is congested if and only if there are more than 10 users be active at the same time).

  You can leave you answer in terms of an expression (e.g., with sums) to be evaluated (i.e., you need not provide a specific numerical value). 

2. Perform "traceroute" from your computer at home to www.google.com at three different hours of the day (If use Windows, please use tracert).

    a). Copy the three times traceroute report in your homework report.

    b). Find the number of routers in the path at each of the three hours. Did the paths change during any of the hours? Did the traceroute go to different IP addresses of www.google.com?

    c). Try to identify the regions or ISP networks that the traceroute packets pass through from the source to the destination. 

3.  True or false? If a statement is false, explain briefly why it is wrong:
    a).  A user requests a Web page that consists of some text and two images. For this page, the client will send one request message and receive three response messages.
    b).  Two distint Web page (e.g., www.mit.edu/research.html and www.mit.edu/students.html can be sent over the same persistent connection.
    c).   With non persistent connections between browser and origin server, it is possible for a single TCP segment to carry two distinct HTTP request messages.
    d).   The Date: header in the HTTP response message indicates when the object in the response was last modified.

4. Suppose you open a startup company "flashNetwork" and want to set up your company network. Your network has the following servers:

  1. DNS server: "dns.flashNetwork.com" with IP as "128.119.12.40"
  2. Web server:  "flashNetwork.com" with two IP as "128.119.12.55" and "128.119.12.56". Internet users can also access the web server by "www.flashNetwork.com".
  3. Email server: "mail.flashNetwork.com" with IP as "128.119.12.60"

Your company's email address is "username@flashNetwork.com".

a). What resource records (RRs) do you need to provide to the upper-level ".com" Registrar?

b). What RRs do you need to put in your company's DNS server?

5.  Consider the networks shown in the figure below. Assume computers in the institution send out 13 requests per second. Each object average size is 100,000 bits. Also assume the internet side delay of a request is 1 seconds. Using M/M/1 queue to model the access delay in the 1.5Mbps access link. That is to say, the average delay time for an object on the access link is E[T] = 1/(x-y), where y is the arrival rate of objects to the access link and x is the service rate of the access link (i.e., on average, the access link could server x objects per second).

 

a).  Find the total average response time when no institutional cache is used. (Hint: total delay includes Internet delay, access link delay, and LAN delay)

b).  Now suppose the institutional cache is used. The hit rate for the cache is 0.6.  Find the total average response time.