Description
Introduction
In this lab, we will do some practice questions on calculating the performance of application layer
protocols. We will also use the Wireshark to sniff packets and examine the HTTP protocol. All work
must be shown for marks. This lab should be electronically submitted on Canvas.
Warm-up Problems: The authors of the text have provided interactive problems to help you with
understanding the concepts presented in chapter 2. Please take the time to review these problems before
attempting the questions. These problems are for your own learning and practice (no marks) but will
allow you to review solutions.
a) HTTP GET message: http://gaia.cs.umass.edu/kurose_ross/interactive/http-get.php
b) HTTP RESPONSE message: http://gaia.cs.umass.edu/kurose_ross/interactive/http-response.php
c) DNS and HTTP delays: http://gaia.cs.umass.edu/kurose_ross/interactive/DNS_HTTP_delay.php
Textbook Questions
Answer the following questions from the textbook:
Question 1) R9. Recall that TCP can be enhanced with SSL (TLS is the upgraded version of SSL) to
provide process-to-process security services, including encryption. Does SSL operate at the transport layer
or the application layer? If the application developer wants TCP to be enhanced with SSL, what does the
developer have to do? (5 marks)
Question 2) R26. In Section 2.7, the UDP server described needed only one socket, whereas the TCP
server needed two sockets. Why? If the TCP server were to support n simultaneous connections, each
from a different client host, how many sockets would the TCP server need? (10 marks)
Question 3) P9. Consider Figure 2.12, for which there is an institutional network connected to the Internet.
Suppose that the average object size is 850,000 bits and that the average request rate from the institution’s
browsers to the origin servers is 16 requests per second. Also suppose that the amount of time it takes
from when the router on the Internet side of the access link forwards an HTTP request until it receives the
response is three seconds on average (see Section 2.2.5).
Model the total average response time as the sum of the average access delay (that is, the delay from
Internet router to institution router) and the average Internet delay. For the average access delay, use Δ/(1
– Δ𝛽), where Δ is the average time required to send an object over the access link and 𝛽 is the
arrival rate of objects to the access link. (15 marks)
a) Find the total average response time.
b) Now suppose a cache is installed in the institutional LAN. Suppose the miss rate is 0.4. Find the
total response time.
© Fatemeh H. Fard. Not to be copied, used, or revised without express written permission from the copyright owner.
Version: September 2020 Page 2 of 3
Question 4) P22. Consider distributing a file of F = 15 Gbits to N peers. The server has an upload rate
of us = 30 Mbps, and each peer has a download rate of di = 2 Mbps and an upload rate of u. For N = 10
and 1,000 and u = 300 Kbps and 2 Mbps, prepare a chart giving the minimum distribution time for each
of the combinations of N and u for both client-server distribution and P2P distribution. (15 marks)
TCP vs UDP (6 marks)
Question 5) Indicate if TCP, UDP, or neither should be used given the following transmission
requirements:
a) Reliable data transfer
b) Minimum transmission overhead
c) Guaranteed transmission rate
d) Bounded limits on packet delay
e) Guarantee in-order delivery of data
f) No transmission setup time
HTTP Request and Response (4 marks)
Question 6) Given the following HTTP request, answer the following questions:
GET /index.html HTTP/1.1
Accept: */*
Accept-Language: en-ca
User-Agent: Mozilla/5.0 (Windows NT 6.1; rv:2.0.1) Gecko/20100101 Firefox/4.0.1
CLR 2.0.50727; InfoPath.1)
Host: localhost:6789
Connection: Keep-Alive
a) What is the full URL of the requested page?
b) Is the browser requesting a persistent or non-persistent connection? How do you know?
Persistent versus non-Persistent Connections (15 marks)
Question 7) Assume you have a HTML file of size 10,000 bits. The HTML file has 4 linked images each
of size 50,000 bits. One of the linked images is on a different server machine. To simplify the discussion,
assume that the round trip time (RTT) from your browser to either server is 100 ms, and the “link” between
browser and server is a 10 Mbps link. Answer these questions:
a) What is the total time to receive the HTML file and all images using non-persistent connections?
b) What is the total time using non-persistent connections but the browser can open up any number
of parallel connections?
c) What is the total time using persistent HTTP with no pipelining and with pipelining? Assume in
both cases the browser can open any number of parallel connections.
© Fatemeh H. Fard. Not to be copied, used, or revised without express written permission from the copyright owner.
Version: September 2020 Page 3 of 3
Caching
Question 8) Given the network diagram below, answer the following questions. Assume that the
transmission delay for HTTP GET/RESPONSE and DNS REQUEST/RESPONSE messages is so small
that it can be ignored. Transmission delays are only calculated when sending the data file. Both routers,
R1 and R2, use store and forward. The transmission delays on the LANs must be calculated, but
propagation delays are ignored. The round-trip time from any machine in a.com to b.com is 50 ms. The
round-trip time from any machine in a.com to the Internet is 100 ms. Answer these questions: (15 marks)
a) Give the sequence and timing of messages sent when m1.a.com requests a 1 Mbit file from
www.b.com. Assume the DNS record and the HTTP file are not cached locally. Also assume that
the top-level domain is not cached.
b) Give the sequence and timing of messages sent when m2.a.com requests the same 1 Mbit file
assuming the DNS records and file have been cached locally.
DNS Wireshark Lab (15 marks): Follow the instructions for the Wireshark lab from your textbook
found here: http://www-net.cs.umass.edu/wireshark-labs/Wireshark_DNS_v7.0.pdf. Provide answers to
the first 15 questions for this lab.
