Computer Networks I  Assignment 2

EECS 325 Computer Networks I
 Assignment 2
Instructions
This assignment is due on Thursday 20th February, 2020 by 1:00pm EST
Please typewrite your answers and submit a pdf file to Canvas. You should complete this
assignment individually.
There are some extra credits for EECS 325 and 325N students. Credit points are shown
in front of each question. Credits are marked as red for EECS 325/325N, and blue
for EECS 425, if they are different.
Questions
1. (6pts) Consider a web page whose base file is of size S1 = 10 KB. Assume that the
web page consists of N = 20 inline objects each of size S2 = 100 KB. Assume that the
round-trip time to the web server is T = 100 ms and the bottleneck capacity is C = 10
Mbps. Ignore any packetization delays and header overhead.
a. (1pt) Assuming non-persistent HTTP is used with a single TCP connection, how
long does it take to download the web page?
b. (1pt) Assuming non-persistent HTTP is used with 4 parallel TCP connections,
how long does it take to download the web page?
c. (2pts) Assuming pipelined, persistent HTTP is used, how long does it take to
download the web page? ”Pipelined” means requests for multiple objects can be sent
back-to-back on the same connection.
d. (2pts) Assuming non-pipelined, persistent HTTP with 2 parallel TCP connections
is used, how long does it take to download the web page? Assume that the 2 parallel
connections equally share the total available bandwidth C.
2. (8pts) Consider the figure used to illustrate Web caching below. Suppose that the access link capacity is C bits/sec and the average object size is S bits and that the average
request rate from the institution’s browsers to the origin servers is A requests/sec. 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 T seconds
on average. Model the total average response time perceived by the institutional users
as the sum of the average access delay (that is, the delay from the Internet router to
institution router) and the average Internet delay. For the average access delay, use
D
1 − DB
, where D is the average time required to transmit an object over the access
lnik and B is the arrival rate of objects to the access link. Note that this access delay
includes both the transmission and queuing delays. (Note also that B = A if there is
no caching.) Ignore the details of persistent or non-persistent HTTP in this question
as well as any LAN delays.
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EECS 325 Computer Networks I
Spring 2020 Assignment 2
origin
servers
public
Internet
institutional
network 100 Mbps LAN
1.5 Mbps
access link
a. (2pts) What is the total average response time T1?
b. (2pts) Now suppose a cache is installed in the institutional LAN and its hit rate is
p. What is the total average response time T2?
c. We didn’t model DNS delays above. Suppose the root, TLD, and authoritative
name servers are located at round-trip latencies of R1, R2, R3 seconds respectively.
What is the additional delay introduced by DNS if
i) (2pts) the institution maintains a local name server internally in its LAN
ii) (2pts) the local name server in maintained by the institution’s ISP at a round-trip
distance of R0
3. a. (5pts) The DNS root hints file is a file that all DNS resolvers have to be able to bootstrap their resolution abilities. It is located at http://www.internic.net/domain/named.root,
and it has entries for each root server as such:
. 3600000 NS A.ROOT-SERVERS.NET.
A.ROOT-SERVERS.NET. 3600000 A 198.41.0.4
A.ROOT-SERVERS.NET. 3600000 AAAA 2001:503:ba3e::2:30
;
. 3600000 NS B.ROOT-SERVERS.NET.
B.ROOT-SERVERS.NET. 3600000 A 199.9.14.201
B.ROOT-SERVERS.NET. 3600000 AAAA 2001:500:200::b
If you have only this file, how can you use the information in it, and dig, to look up
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EECS 325 Computer Networks I
Spring 2020 Assignment 2
www.facebook.com? List the steps you need to take (along with the answer you might
get), using the syntax dig <name> @<server> <record type> +norecursive.
b. Suppose you run a website at www.foo.net:
i (2pts) Which name servers need to change their databases if you decide to change
the name to www.foo.com?
ii (1pt) To web.foo.net?
iii (2pts) What needs to change if you decide to keep the name www.foo.net, but
change the IP address?
c. (1pt) Explain why using a DNS resolver out on the Internet (i.e., open resolver) can
result in poor choice of CDN servers.
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