CSE 489/589Homework Assignment 2
Due at the end of class on Monday, Oct 18.
*
October 15, 2010
Conventions:
•
bps is bits per second
•
Bps is Bytes per second or 8 bits per second
•
1
kilo =
10
3
(forget
1024
)
•
1
mega =
10
6
•
1
giga =
10
9
Problem 1
(Birthday attack)
.
To poison the cache of a DNS server, in the old days the attacker only
needed to match the Query ID, which is a 16bit number. Let
n
= 2
16
be the size of this ”sample space”.
The attacker sends
m
DNS requests (for the same domain), which will trigger
m
DNS queries with
different (presumably randomized) IDs being sent, and then the attacker sends
k
faked replies whose IDs
are chosen randomly in the set
[
n
] =
{
1
,
· · ·
, n
}
. The probability that no (faked) reply has a matching
ID with a query was shown in class to be
(
1

m
n
)
k
≤
e

mk/n
. So the attacker’s failure probability can
be approximated to be
e

mk/n
.
Generally, the attacker wants to minimize the total number of packets he has to send (that’s
m
+
k
)
and mimimize the chance of failure too. For
n
= 2
16
, to guarantee at most a
1
/
1000
chance of failure,
what’s the minimum number of packets he has to send? (Use
e

mk/n
as the failure probability.)
Repeat the quesiton for
n
= 2
32
, which is the case with randomized port numbers and Query IDs.
Problem 2.
Suppose we run the GoBackN protocol for a channel with endtoend delay
30
ms (typical
for coasttocoast). Suppose the window size (in bytes) is
2
16
bytes. (This number is indeed the max
imum TCP window size without window scale option). The channel is perfectly reliable with bit rate
10
Gbps. What is the maximum utilization theoretically achievable? (Assume ACK transmission time is
negligible.)
Problem 3.
In this problem, we attempt to derive a highly simplified “macroscopic” model for the
steadystate behavior of TCP. The major question we’d like to answer is: given a certain loss rate, what’s
the maximum throughput TCP (Reno) can achieve. For simplicity, we will assume that all losses are
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 Fall '10
 hungngo
 Networking, Bit rate, Channel capacity, Transmission Control Protocol, Parity bit, Measuring network throughput, window size

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