a2 - CSE 489/589Homework Assignment 2 Due at the end of...

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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 16-bit 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 Go-Back-N protocol for a channel with end-to-end delay 30 ms (typical for coast-to-coast). 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 steady-state behavior of TCP. The major question we’d like to answer is: given a certain loss rate, what’s
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This note was uploaded on 12/03/2010 for the course CS 489 taught by Professor Hungngo during the Fall '10 term at SUNY Buffalo.

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a2 - CSE 489/589Homework Assignment 2 Due at the end of...

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