assignment 1 - nus computing
TRANSCRIPT
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Assignment 1
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Collect TracesCompute Throughput/Loss RateCompare Different Scenarios
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How to tell if a packet is lost fromreceiver’s TCP trace?
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A lost packet +retransmission leads toout-of-order packets(most of the time)
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out-of-order packetdoes not imply
loss+retransmission
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what triggerretransmission?
timeout or 3 dup ACKs
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are there 3 dup ACKs asking for seqno X?
if yes, it’s a fast retransmissionseqno=X
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If not, is t large enough?
t
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if t is large (>=3ms in Wireshark)the packet is a retransmission, else
it’s an out-of-order packet
t
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A lost packet leads toretransmission
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But retransmission doesnot imply packet loss
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did we receive anotherpacket with same seq no?
if yes, the packet is not lost
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OK, we have to stopsomewhere..
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Loss packets:retransmitted packets
without duplicates
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upper or lower bound ofthe actual loss rate?
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Some lost packets are notcounted
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Some counted packets arenot lost (just out-of-order)
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Other Methods
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Count number of timeswindows is halved
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Use TCP throughputEquation
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Collect TracesCompute Throughput/Loss RateCompare Different Scenarios
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The ExpectedWired > Wireless
Strong Signal > Weak SignalNUSOPEN > NUS
Evening > Day (at work)
Day > Evening (at home)
Starhub’s service varies (640-1350Kbps)
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The UnexpectedWireless@SG is excellent
(~DSL, 350kbps, no loss)
Boon Lay Hostel not too good(40 kbps, 10% loss)
I2R off-peak 1500kbps !
M1 broadband day > evening
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Interesting FindingsNUS VPN uses small Wmax?RTT is as small as 2ms (bug?)
Faster “slow start”?
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Date: 07/12/2007Time: 7:30 PMDuration: 2 hLocation: SR3A/BOpen Book: YAnswer Book: Y
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Final Exam ReminderBring the papersBring calculator
Lecture 1-12 (except 9)
Date: 07/12/2007Time: 7:30 PMDuration: 2 h Location: SR3A/BOpen Book: YAnswer Book: Y
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Assignment 2
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Max threshold increases,drop probability reduces,queue length increases
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Min threshold increases,queue length increases
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Q weight increases,avg Q length fluctuates,
drop more
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