measuring network performance Flashcards

1
Q

what do we mean by the internet is a “best effort” delivery service

A

some packets wont get delivered
the can get reordered or delayed
there is no guarantee on the condition theyll arrive in

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2
Q

store and forward

A

the entire packet must arrive at the router before it can be transferred on the next link

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3
Q

where does reliability over the internet come from

A

protocols

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4
Q

what is a delay and how is it measured

A

the time taken for a packet to travel across the network
measured in fractions of a second

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5
Q

transmission delay

A

the time it takes to push all the bits of the packet into the link
bits have to be pushed one at a time

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6
Q

how do you calculate the transmission delay

A

packet length(bits) / link transmission rate(bps)

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7
Q

queuing delay

A

when work travels faster than it can be processed packets have to be stored in the router buffer until the router is ready to transmit them

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8
Q

packet losses

A

when the packets in the router buffers reach capacity, the router starts to discard them

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9
Q

in which other ways can packet losses occur

A

packets can get lost in the physical medium
hardware and software malfunctions; errors, corruption, failed checksums
attacks

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10
Q

what are the 4 types of delay

A

transmission delay
queuing delay
processing delay
propagation delay

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11
Q

what are the 3 ways that the router buffers choose which packets to drop

A

tail end drop
random drop
quality of service (qos) aware (priority)

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12
Q

random drop

A

buffer drops any packet in the queue

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13
Q

qos aware

A

packets will be dropped according to their priority

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14
Q

what is the benefit of qos aware drop

A

provides fairness
guaranteed throughput for sensitive services e.g. voice calls and live videos

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15
Q

how do dos attacks cause packet losses

A

bombarding a device with packets so its buffers are filled up which can prevent other hosts from sending packets

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16
Q

processing delay

A

the time taken for a device to examine the packets header and decide where to direct it

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17
Q

usually how long is the processing delay

A

usually microseconds bit can vary depending on how busy the device is

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18
Q

propagation delay

A

time needed to travel from one end of the link to another

19
Q

what is propagation delay dependant on

A

the physical type of the link

20
Q

how do you calculate the propagation delay

A

physical link length / link’s propagation speed

21
Q

what is the difference between the propagation speed for wired and wireless links

A

wired; approx 2/3 the speed of light
wireless; v close to the speed of light

22
Q

nodal delay

A

the total of all the different types of delays measured per node

23
Q

throughput

A

the rate in bits/s at which bits are transferred from a sender to a receiver

24
Q

end-to-end delay

A

total of all nodal delays from one end to another

25
round- trip time (rtt)
end to end delay measured in both directions from one end to another and back but doesn't have to be the same route
26
what does the traceroute program do (not how)
it measures delay from the source to each router on the path to the destination and records the rtt for each hop
27
during traceroute, for each router, what does the source do
sends 3 packets/probes to a router that lies on the path to the destination the router returns the packets to the sender the sender measures the lag between the transmission and the reply
28
what does *** mean in tracerotue
no response; router not responding / probe lost
29
what do we use to measure delay
traceroute
30
how do we calculate the traffic intensity (queuing delay)
(l * a) / r = arrival rate of bits / service rate of bits
31
instantaneous throughput
the rate at a given point in time
32
average throughput
rate over a period of time
33
peak throughput
highest instantaneous throughput rate seen so far
34
bottleneck link
link on the end-to-end path that constraints end-to-end throughput
35
what does it mean if traffic intensity is ~ 0
average delay is small
36
what does it mean if traffic intensity is -> 1
average delay is large
37
what does it mean if traffic intensity is > 1
more work is arriving than can be processed average delay is infinite
38
what is usually the bottleneck link
Rc (network capacity) or Rs (sending rate)
39
what do we use to measure throughput
using iperf; provides standardized throughput measures
40
how does iperf normally run (model)
as a client server model
41
how does iperf work
client host requests sample data and the server host serves it which generates a stream of sample data across the network because we know how large the sample data is and how long it takes to retrieve it we can calculate throughput
42
what are some benefits of iperf
v configurable; can use different protocols and data sample sizes can do bidirectional transfer to test both directions and check for inequalities
43
how do we measure loss
ping
44
how does ping work
creates messages to be sent out to specific hosts measures the rtt measures packet loss by keeping track of how many messages were sent and how many responses were received