measuring network performance

Question 1

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

Answer 1

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

Question 2

store and forward

Answer 2

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

Question 3

where does reliability over the internet come from

Answer 3

protocols

Question 4

what is a delay and how is it measured

Answer 4

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

Question 5

transmission delay

Answer 5

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

Question 6

how do you calculate the transmission delay

Answer 6

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

Question 7

queuing delay

Answer 7

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

Question 8

packet losses

Answer 8

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

Question 9

in which other ways can packet losses occur

Answer 9

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

Question 10

what are the 4 types of delay

Answer 10

transmission delay
queuing delay
processing delay
propagation delay

Question 11

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

Answer 11

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

Question 12

random drop

Answer 12

buffer drops any packet in the queue

Question 13

qos aware

Answer 13

packets will be dropped according to their priority

Question 14

what is the benefit of qos aware drop

Answer 14

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

Question 15

how do dos attacks cause packet losses

Answer 15

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

Question 16

processing delay

Answer 16

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

Question 17

usually how long is the processing delay

Answer 17

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

Question 18

propagation delay

Answer 18

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

Question 19

what is propagation delay dependant on

Answer 19

the physical type of the link

Question 20

how do you calculate the propagation delay

Answer 20

physical link length / link’s propagation speed

Question 21

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

Answer 21

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

Question 22

nodal delay

Answer 22

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

Question 23

throughput

Answer 23

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

Question 24

end-to-end delay

Answer 24

total of all nodal delays from one end to another

Question 25

round- trip time (rtt)

Answer 25

end to end delay measured in both directions
from one end to another and back but doesn’t have to be the same route

Question 26

how does the traceroute program work

Answer 26

it measures delay from the source to each router on the path to the destination and records the rtt for each hop

Question 27

during traceroute, for each router, what does the source do

Answer 27

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

Question 28

what does *** mean in tracerotue

Answer 28

no response; router not responding / probe lost

Question 29

how do we measure delay

Answer 29

using traceroute

Question 30

how do we get the traffic intensity (queuing delay)

Answer 30

(l * a) / r = arrival rate of bits / service rate of bits

Question 31

instantaneous throughput

Answer 31

the rate at a given point in time

Question 32

average throughput

Answer 32

rate over a period of time

Question 33

peak throughput

Answer 33

highest instantaneous throughput rate seen so far

Question 34

bottleneck link

Answer 34

link on the end-end path that constraints end-end throughput

Question 35

what does it mean if traffic intensity is ~ 0

Answer 35

average delay is small

Question 36

what does it mean if traffic intensity is -> 1

Answer 36

average delay is large

Question 37

what does it mean if traffic intensity is > 1

Answer 37

more work is arriving than can be processed
average delay is infinite

Question 38

what is usually the bottleneck link

Answer 38

Rc (network capacity) or Rs (sending rate)

Question 39

how do we measure throughput

Answer 39

using iperf; provides standardized throughput measures

Question 40

how does iperf normally run

Answer 40

as a client server model

Question 41

how does iperf work

Answer 41

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

Question 42

what are some benefits of iperf

Answer 42

v configurable; can use different protocols and data sample sizes
can do bidirectional transfer to test both directions and check for inequalities

Question 43

how do we measure loss

Answer 43

ping

Question 44

how does ping work

Answer 44

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