Lecture 8
Performance Measurements and Metrics
Kurose-Ross: 1.2-1.4
(Hassan-Jain: Chapter 3 ”Performance
Measurement of TCP/IP Networks”)
2010-02-17 Sid 2 David Gundlegård, ITN
Outline
¾
Performance Metrics
¾
Performance Measurements
Performance Metrics
Performance Metrics
¾
Asessment of communication network
performance
¾
How good is our network?
¾ (How much better than competitors?)
¾ Classical: Internet access bandwidth
¾
What can it be used for?
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Performance Metrics
¾
Main quantitative metrics for packet switched
networks
¾ Packet loss
¾ Due to congestion and bit errors ¾ End-to-end delay
¾ From sender to (final) receiver ¾ Delay-variation (A.k.a. Jitter)
¾ Variation in delay in a packet session ¾ Throughput (Bandwidth)
¾ Bits per second that can be sent through the network
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Performance Metrics
¾
Other aspects
¾ Fairness ¾ CPU usage ¾ Memory usage ¾ Battery consumption ¾ Response time ¾ Availability ¾ Packet reordering ¾ Number of users ¾ Blocking rate ¾ …¾
Qualitative metrics
¾ Service experience: Good sound, bad picture, slow response…
2010-02-17 Sid 7 David Gundlegård, ITN
Switching and Delay
¾
Circuit switching
¾ Connection establishment delay ¾ Propagation delay ¾ Processing delay (typically small) ¾ Transmission delay¾
Packet switching
¾ Store-and-forward delay ¾ Propagation delay ¾ Processing delay ¾ Transmission delay ¾ Queuing delay2010-02-17 Sid 8 David Gundlegård, ITN
2010-02-17 Sid 9 David Gundlegård, ITN
Link Bandwidth and Delay
¾
Left: high bandwidth
and/or
propagation delay
¾
Right:
low
propagation
delay
and/or
bandwidth
¾
Bandwidth x delay
product
¾
Link utilisation and max
throughput
¾ Rmax=W/RTT
¾ R = datarate
¾ W = window size
2010-02-17 Sid 10 David Gundlegård, ITN
Link Bandwidth and Delay
¾
Sliding Window flow control
¾
Utilisation
¾ Stop and wait
¾ Sliding window ( 2 1) 2 1+ < + = W a a W U a U 2 1 1 + = frame prop t t a=
Delay
¾
Store-and-forward-delay
¾
Prop. to packet size
(L)
¾
In a switch: L/R
¾
Route with Q links:
End-to-end delay (PS)
¾
Store-and-forward: d=QL/R
¾
Propagation: d=distance/velocity
¾ ~3*10^8 m/s (factor 0.7-1 depending on transmission media)
¾
Processing: time for process on every node –
proportional to the number of hops
¾ Error control, routing decisions etc.
¾
Transmission: d=L/R
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Queuing Delay
¾
Queues occur when incoming traffic to a
node (interface) is larger than the forwarding
capacity (for some period of time)
¾
Forwarding capacity VS link/interface capacity
¾
Number of links/interfaces on a node
A
B
C
100 Mb/s Ethernet 1.5 Mb/s statistical multiplexing queue of packets waiting for outputlink
2010-02-17 Sid 14 David Gundlegård, ITN
Queuing Delay
¾
Traffic intensity:
¾ a = average rate that packets arrive to the queue
¾ L = packet length, R = datarate (service rate, capacity)
¾
What happens when La/R>1?
¾ Infinite buffer size?
¾ Finite buffer size?
¾
Analysed with
¾ Queuing theory ¾ Simulation R La = ρ2010-02-17 Sid 15 David Gundlegård, ITN
Queuing Models
¾
M/M/1 with and without priority
¾
The Markov assumption
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End-to-end Delay Metrics
¾
Round trip time (RTT)
¾ The time needed to travel from source to destination, plus the time to travel from destination back to the source.
¾
One way delay
¾ The time needed to travel from source to destination, or from destination to source
2010-02-17 Sid 17 David Gundlegård, ITN
Delay Variation (Jitter)
¾
The end-to-end delay
variation over time
(between packets)
¾
Important in real-time
applications (buffer size
and playout delay)
¾
Defined for a session of
packets (>2)
¾ Max difference in delay
¾ Mean difference in delay
¾ Standard deviation
¾ ...
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Throughput
¾
Number of bits per time unit that can be pushed
through the link/network (A.k.a. bandwidth:
ambiguity…)
¾
Data rate
¾ Often referred to as max transmitting rate at a link
¾
Throughput
¾ The actual data rate at the (final) receiver
¾ Taken into account flow control, bottlenecks, retransmissions, FEC, cross traffic (other users) etc.
¾
Throughput variation
Packet Loss
¾
Percentage of packets lost
¾
Packet loss distribution
¾ Bursty
¾ Uniform
¾
TCP VS UDP
¾
Reasons
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Measurement Tasks
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Data collection
¾ Typically raw data from live network
¾
Analysis
¾ Statistical analysis of data
¾ Use collected data in simulation
¾ …
¾
Presentation
¾ Visualisation through graphs and charts
¾
Interpretation
¾ What do the results tell us?
¾ ”We need new equipment”
¾ ”Ip telephony can/cannot be used in current network”
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Measurement Tools
¾
Monitoring tools
¾
Monitors existing traffic
¾
Ethereal,
Tcpdump, Tcpstat…
¾
Benchmarking tools
¾
Generates traffic used for analysis
¾
Often both client and server needed
¾
TPtest, Iperf
, Netperf, Netpipe, DBS…
¾
Standard tools
¾
Ping, traceroute, netstat
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Analysis Approaches
¾
Live network measurements
¾ Often difficult/expensive
¾ Does the system exist yet?
¾
Lab experiments
¾ Requires generalisations to more realistic conditions
¾
Simulations
¾ Time consuming/validation/verification
¾
Analytical evaluation
¾ E.g. queuing theory
¾ Quick but often many simplifications
¾
Combination
¾ For validation purposes
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Data Analysis
¾
Number of measurements
¾ Purpose of measurements?
¾ Variation in results?
¾
Length of every measurement
¾ Transient behaviour?
¾ Variability?
¾
Assumptions and configurations
¾ Generalisations possible?
¾
Statistical measures
¾ Average, min, max, standard deviation, variance, confidence intervals, hypothesis trial, correlation etc.
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Internet Example
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