EECS122

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EECS122 Lecture 5

• Department of Electrical Engineering and Computer
  Sciences
• University of California
• Berkeley

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TOC Network Performance

• Motivation
• Timing Diagrams
• Metrics
• Evaluation Techniques

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Performance Motivation

• Understanding Network Behavior
• Improving Protocols
• Verifying Correctness of Implementation
• Detecting Faults
• Choosing Provider
• Feasibility of Applications
• Monitoring Service Level Agreements
• Billing

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Performance Timing

• Sending one packet
• Illustration
• Examples
• Queuing
• Queuing example
• Store and forward
• SFMultiple packets
• Cut-Through
• Fluid View

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Timing One Packet
R bps
T seconds
P bits
T
P/R
Time
P/R Transmission time T Propagation time
L/speed L(km)x(time/km)
Time/km 3.3ms in free space 4
ms in copper 5 ms in fiber
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Timing Illustration
Transmission Line
Transmitter ?
Receiver
Signal ?
One bit
Each bit takes 1/R seconds to be transmitted The
bits take T seconds to propagate
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Timing Examples
P 1KByte R 1Gbps 100km, fiberT 500ms P/R
8ms
T gtgt P/R
T
P/R
T
P 1KByte R 100Mbps 1km, fiberT 5ms P/R
80ms
P/R
T ltlt P/R
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Timing Queuing

• Link

R bps
T seconds
P bits
Q
Q/R
T
Q/R queuing delay (load-dependent)
P/R
Time
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Timing Queuing Example

• Link

1-kbit packets R 1Mbps
1ms
Time
T(t) Q/R P/R for a packet that arrives at t
1ms
Time t
Let Tn Q/R P/R for packet n
T1 1ms T2 1.5ms
T3 2ms T4 1ms
T5 1.5ms T6 1ms
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Timing Store and Forward

• System

10Mbps
5Mbps
100Mbps
10Mbps
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Timing SF - Multiple

• System

10Mbps
5Mbps
100Mbps
10Mbps
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Timing Cut-Through

• System

R1 10Mbps
R2 10Mbps
Header
Start forwarding as soon as the header is
received
Note What if R2 gt R1?
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Timing A Fluid View
Rate R

• System

A(t) rate a(t)
D(t) rate d(t)
X(t)
bits in 0, t
bits in 0, t
a(t)
d(t)
X(t)
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Performance Metrics

• Throughput
• Delay

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Metrics Throughput

• Definitions
• Example 1 Connection
• Example 2 Link
• Fluctuations
• Measurements

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Throughput Definitions

• Roughly throughput bit rate (e.g.,
  120Kbits/second)
• More precisely
• Throughput of a connection or of a link total
  number of bits during some period t, t T
  divided by T
• Bandwidth of a link link rate bit rate of
  the transmitterNote misnomer, but common usage
• Utilization of a link throughput of the link /
  link rate
• Bit rate units 1Kbps 103bps, 1Mbps 106bps,
  1Gbps 109bpsFor memory 1Kbytes 210 bytes
  1,024 bytes 1MBytes 220 bytes
• Some rates are expressed in packet per second
  (pps)? relevant when the bottleneck is the
  header processing

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Throughput Connection

• Connection
• Send W bits (window size)
• Wait for ACKs
• Repeat
• Assume that the round-trip time is RTT seconds
• Throughput W/RTT bps
• Numerical Example
• W 64KBytes 512 kbits 512x1,024
  524,288 bits
• RTT 200ms
• Throughput W/T 2.6Mbps

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Throughput Link

• 1Mbps link sends 1,000-bit packets

Rate every ms
Mbps
1
Rate every 40 ms
0.5
Utilization 50
Time t
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Throughput Fluctuations

• Rate varies over time

Throughput over last T seconds
max
mean
min
Time t
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Throughput Measurements

• TCP Keep track of number of bytes receivedLet
  R(t) number of bits in 0, tThroughput over
  last T seconds R(t) R(t T)/T
• Link Device has counter with number of bytes
  received calculate as above

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Metrics Delay

• Definitions
• Illustration 1
• Illustration 2
• Littles Result
• Measurements Example 1
• Measurements Example 2

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Delay Definitions

• Delay/Latency of bit (packet, file) from A to B
• The time required for bit (packet, file) to go
  from A to B
• Jitter
• Variability in delay
• Round-Trip Time (RTT)
• Two-way delay from sender to receiver and back
• Bandwidth-Delay Product
• Product of bw and delay, indicates storage
  capacity of network

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Delay Illustration 1
S
D
Latest bit seen by time t
at point 1
at point 2
n
Delay of bit n
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Delay Illustration 2
Max delay 100 ms Min delay 40 ms Jitter 60
ms
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Delay Littles Result
S area
S T(1) T(N) integral of X(t)
T
? Average occupancy (average delay)x(average
arrival rate)
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Delay Measurements 1

• A Good Epoch

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Delay Measurements 2

• A Worse Epoch

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Performance Evaluation Techniques

• Models Analysis
• Models Simulations
• Measurements

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Evaluation Analysis

• Example M/M/1 Queue
• Arrivals are Poisson with rate l
• Service times are exponentially distributed with
  mean 1/m
• Average delay per packet
• T 1/(m l) (1/m)/(1 r) where r l/m
  utilization
• For instance, 1/m 1ms and r 80 gt Q 5ms

l
m
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Evaluation Simulation

• Model of traffic
• Model of routers, links, .
• Simulation
• Time Driven X(n) state at time neX(n1)
  f(X(n), event at time ne)
• Event Driven Y(n) state after event nE(n)
  n-th eventT(n) time when event n
  occursY(n1), T(n1) g(Y(n), T(n), E(n)
• Key Step Output analysis (estimates, confidence
  intervals.)

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Evaluation Measurements

• Design Experiment
• Representative?
• Output Analysis