CSIT560
Internet Infrastructure:
Switches and Routers
Active Queue Management
Presented By: Gary Po, Henry Hui and Kenny Chong
Agenda
 Why
AQM?
 In Considerations
 AQM Algorithms
 Commercial Effort
 Conclusions
Why AQM?
 Two
Classes of Router Algorithms for
Congestion Control
 What is Congestion?
 Congestion avoidance in TCP, is it
good enough?
 Our Choice :
Active Queue Management
What do we consider when
implementing AQM?

QoS
– Keep Average queue size small
– Bounded Delay

Link Utilization
– Avoid Global Synchronization
– Absorbs bursts without dropping packets

Fairness
– Punishes misbehaving flows
– Prevent bias against bursty connections

Implementation
–
–
–
–
Ease of Configurations
Buffer Size Requirement (Large or Small)
Per-flow State Information
Computational Overhead
AQM Algorithms
FIFO + Drop Tail
 Problems:
– No isolation
– No policing
– Large queues for high utilizations
– Synchronization problem
– Lock-out problem
RED (Random Early Detection)
 FIFO
scheduling
Max thresh
Min thresh
Average queue
length
Case 1:
Make
Define
Use
Admit
of
Two
Average
the
Threshold
New Queue
Packet
Values
Length
Average Queue Length < Min. Thresh Value
RED (Cont’d)
Max thresh
Min thresh
p p
1-p1-p
Average queue
length
Case 2: Average Queue Length between
Admit
Or
Dropthe
theNew
NewPacket
PacketWith
WithProbability
Probabilityp…
1-p
Min. and Max. Threshold Value
RED (Cont’d)
Max thresh
Min thresh
Average queue
length
UntilAs
the
New
noaverage
new
packet
orpackets
being
queue
Case
could
dropped
can
3:length
be be
admitted
admitted,
drops below
New Packet will be dropped
with
with
aqueue
aprobability
probability
1-p…
p…
Avg.
the
Queue
average
the
Length
max
threshold
> Max.
length
Threshold
value
decreases.
Value
RED Flow Diagram
RED (Cont’d)
Queue Size versus Time
RED: Queue
CHOKe:
QueueSize
Size
250
Average Queue Size
Current Queue Size
200
Size of Queue (No. of Packets)

Delay is
bounded
150
100
50
Global Synchronization solved
0
0
5
10
15
20
25
30
Time (Seconds)
35
40
45
50
Unfairness of RED
RED's Throughput
1000
Idea Fair Share
RED's Throughput
Unresponsive
Flow (such as
UDP)
900
800
Throughput (Kbps)
An unresponsive
700
flow occupies over
600
95% of bandwidth
500
400
300
32 TCP Flows
1 UDP Flow
200
100
0
0
5
10
15
20
Flow Number
25
30
35
CHOKe
(CHOose and Keep)
 Based
on RED
 Simple
 Designed for fairness
 Penalize the unresponsive flow
CHOKe (Cont’d)

Mechanism
CHOKe (Cont’d)
Max thresh
Min thresh
Average queue
length
Case 1:
Admit the New Packet
Average Queue Length < Min. Thresh Value
CHOKe (Cont’d)
Max thresh
Min thresh
p
1-p
Average queue
length
A If
packet
Case
they
If 2:
they
are
is Avg.
randomly
from
are
Queue
from
different
chosen
Length
the same
flows,
from
is between
flow,
the
the same
queue
to compare
Min.
both
and
logic
packets
with
Max.
inthe
RED
Threshold
will
new
applies
be arrival
dropped
Values
packet
CHOKe (Cont’d)
Max thresh
Min thresh
Average queue
length
AIf
If
random
they
they are
are
packet
from
from
Case
will
different
the
3: be
same
chosen
flows,
flow,for
Avg. Queue
the
both
new
Length
packets
packet
comparison
>will
Max.
willbe
be
Threshold
dropped
dropped Value
Evaluate CHOKe’s
performance using NS-2
Simulation Scenario
source
destination
10Mbps
10Mbps
UDP
UDP
1Mbps
router
router
TCP
• Topology: Dumb-bell
• Metrics: throughput and queue size
TCP
Performance of CHOKe
Unresponsive
Flow (UDP)
CHOKe: Throughput Per Flow
40
Ideal Fair Share
CHOKe
35
Fair Share
Level
Throughput (Kbps)
30
25
20
15
Bandwidth is evenly
shared
10
5
0
0
32 TCP Flows
1 UDP Flow
5
10
15
20
Flow Number
25
30
35
Parameters
 Number
of responsive/unresponsive
flows
 Transfer rate of different flows
 Number of random candidates
chosen for comparison
CHOKe Simulation

Different Parameters, different performance
CHOKe 1: Throughput Per Flow
CHOKE 2: Throughput with 3 UDP with Different Rate
250
150
Ideal Fair Share
CHOKe 1
CHOKe-2
32 TCPs,
3 UDPs of
different rate
200
100
Throughput (Kbps)
150
Throughput
CHOKe-1
32 TCPs
1 UDP
Ideal Fair Share
CHOKE 2
100
50
50
0
0
5
10
15
20
25
30
0
35
0
5
10
15
Flow Number
CHOKe 2: Throughput Per Flow (with 4x UDP rate)
20
Flow Number
25
30
35
CHOKe 2: Throuhput Per Flow (with 3 UDP Flows)
50
250
Ideal Fair Share
CHOKe 2
Ideal Fair Share
CHOKe 2
45
40
200
Throughput (Kbps)
CHOKe-2
32 TCPs,
1 UDP of
high rate
Throughput (Kbps)
35
30
25
20
CHOKe-2
32 TCPs,
3 UDPs of
same rate
150
100
15
10
50
5
0
0
5
10
15
20
Flow Number
25
30
35
0
0
5
10
15
20
Flow Number
25
30
35
Evolutions of AQM Algorithms
FIFO+
DropTail
RED
SRED
FRED
BLUE
REM
REM, AVQ,LDC
PI Controller


–
– Merits
Merits
Merits
BLUE
–
–
–
RED
CHOKe
–
–
–
SRED
FRED
Early
congestion
Low
Sensitivity
delay
and
to
traffic
small
Stabilized
Good
Simplicity
protection
queue
from
detection
queues
load
and
drain
rate
occupancy
flows
 misbehaving
High
throughput
SFB

No
bias
against
 Low
Independence
delay
of the
Drawbacks

Protection
from
Drawbacks
bursty
traffic
of users
 number
Target delay
achieved
misbehaving
flows

Per-flow
state
global

early
congestion
Drawbacks
 No
Intuitive
parameters,
Drawbacks
synchronization

RED
disadvantages
(Pdrop
 detection
Some
meaningful
complexity
to users
due
SAC
 (target
Some
updated
per-flow
only onstate
to
parameters
delay)
Drawbacks
(zombie
overflow
list)
orWeb
Drawbacks

Low
throughput
for
 queue
Difficulty
in
link
events)
parameter
setting

Some
complexity
due
 traffic
RED idle
disadvantages

Inconsistency
parameters
with
TCP
Insensitivity
to and
traffic
 to
Slow
response
mechanism;
loadthroughput
and
drain
rates
 sender
Low
dependence
onin
works
best with ECN
some
cases
history



Commercial Efforts &
Conclusion
Commercial Efforts &
Conclusion (Cont’d)
AQM over 3G wireless
network” – a paper supported by
Motorola Canada Ltd. (Mar. 2003)
 3G network, real-time applications
have hard time deadlines for packet
delivery at the receiver.
 Use AQM to avoid long queuing delay
and prevent expiring packets.
 “Applying
Commercial Efforts &
Conclusion (Cont’d)
 AQM
improves overall system
performance by increasing
throughput and reducing end-to-end
delay.
Commercial Efforts &
Conclusion (Cont’d)
of AQM on Web
Performance” – a paper supported by
Cisco Systems and IBM. (Aug. 2003)
 Proportional Integrator (PI) controller
 “Effect
Random Exponential Marking (REM) controller
 Adaptive Random Early Detection (ARED).
 IETF proposed standard :
Explicit Congestion Notification (ECN)

Commercial Efforts &
Conclusion (Cont’d)
 ECN
has significant impact with AQM
scheme in web performance.
 Many researches and efforts are
going on in the field of AQM.
 Simple and Easy to implement