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The Earliest Deadline First
Scheduling for Real-Time Traffic
in the Internet
Danny H. K. Tsang, Xiaojun Hei
July 10, 2001
http://www.ee.ust.hk/~eetsang/
Department of Electrical and Electronic Engineering
Hong Kong University of Science and Technology
Outline
•
•
•
•
Introduction
Hierarchical scheduling framework
EDF with active buffer management
Conclusion and future work
Broadband Network Lab
2
Network Convergence
• Telephone Networks
• Computer Networks
• Cable Television Networks
• Wireless Networks
• Convergence: IP-centric networks
Broadband Network Lab
3
Next Generation Internet
• Broadband networks (WDM, 3G, 802.11, …)
• Multimedia communication
• Quality of Service provision
– Integrated Services (IntServ)
– Differentiated Services (DiffServ)
Broadband Network Lab
4
Packet Scheduling
•
Service Policies
•
•
First Come First Serve
Generalized Processor Sharing
•
Earliest Deadline First
•
Buffer Management
•
•
Drop-Tail
Random Early Detection
Broadband Network Lab
5
Real Time Traffic
• Requirements:
– High bandwidth
– Time-sensitive
– Packet loss
• Transport candidates
– User Datagram Protocol (UDP)--aggressive
– Transmission Control Protocol (TCP)--cooperative
Broadband Network Lab
6
Hierarchical Generalized Processor Sharing
Link
GPS
GP S
…

GP S

Class 1
Class N
Broadband Network Lab
7
Hierarchical Scheduling Framework
Link
GPS
Class 1
Class M
GP S
………
…
Best-effort Traffic
EDF
…
Figure 1
Broadband Network Lab
Real-time traffic
8
Hierarchical Scheduling Structure
Link
PGPS
EDF
CHOKe
CHOKe
P GP S
Best-effort
Traffic
Real-time
Traffic
Figure 2
Broadband Network Lab
9
Queueing Model for the EDF-CHOKe Scheme
flow 1
C
CHOKe
EDF
…
flow 2
flow N
Broadband Network Lab
10
Dropping
Probability (pa)
RED
1
Average
Queue length
0
min_th
max_th
Broadband Network Lab
11
CHOKe
s tart
Y
AvgQsize<=M in _ t h ?
N
Draw a packet at
random from queue
Admit the new
packet
Y
end
Drop both
packets
Both packets
from s ame flow?
Y
N
AvgQsize<=M ax _ th?
N
Admit packets
with a probability p
Drop the new
packet
end
end
end
Broadband Network Lab
12
Network Topology
1
1
10
M
bp
s
UDP
TCP Sink
s
p
b
3
R2
…
M
10
1 Mbps
s
bp
M
33
R1
10
…
TCP
3
2
10
s
bp
M
2
UDP Sink
33
Figure 3
Broadband Network Lab
13
UDP Throughput Comparison
5
10
x 10
FCFS-DropTail
FCFS-RED
FCFS-CHOKe
EDF-CHOKe
9
8
Throughput(bps)
7
6
5
4
3
2
1
0
0
50
100
150
200
250
Time (sec)
Figure 4
Broadband Network Lab
14
Per Flow Throughput Comparison
1000000
10000
Ideal
1000
FCFS-CHOKe
EDF-CHOKe
100
10
31
28
25
22
19
16
13
10
7
4
1
1
Throughput(bps)
100000
Flow ID
Figure 5
Broadband Network Lab
15
Fairness Index
• Definition
f ( x1 , x2 ,, xn ) 
2
n
(  xi )
i 1
n
n  xi
i 1
2
Ideal
1.0
FCFS-DropTail
0.0305
FCFS-RED
0.0309
FCFS-CHOKe
0.3744
EDF-DropTail
0.0304
EDF-RED
0.0304
EDF-CHOKe
0.2838
Table 1
Broadband Network Lab
16
Packet Delay Distribution of the TCP Connections
Figure 6
Broadband Network Lab
17
Statistics about the Delay Distribution(sec)
TCP
UDP
Avg
Std
Avg
Std
FCFSCHOKe
0.9378
0.1203
0.0859
0.2743
EDFCHOKe
0.5665
0.0398
0.0476
0.1564
Table 2
Broadband Network Lab
18
Power of the Network
Definition:
throughput
power 
delay
TCP
UDP
FCFS-CHOKe
2.45 x 104
3.44 x 106
EDF-CHOKe
3.78 x 104
7.65 x 106
Table 3
Broadband Network Lab
19
Conclusion
• EDF: Schedule real-time traffic
• Hierarchical Scheduling Structure
– Best-effort service together with real–time
traffic
• EDF with active buffer management
– Good delay performance
– Fair bandwidth allocation between UDP and
TCP flows
Broadband Network Lab
20
Future Work
• Hierarchical scheduling framework
• More complex networking scenarios
Broadband Network Lab
21
Q&A
Broadband Network Lab
22