Katz, Stoica F04 EECS 122: Introduction to Computer Networks Packet Scheduling and QoS Computer...
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Transcript of Katz, Stoica F04 EECS 122: Introduction to Computer Networks Packet Scheduling and QoS Computer...
![Page 1: Katz, Stoica F04 EECS 122: Introduction to Computer Networks Packet Scheduling and QoS Computer Science Division Department of Electrical Engineering and.](https://reader033.fdocuments.in/reader033/viewer/2022052509/56649d4b5503460f94a29309/html5/thumbnails/1.jpg)
Katz, Stoica F04
EECS 122: Introduction to Computer Networks
Packet Scheduling and QoS
Computer Science Division
Department of Electrical Engineering and Computer Sciences
University of California, Berkeley
Berkeley, CA 94720-1776
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2Katz, Stoica F04
Today’s Lecture: 15
Network (IP)
Application
Transport
Link
Physical
2
7, 8, 9
10,11
17, 18, 19
14, 15, 16
21, 22, 23
25
6
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3Katz, Stoica F04
Packet Scheduling
Decide when and what packet to send on output link- Usually implemented at output interface of a router
1
2
Scheduler
flow 1
flow 2
flow n
Classifier
Buffer management
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4Katz, Stoica F04
Goals of Packet Scheduling
Provide per flow/aggregate QoS guarantees in terms of delay and bandwidth
Provide per flow/aggregate protection Flow/Aggregate identified by a subset of following fields in
the packet header - source/destination IP address (32 bits)
- source/destination port number (16 bits)
- protocol type (8 bits)
- type of service (8 bits) Examples:
- All packets from machine A to machine B
- All packets from Berkeley
- All packets between Berkeley and MIT
- All TCP packets from EECS-Berkeley
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5Katz, Stoica F04
Outline
QoS guarantees Link sharing Service curve (short)
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6Katz, Stoica F04
Recap: Token Bucket and Arrival Curve
Parameters- r – average rate, i.e., rate at which tokens fill the bucket- b – bucket depth- R – maximum link capacity or peak rate (optional parameter)
A bit is transmitted only when there is an available token Arrival curve – maximum number of bits transmitted within an
interval of time of size t
r bps
b bits
<= R bps
regulatortime
bits
b*R/(R-r)
slope R
slope r
Arrival curve
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7Katz, Stoica F04
How Is the Token Bucket Used?
Can be enforced by - End-hosts (e.g., cable modems)
- Routers (e.g., ingress routers in a Diffserv domain)
Can be used to characterize the traffic sent by an end-host
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8Katz, Stoica F04
Source Traffic Characterization
Arrival curve – maximum amount of bits transmitted during an interval of time Δt
Use token bucket to bound the arrival curve
Δt
bits
Arrival curve
time
bps
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9Katz, Stoica F04
Source Traffic Characterization: Example
Arrival curve – maximum amount of bits transmitted during an interval of time Δt
Use token bucket to bound the arrival curve
bitsArrival curve
time
bps
0 1 2 3 4 5
1
2
1 2 3 4 5
1
2
3
4
(R=2,b=1,r=1)
Δt
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10Katz, Stoica F04
QoS Guarantees: Per-hop Reservation
End-host: specify- the arrival rate characterized by token-bucket with parameters (b,r,R)- the maximum maximum admissible delay D
Router: allocate bandwidth ra and buffer space Ba such that - no packet is dropped- no packet experiences a delay larger than D
bits
b*R/(R-r)
slope rArrival curve
DBa
slope ra
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11Katz, Stoica F04
Packet Scheduling
Make sure that at any time the flow receives at least the allocated rate ra
The canonical example of such scheduler: Weighted Fair Queueing (WFQ)
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12Katz, Stoica F04
Recap: Fair Queueing
Implements max-min fairness: each flow receives min(ri, f) , where
- ri – flow arrival rate
- f – link fair rate (see next slide)
Weighted Fair Queueing (WFQ) – associate a weight with each flow
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13Katz, Stoica F04
Fair Rate Computation: Example 1
If link congested, compute f such that
Cfri
i ),min(
8
6
244
2
f = 4: min(8, 4) = 4 min(6, 4) = 4 min(2, 4) = 2
10
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14Katz, Stoica F04
Fair Rate Computation: Example 2
Associate a weight wi with each flow i If link congested, compute f such that
Cwfr ii
i ),min(
8
6
244
2
f = 2: min(8, 2*3) = 6 min(6, 2*1) = 2 min(2, 2*1) = 2
10(w1 = 3)
(w2 = 1)
(w3 = 1)
If Σk wk <= C, flow i is guaranteed to be allocated a rate >= wi
Flow i is guaranteed to be allocated a rate >= wi*C/(Σk wk)
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15Katz, Stoica F04
Fluid Flow System
Flows can be served one bit at a time WFQ can be implemented using bit-by-bit weighted
round robin- During each round from each flow that has data to send, send a
number of bits equal to the flow’s weight
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16Katz, Stoica F04
Fluid Flow System: Example 1
1 2 3 4 5
1 2 3 4
1 2 31 2
43 4
55 6
5 6Flow 2(arrival traffic)
Flow 1(arrival traffic)
Servicein fluid flow
system
time
time
time (ms)
Packet Size (bits)
Packet inter-arrival time (ms)
Rate (Kbps)
Flow 1 1000 10 100
Flow 2 500 10 50
100 KbpsFlow 1 (w1 = 1)
Flow 2 (w2 = 1)
0 10 20 30 40 50 60 70 80
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17Katz, Stoica F04
Fluid Flow System: Example 2
0 152 104 6 8
5 1 1 11 1
Red flow has packets backlogged between time 0 and 10
- Backlogged flow flow’s queue not empty
Other flows have packets continuously backlogged
All packets have the same size
flows
link
weights
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18Katz, Stoica F04
Implementation In Packet System
Packet (Real) system: packet transmission cannot be preempted. Why?
Solution: serve packets in the order in which they would have finished being transmitted in the fluid flow system
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19Katz, Stoica F04
Packet System: Example 1
1 2 1 3 2 3 4 4 55 6Packetsystem time
1 2 31 2
43 4
55 6
Servicein fluid flow
system time (ms)
Select the first packet that finishes in the fluid flow system
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20Katz, Stoica F04
Packet System: Example 2
0 2 104 6 8
0 2 104 6 8
Select the first packet that finishes in the fluid flow system
Servicein fluid flow
system
Packetsystem
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21Katz, Stoica F04
Implementation Challenge
Need to compute the finish time of a packet in the fluid flow system…
… but the finish time may change as new packets arrive!
Need to update the finish times of all packets that are in service in the fluid flow system when a new packet arrives
- But this is very expensive; a high speed router may need to handle hundred of thousands of flows!
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22Katz, Stoica F04
Example
Four flows, each with weight 1
Flow 1
time
time
ε
time
time
Flow 2
Flow 3
Flow 4
0 1 2 3
Finish times computed at time 0
time
time
Finish times re-computed at time ε
0 1 2 3 4
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23Katz, Stoica F04
Solution: Virtual Time
Key Observation: while the finish times of packets may change when a new packet arrives, the order in which packets finish doesn’t!
- Only the order is important for scheduling
Solution: instead of the packet finish time maintain the number of rounds needed to send the remaining bits of the packet (virtual finishing time)
- Virtual finishing time doesn’t change when the packet arrives
System virtual time – index of the round in the bit-by-bit round robin scheme
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24Katz, Stoica F04
System Virtual Time: V(t) Measure service, instead of time V(t) slope – normalized rate at which every backlogged flow receives
service in the fluid flow system- C – link capacity- N(t) – total weight of backlogged flows in fluid flow system at time t
1 23
1 24
3 45
5 6Servicein fluid flow
systemtime
time
V(t)
)(
)(
tN
C
t
tV
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25Katz, Stoica F04
System Virtual Time (V(t)): Example 1
V(t) increases inversely proportionally to the sum of the weights of the backlogged flows
1 2 31 2
43 4
55 6
Flow 2 (w2 = 1)
Flow 1 (w1 = 1)
time
time
C
C/2V(t)
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26Katz, Stoica F04
System Virtual Time: Example
0 4 128 16
w1 = 4
w2 = 1
w3 = 1
w4 = 1w5 = 1
C/4
C/8C/4V(t)
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27Katz, Stoica F04
Fair Queueing Implementation
Define- - virtual finishing time of packet k of flow i
- - arrival time of packet k of flow i
- - length of packet k of flow i
- wi – weight of flow i
The finishing time of packet k+1 of flow i is
kiL
kia
kiF
111 )),(max( ki
ki
ki
ki LFaVF / wi
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28Katz, Stoica F04
Properties of WFQ
Guarantee that any packet is transmitted within packet_lengt/link_capacity of its transmission time in the fluid flow system
- Can be used to provide guaranteed services
Achieve max-min fair allocation- Can be used to protect well-behaved flows against
malicious flows
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29Katz, Stoica F04
Outline
QoS guarantees Link sharing Service curve (short)
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30Katz, Stoica F04
Hierarchical Link Sharing
Resource contention/sharing at different levels
Resource management policies should be set at different levels, by different entities
- Resource owner
- Service providers
- Organizations
- Applications
Link
Provider 1
seminar video
Math
Stanford.Berkeley
Provider 2
WEB
155 Mbps
50 Mbps50 Mbps
10 Mbps20 Mbps
100 Mbps 55 Mbps
Campus
seminar audio
EECS
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31Katz, Stoica F04
Hierarchical WFQ (H-WFQ) Example
4 1
1 11 1
Red session has packets backlogged at time 5
Other sessions have packets continuously backlogged
5
0 10 20
10
1
First red packet arrives at 5 …and it is served at 7.5
Servicein fluid flow
system
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32Katz, Stoica F04
Packet Approximation of H-WFQ
Idea 1- Select packet finishing first in H-
WFQ assuming there are no future arrivals
- Problem:
• Finish order in system dependent on future arrivals
• Virtual time implementation won’t work
Idea 2- Use a hierarchy of WFQ to
approximate H-WFQ
6 4
321
WFQ WFQ WFQ
WFQ WFQ
WFQ
10
Packetized H-WFQFluid Flow H-WFQ
6 4
321
WFQ WFQ WFQ
WFQ WFQ
WFQ
10
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33Katz, Stoica F04
Problems with Idea 1
The order of the 4th blue packet finish time and of the first green packet finish time changes as a result of a red packet arrival
Make decision here
Blue packet finish first
Green packet finish first4 1
1 11 1
5
10
1
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34Katz, Stoica F04
Problem with Idea 2
A packet on the second level can miss its deadline (finish time)
4 1
1 11 1
5
10
1
First red packet arrives at 5 …but it is served at 11 !
First level packet schedule
Second level packet schedule
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35Katz, Stoica F04
Solution
Hierarchical-WFQ with a better implementation of WFQ, called Worst-Case Weighted Fair Queueing (WF2Q)
Main idea of WF2Q- Consider for scheduling only eligible packets
- Eligible packet at time t: a packet that has started being serviced in the fluid flow system at time t
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36Katz, Stoica F04
Example
Fluid-Flow System
WFQ (smallest finish time first)
WF2Q (smallest eligible finish time first)
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37Katz, Stoica F04
Hierarchical-WF2Q Example
In WF2Q, all packets meet their deadlines modulo time to transmit a packet (at the line speed) at each level
4 1
1 11 1
5
10
1
First red packet arrives at 5 ..and it is served at 7
First level packet schedule
Second level packet schedule
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38Katz, Stoica F04
Outline
QoS guarantees Link sharing Service curve (short)
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39Katz, Stoica F04
Bandwidth-Delay Coupling
Assume- An arrival curve specified by token bucket (R, r, b) - A reservation request (rmin, D), where rmin – minimum bandwidth, D – maximum delay
WFQ can be inefficient: to satisfy delay D, the allocated rate ra may need to be much higher than r !
bits
b*R/(R-r)
slope rArrival curve
slope ra
D
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40Katz, Stoica F04
Example
End-host: request a worst-case delay D=10ms for a flow characterized by token bucket (R=1Mbps, b=10Kb, r=100Kbps)
Router: allocate rate ra, where
Thus, the router needs to allocate to the flow more than 5 times its average rate r !
ra = b*R / (D*(R-r) + b) = 0.52Mbps
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41Katz, Stoica F04
Solution: Service Curve
Generalize the service allocated to a flow- Assume a flow that is idle at time s and it is backlogged
during the interval (s, t)- Service curve: the minimum service received by the flow
during the interval (s, t)
t
S (t) = service curve
Arrival Curve
D
B
bits
WFQ implements a linear service curve
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42Katz, Stoica F04
Linear Service Curves: Example
t
bits
t
bits
t
Arrival traffic
t
bits
t t
bitsbits
bits
Service curves
Arrival process
Service
VideoFTP
t
Video packets have to wait after ftp packets
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43Katz, Stoica F04
Non-Linear Service Curves: Example
t
bits
t
bits
t
Arrival traffic
t
bits
t t
bitsbits
bits
Service curves
Arrival process
Service
t
Video FTP
Video packets transmittedas soon as they arrive
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44Katz, Stoica F04
What You Need to Know
Basic concepts- Arrival & service curve- Token-bucket specification- System virtual time / finish virtual time- WFQ properties - Link sharing requirements and challenges- Bandwidth-delay coupling problem
Mechanisms: - WFQ implementation in the fluid flow & packet system
You don’t need to know- Details of WF2Q- How service curve works