1. 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

2. Packet Scheduling Decide when and what packet to send on output link
Usually implemented at output interface of a router
flow 1
Classifier
flow 2 1
Scheduler 2
flow n
Buffer management

3. 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

4. Outline
QoS guarantees
Link sharing
Service curve (short)

5. 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
Arrival curve
r bps
bits
slope r
b*R/(R-r)
b bits
slope R
<= R bps
time
regulator

6. 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

7. Source Traffic Characterization
Arrival curve – maximum amount of bits transmitted during an interval of time Δt
Use token bucket to bound the arrival curve
bps
bits
Arrival curve Δt
time

8. 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
(R=2,b=1,r=1)
Arrival curve
bits 4
bps 3 2 2 1 1 Δt 1 2 3 4 5
time 1 2 3 4 5

9. 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
slope ra
slope r
bits
Arrival curve
b*R/(R-r) D Ba

10. 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)

11. 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

12. Fair Rate Computation: Example 1
If link congested, compute f such that
f = 4:
min(8, 4) = 4
min(6, 4) = 4
min(2, 4) = 2 8 10 4 6 4 2 2

13. Fair Rate Computation: Example 2
Associate a weight wi with each flow i
If link congested, compute f such that
f = 2:
min(8, 2*3) = 6
min(6, 2*1) = 2
min(2, 2*1) = 2
(w1 = 3) 8 10 4
(w2 = 1) 6 4 2
(w3 = 1) 2
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)

14. 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

15. Fluid Flow System: Example 1
Packet Size (bits)
Packet inter-arrival time (ms)
Rate (Kbps)
Flow 1
1000 10
100
Flow 2
500 50
Flow 1 (w1 = 1)
100 Kbps
Flow 2 (w2 = 1)
Flow 1
(arrival traffic) 1 2 3 4 5
time
Flow 2
(arrival traffic) 1 2 3 4 5 6
time
Service
in fluid flow
system 3 4 5 1 2 1 2 3 4 5 6 10 20 30 40 50 60 70 80
time (ms)

16. Fluid Flow System: Example 2
link
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
weights 5 1 1 1 1 1 2 4 6 8 10 15

17. 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

18. Packet System: Example 1
Service
in fluid flow
system 3 4 5 1 2 1 2 3 4 5 6
time (ms)
Select the first packet that finishes in the fluid flow system
Packet
system 1 2 1 3 2 3 4 4 5 5 6
time

19. Packet System: Example 2
Service
in fluid flow
system 2 4 6 8 10
Select the first packet that finishes in the fluid flow system
Packet
system 2 4 6 8 10

20. 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!

21. Example Four flows, each with weight 1 Flow 1 time Flow 2 time Flow 3ε 1 2 3
Finish times computed at time 0
time
time
Finish times re-computed at time ε 1 2 3 4

22. 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

23. 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
V(t)
time
Service
in fluid flow
system 1 2 3 4 5 6 1 2 3 4 5
time

24. System Virtual Time (V(t)): Example 1
V(t) increases inversely proportionally to the sum of the weights of the backlogged flows
Flow 1 (w1 = 1)
time
Flow 2 (w2 = 1)
time 3 4 5 1 2 1 2 3 4 5 6
V(t)
C/2 C

25. System Virtual Time: Example
w1 = 4
w2 = 1
w3 = 1
w4 = 1
w5 = 1
V(t)
C/4
C/8
C/4 4 8 12 16

26. 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
/ wi

27. 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

28. Outline
QoS guarantees
Link sharing
Service curve (short)

29. 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
155 Mbps
Link
100 Mbps
55 Mbps
Provider 1
Provider 2
50 Mbps
50 Mbps
Berkeley
Stanford.
20 Mbps
10 Mbps
EECS
Math
Campus
seminar
video
seminar
audio
WEB

30. Hierarchical WFQ (H-WFQ) Example10
Red session has packets backlogged at time 5
Other sessions have packets continuously backlogged 5 1 1 1 1 1 4 1
First red packet arrives at 5
…and it is served at 7.5
Service
in fluid flow
system 10 20

31. 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
Fluid Flow H-WFQ
Packetized H-WFQ 10 10
WFQ
WFQ 6 4 6 4
WFQ
WFQ
WFQ
WFQ 1 3 1 3 2 2
WFQ
WFQ
WFQ
WFQ
WFQ
WFQ

32. Green packet finish first
Problems with Idea 1 10
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 5 1 1 1 1 1 4 1
Green packet finish first
Make decision here
Blue packet
finish first

33. Problem with Idea 2 10
A packet on the second level can miss its deadline (finish time) 5 1 1 1 1 1 4 1
First level packet schedule
Second level packet schedule
First red packet arrives at 5
…but it is served at 11 !

34. 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

35. Example Fluid-Flow System WFQ (smallest finish time first)
WF2Q (smallest eligible finish time first)

36. Hierarchical-WF2Q Example10
In WF2Q, all packets meet their deadlines modulo time to transmit a packet (at the line speed) at each level 5 1 1 1 1 1 4 1
First level packet schedule
Second level packet schedule
First red packet arrives at 5
..and it is served at 7

37. Outline
QoS guarantees
Link sharing
Service curve (short)

38. 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 !
slope ra
slope r
bits
Arrival curve
b*R/(R-r) D

39. 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

40. 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)
bits
Arrival Curve D B
S (t) = service curve t
WFQ implements a linear service curve

41. Linear Service Curves: Example
bits
bits
Service
curves
FTP
Video t t
Arrival
process
bits
bits
Arrival
traffic t t
bits
bits
Service t t
Video packets have to wait after ftp packets t

42. Non-Linear Service Curves: Example
bits
bits
Service
curves
FTP
Video t t
Arrival
process
bits
bits
Arrival
traffic t t
bits
bits
Service t t
Video packets transmitted
as soon as they arrive t

43. What You Need to Know Basic concepts Mechanisms:
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