1. EECS 122: QoS and Packet Scheduling: Token-Bucket, WFQ, Hierarchical Link Sharing
Computer Science Division
Department of Electrical Engineering and Computer Sciences
University of California, Berkeley
Berkeley, CA

2. Quality of Service (QoS)
IP provides only best effort service
Many applications want better services
IP telephony
Video teleconferencing ….
Design a network that provides better than best-effort service
Basic service parameters
Delay
Bandwidth
Loss rate

3. QoS Service A contract between end-hosts and the network
End-host specify
Traffic characteristics (e.g., send no more than 100 Kbps)
Service requirements (e.g., delay < 50ms, loss < 0.1%)
Network performs:
Admission control: check whether it can grant end-host’s wish
Reserve resources for the traffic sent by end-host
Schedule packets to make sure that end-host’s requirements are met
If end-host violates the contract (e.g., sends more traffic than it said) all bets are off!

4. QoS Granularity
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

5. Token Bucket A simple traffic model used to:
Enforce traffic sent by a sender (e.g., cable modem)
Characterize the traffic sent by a sender

6. Token Bucket 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
Maximum # of bits sent
r bps
bits
slope r
b*R/(R-r)
b bits
slope R
<= R bps
time
regulator

7. Traffic Enforcement: Example
r = 100 Kbps; b = 3 Kb; R = 500 Kbps
2.2Kb
T = 2ms : packet transmitted
b = 3Kb – 1Kb + 2ms*100Kbps = 2.2Kb
(b)
3Kb
T = 0 : 1Kb packet arrives
(a)
2.4Kb
T = 4ms : 3Kb packet arrives
(c)
3Kb
T = 10ms : packet needs
to wait until enough
tokens are in the
bucket!
(d)
0.6Kb
T = 16ms : packet
transmitted
(e)

8. Source Traffic Characterization: Arrival Curve
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

9. Arrival Curve: 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

10. 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, no losses
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 R

11. Packet Scheduling Decide when and what packet to send on output link
Make sure that the flow gets the allocated bandwidth
Usually implemented at output interface of a router
flow 1
Classifier
flow 2 1
Scheduler 2
flow n
Buffer management

12. Packet Scheduling: Example
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)

13. Weighted Fair Queueing (WFQ)
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

14. 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

15. 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)

16. 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

17. Fluid Flow System: Example 1
Packet Size (bits)
Packet inter-arrival time (ms)
Rate (C) (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
transmission time
Service
in fluid flow
system C 1 2 3 1 2 4 3 5 6 10 20 30 40 50 60 70 80
time (ms)
Area (C x transmission_time) =
packet size

18. Fluid Flow System: Example 2
link
Red flow has sends packets between time 0 and 10
Backlogged flow  flow’s queue not empty
Other flows send packets continuously
All packets have the same size
flows
weights 5 1 1 1 1 1 2 4 6 8 10 15

19. Implementation In Packet System
Packet (Real) system: packet transmission cannot be preempted.
Solution: serve packets in the order in which they would have finished being transmitted in the fluid flow system

20. 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 5 6
time

21. 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

22. 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!

23. 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

24. 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

25. 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

26. 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

27. 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

28. 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

29. 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

30. 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

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