Rate Limiting and Traffic Shaping

Question 1

Traffic Shaping Approaches (3)

Answer 1

Leaky Bucket

(r, T) Traffic Shaping

Token Bucket

Question 2

Motivation for traffic shaping

Answer 2

• resource control

- ensure flows don’t exceed drain rate

Question 3

Source Classifications and attributes for each one

Answer 3

Data: regular, bursty, periodic

Audio: continuous, periodic

Video: continuous, bursty, periodic

Question 4

Sourse Classes

Answer 4

CBR: constant bit rate

VBR: variable bit rate

Question 5

CBR

Answer 5

constant bit rate: arrives at regular intervals and at regular lengths (audio)

Question 6

VBR

Answer 6

variable bit rate: like video and data. Arrives at variables intervals and lengths

Question 7

What is constant bit rate shaped by?

Answer 7

peak rate

Question 8

What is variable but rate shaped by?

Answer 8

average and peak rate

Question 9

leaky bucket shaping attributes

Answer 9

beta: size of bucket
row: drain rate of bucket (maximum smooth rate)

Size of bucket controls the burst rate. As long as size of the burst does not exceed size of bucket, sender can send at faster rate.

• smooths bursty traffic
• priority policies for flows that exceed smoothing rate

Question 10

(r, T) traffic shaping attributes

Answer 10

• traffic divided into T-bit frames
• flow can inject <= r bits in any T-bit frame
• typically used for fixed flows

Question 11

what is “(r, T) smooth”?

Answer 11

optimal property achieved when (r, T) traffic shaping is implemented

Question 12

what is policing?

Answer 12

it is process by which packets are given priority assignments by the sender or within the network

Question 13

token bucket attributes

Answer 13

• row = rate at which tokens are placed in the bucket
• traffic is sent by the regulator as long as there are tokens in the bucket
• permits burstiness, but bounds it
• long term rate < row
• no discard or priority/policing

Question 14

problem with standard token bucket

Answer 14

lack of policing means that it can monopolize network resources by interfering with other high priority traffic

Question 15

How to police with a token bucket?

Answer 15

You must use a composite shaper (leaky bucket + token bucket)

Question 16

Composite shaper attributes

Answer 16

• combination of leaky bucket and token bucket

- more complex because you need 1 counter and 1 timer for each bucket

Question 17

token bucket shaper max rate calculation

Answer 17

max = beta + (timeInterval * row)

Question 18

what is power boost?

Answer 18

Technology which allows a subscriber to temporarily send @ higher rate than the sustained rate

Question 19

2 types of power boost are:

Answer 19

capped and uncapped

• capped uses a second token bucket w/ additional ‘row’; value
• uncapped: power boost window is the bucket size

Question 20

power boost variables

Answer 20

• sending rate: r
• sustained rate: Rsus
• power boost bucket size: beta
• time: d

Question 21

power boost bucket formula

Answer 21

beta = d * (r - Rsus)

Question 22

power boost time formula

Answer 22

d = beta / (r- Rsus)

Question 23

potential problem and solution to power boost

Answer 23

rate at which data sent during the PB may exceed the access link’s ability to service the packets. So buffer may fill up.

The solution to this is to place a traffic shaper in front of the power boost so that the rate never exceeds Rsus

Question 24

what is buffer bloat?

Answer 24

In the presence of a network buffer, delay experienced by sender. This is because if buffer is large, sender won’t see dropped packets until the buffer is full, causing delay.

Question 25

solutions to buffer bloat

Answer 25

• smaller buffers (hard to do since network infrastructure already deployed)
• shaping: ensure sending rate never exceeds the drain rate of the access link

Question 26

types of network measurement

Answer 26

passive and active

passive: collection of packets, flow states already on network
active: inject additional traffic to measure characteristics (ping, traceroute)

Question 27

why measure

Answer 27

• billing (95th percentile billing)

- security (compromised hosts, botnets, DOS)

Question 28

packet delay calculation

Answer 28

data in buffer / Rsus