7 - Rate Limiting and Traffic Shaping

Question 1

Leaky Bucket with a Token bucket

Answer 1

Composite Shaper

Question 2

Motivation for Traffic classification and shaping

Answer 2

resource control on network

ensure no traffic flow exceeds particular pre-specified rate

Question 3

Data traffic

Answer 3

bursty, periodic, regular

Question 4

Audio traffic

Answer 4

continuous, periodic

Question 5

Video traffic

Answer 5

continuous, bursty (compression), periodic

Question 6

Two kinds of traffic (sources)

Answer 6

1) Constant bit rate source(CBR) (i.e audio) -> shape it according to a “peek” rate
2) Variable bit rate (VBR) (i.e. video, data) -> shape it according to both an average rate and a peek rate

Question 7

A way of shaping traffic. Each flow has its own bucket (Buffer)

Answer 7

Leaky Bucket

Question 8

Traffic is divided into t-bit frames and a flow can inject <= r-bits in any t-bit frame

Answer 8

(r,t) traffic shaping

Question 9

A flow that obeys the (r,t) traffic shaping rule

Answer 9

(r,t) smooth

Question 10

Priorities can be assigned at the

Answer 10

Sender: Application may mark its own packets since it knows best which packets may be less important
or
Network: The routers may mark packets with lower priority - which is sometimes called “policing”

Question 11

Use this for shaping bursty traffic patterns:

Answer 11

Token bucket

Question 12

1) Permits burstiness, but bounds it by the rate rho
- > in any interval T, rate < beta (bucket size) + t * rho
2) No discard or priority policies
3) difficulty with policing (but composite shaper helps - which is combining Token bucket with leaky bucket)

Answer 12

Token Bucket

Question 13

1) Forces the bursty traffic to be smooth

2) Priority policies

Answer 13

Leaky Bucket

Question 14

Token Bucket Shaper:
Size of bucket (beta) = 100kb
Rho = 10 packets per second
packet size = 1kb or ~8000 bits
Interval T = 1 second

Remember that in any given interval…
a flow can never send more than beta plus T * rho bits of data (B+T*P)

Answer 14

100kb + 10kb = 110kb

880kb/sec

Question 15

Traffic shaping mechanism first deployed by Comcast.

allows a subscriber to send at a higher rate for a brief period of time (June 2006)

Answer 15

Power Boost
2 types of power boosts
Capped - limits the peek sending rates
uncapped - configuration is simple

Question 16

Calculating powerboost rates

Answer 16

Beta = d*(r - Rsus)
d = Beta/(r - Rsus)

Question 17

Rsus = 10mbps
r = 15mbps
Beta = 1 MB or 8 Megabits
How long can the sender send in the higher rate (decimal form in seconds)

Answer 17

d = Beta/r-Rsus)
d = 8megabits/15 - 10)
d = 8/5
d = 1.6 seconds

Question 18

Also has effect on latency

Answer 18

Powerboosts

Question 19

Delay = Data in Buffer/Rsus

Answer 19

BufferBloat

Question 20

Solutions to buffer bloat problem:

Answer 20

1) use smaller buffers (hard to do in most devices)

2) Shaping traffic

Question 21

Network Measurement (How to see what traffic is being sent on the network)

Answer 21

Passive Measurement - Collect packets, flow statistics, etc… of traffic already being sent on the network
Active Measurement - Inject additional traffic to measure various characteristics (ping, traceroute)

Question 22

Often used to measure the delay to a particular server

Answer 22

Ping

Question 23

often used to measure the network level or the IP level path between two hosts on a network

Answer 23

Traceroute

Question 24

Why measure?

Answer 24

• Billing - 95th percentile (CIR = Committed information rate)
• Security - What type of traffic to detect rogue behavior (Botnets, DDOS, etc…)

Question 25

How to measure (Passively)

Answer 25

SNMP (Simple Network Management Protocol) ubiquitous but also coarse

Question 26

2 ways to measure passively

Answer 26

1) Packet monitoring - monitors can see full packet or header contents
2) Flow monitoring - monitor can see specific statistics about individual flows in the network

Question 27

tcpdump. thereal, wireshark

Sometimes can be done with hardware

Answer 27

Packet monitoring

Question 28

Advantages and disadvantages of Packet monitoring

Answer 28

Advantage: Lots of detail (timing, header info)
Disadvantage: High overhead

Question 29

A Monitor (may even run on a router) records statistics per flow and has less overhead

Answer 29

Flow monitoring

Question 30

flow information can contain

Answer 30

timing, header info
Next hop IP
src/dst AS and Prefix

Question 31

Advantages and disadvantages of Flow monitoring

Answer 31

Advantage: less overhead
Disadvantage: Even more coarse than Packet monitoring - no packets/payloads

Question 32

Flow stats based on samples of packets

Answer 32

Sampling

Question 33

What Packet and flow monitoring will show…

Answer 33

Packet monitoring = Timing information (packet level)
Packet monitoring = Packet headers
Packet monitoring and flow monitoring = number of bytes in each flow