Lesson 7 - Rate Limiting & Traffic Shaping Mike

Question 1

What classification rate is Data?

Answer 1

Regular, periodic & bursty

Question 2

What classification rate is Audio?

Answer 2

Continuous and periodic

Question 3

What classification rate is Video?

Answer 3

Continuous, periodic & bursty

Question 4

What two classes are there for traffic?

Answer 4

• Constant Bit Rate (CBR)

* Variable Bit Rate (VBR)

Question 5

What is the difference between CBR and VBR and what is each traffic type good for?

Answer 5

Constant Bit Rate (CBR) is…
shaped according to PEAK rate
good for audio

Variable Bit Rate (VBR) is…
shaped according to avg rate + peak rate (peak rate can be higher at times)
good for data transfers

Question 6

What is the difference between a Token Bucket and a Leaky Bucket?

Answer 6

Token buckets permit burstiness but are difficult to police

Leaky buckets smooth burstiness and implement priority policing

• Both easy to implement, but Token bucket more flexible because of additional parameters to configure burst size
• Token buckets allow for long bursts that are difficult to police and may interfere with other high priority traffic

Question 7

What is Policing in a network?

Answer 7

If flow exceeds the rate then the network can decide to assign certain packets a lower priority

Question 8

What is a Composite Shaper?

Answer 8

A Token/Leaky bucket combo that uses two timers and allows for policing

Question 9

• In a Token Bucket Shaper, if you have…
  
  • T = 1 second
  • β = 100 KB
  • ϱ = 10 packets / second
  • packet size = 1 KB
  • 1 Byte = 8 bits
  • ….what is the Max Rate? Give answer in Kb per second

Answer 9

• rate is β + T * ϱ
• so…
• rate = 100 + 1 * 10 ==> 110
• 110 KB / second
• (110 KB) * (8 kilobits/1KB) = 880 kilobits per second

Question 10

What is Power Boost?

Answer 10

• It allows for the subscriber to send at a faster rate for a brief period of time
• spare capacity is for users who do not put a sustained load on the network

Question 11

What is the difference between Capped and Uncapped Power Boost?

Answer 11

Capped Powerboost = the rate can’t go beyond a fixed point

Uncapped Powerboost = limited only by the bucket size because it can’t send more packets than it has (otherwise it is limitless)

Question 12

What is the formula for calculating power boost?

Answer 12

d = (beta / r - Rsus)

Question 13

If the sustained rate is 10Mbps/but they like to burst at 15Mbps and the bucket size is 1 MByte (8 Mbits) then how long can the sender send at rate r?

Answer 13

1Mbyte / (15Mbps - 10Mbps)

Which is…

8Mbits / 5Mbps = 1.6Mbits

Question 14

How can Powerboost effect latency?

Answer 14

• Increases latency
• It can cause buffers to fill up in the system -> just cause we can send faster doesn’t mean the receiver can process it faster
• Has high overhead

Question 15

What is Passive Measurement?

Answer 15

A collection of packets, flow stats and other metrics already sent in the network

Question 16

What is Active Measurement?

Answer 16

Injecting additional traffic into the network to measure various characteristics (ping, traceroute, etc)

Question 17

What are the 3 main types of Passive Measurement?

Answer 17

• Simple Network Management Protocol (SNMP)
• Packet Monitoring
• Flow Monitoring

Question 18

What is the Simple Network Management Protocol (SNMP)?

Answer 18

• devices have a Management Information Base (MIB) that stores the info related to that device that can be queried
• you can query the MIB for rate measurements
• COARSE

Question 19

What is Packet Monitoring?

Answer 19

• Wireshark, tcpdump, ethereal
• sees packet contents (or at least headers)
• provides timing info (at a packet level), Packet Headers & the # of Bytes in each flow

Question 20

What is Flow Monitoring?

Answer 20

• Monitors in the network that record stats on a per flow basis
• contains info like next-hop IP, src/dst AS & prefix
• less overhead than Packet Monitoring
• CAN’T PROVIDE HEADERS OR PACKET INFO, just the # of bytes per flow

Question 21

What are two reasons for measuring networks?

Answer 21

Billing and security