Lesson 7: Rate Limiting and Traffic Shaping

Question 1

Motivation for Rate Limiting and Traffic Shaping

Answer 1

Control network resources and ensure no traffic flow exceeds pre-specified rate

Question 2

Classifications of Sources

Answer 2

Data, Audio, Video

Question 3

What is CBR and what does it look like?

Answer 3

Constant Bit Rate: Arrives at regular intervals, packets are same size as they arrive. Example: Audio

Question 4

What is VBR and what does it look like?

Answer 4

Variable Bit Rate: Shaped according to the average rate and peak rate. Average rate is a small fraction of the peak rate. Example: Video and Data

Question 5

How does Leaky Bucket Traffic Shaping Work?

Answer 5

Each flow has its own bucket, and traffic arrives in bucket of a size beta. Data can arrive faster or slower into bucket, but will not exceed the RHO drain rate.

Question 6

In a leaky bucket, what controls the max burst size?

Answer 6

The size of the bucket controls the max burst size that can be sent at once.

Question 7

How does (r, T) traffic shaping work?

Answer 7

There are T-bit frames, and the flow can only inject less than or equal to r bits into each T-bit frame.

Question 8

What happens if the sender wants to send more than one packet of r bits in a T-bit frame in (r,T) traffic shaping?

Answer 8

They must wait until the next T-bit frame to do so.

Question 9

What happens in (r, T) traffic shaping if a flow exceeds a certain rate?

Answer 9

The excess packets for that flow are given lower priority, which can be done at the sender or the network.

Question 10

What is it called when the network marks packets with lower priorities?

Answer 10

Policing

Question 11

What is it called when the sender marks packets with priorities?

Answer 11

Marking

Question 12

What is a token bucket, and how does it work?

Answer 12

In a token bucket, tokens are placed into a bucket at rate rho, where beta is the capacity of the bucket. Traffic can be sent through regulator as long as there are token in the bucket. Otherwise, it will have to wait for token to be placed in bucket to be sent.

Question 13

What is the formula for the maximum rate that can be sent in a token bucket?

Answer 13

Beta + T*rho, where beta is the bucket size, T is the time interval, and rho is the rate at which tokens are placed in the bucket.

Question 14

In what way is a token bucket worse than a leaky bucket?

Answer 14

Token buckets are much harder to police than leaky buckets, since they have no discards or priorties

Question 15

What are the main differences between a leaky bucket and a token bucket?

Answer 15

A token bucket allows burstiness, but will bound it to rate rho. A leaky bucket will force the bursty traffic to be smoothed.

Question 16

What is a way that you can police packets in a token bucket?

Answer 16

Use a composite shaper, which is where you combine a token bucket and a leaky bucket. The Token bucket fills into a leaky bucket, which allows for good policing by confirming that the flow data rate does not exceed the avg data rate allowed by the leaky bucket.

Question 17

What are the two main types of power boost?

Answer 17

Capped power boost, where the rate at which a user can achieve during a boost window cannot exceed a particular rate. Uncapped Power Boost, where the rate at which the user can achieve is theoretically unlimited, or only limited by the bucket size.

Question 18

What is the equation to find the time(d) at which a sender can send at a rate r, which exceeds the sustained rate Rsustained?

Answer 18

d = beta/(r - Rsustained)

Question 19

What effect does powerboost have on latency?

Answer 19

Powerboost has a negative effect on latency. Although it allows a higher traffic rate, users may experience high latency and loss over the duration of the power boost at a higher sending rate.