Traffic Access Control - Sharif University of Technologyce.sharif.edu/courses/90-91/2/ce873-1/resources/root/Class Notes/MM… · The components of traffic access control are directly

Traffic Access Control

Instructor: Hamid R. Rabiee

Spring 2012

Digital Media Lab - Sharif University of Technology2

Outlines

Traffic Access Control

Definition

Traffic Shaping

Traffic Policing

The Leaky Bucket

The Token Bucket

What is Traffic Access Control?

A collection of specification techniques and mechanisms to:

specify the expected traffic characteristics and service requirements (e.g. peak

rate, required delay bound, loss tolerance) of a data stream

shape data streams (e.g. reducing their rates and/or Traffic Burstiness) at the

edges and selected points within the network

police data streams and take corrective actions (e.g. discard, delay, or mark

packets) when traffic deviates from its specification

The components of traffic access control are directly related to the

mechanisms of admission control and scheduling that implement QoS

controlled services:

Most real-time queuing and scheduling mechanisms require some control of the

rate and burstiness of data moving through the system.


Traffic Access Control Functions

Flow specification function

Provides the common language by which applications and network elements

communicate service requirements

Shaping function

Policing function

Monitors traffic flows and takes corrective actions when the observed

characteristics deviate from those specified

The location of policing functions (e.g. at the network edge and at stream

merge points) are usually determined by the network providers


Traffic Shaping

Goal: limit traffic to not exceed declared parameters

Traffic shaping is usually done in the egress line card to shape and

smooth the outgoing traffic

Retains excess packets in a queue and then schedules the excess for later

transmission over increments of time

The result of traffic shaping is a smoothed packet output rate


6

Traffic Shapers-Schemes

Leaky Bucket (LB)

Token Bucket

Window shapers

Jumping window

Moving window

Composite Shapers

Composite Leaky Bucket

Dual LB

Triple LB

Composite windows

6Digital Media Lab - Sharif University of Technology

Traffic Policing

Traffic policing propagates bursts

When the traffic rate reaches the configured maximum

rate, excess traffic is dropped (or remarked)

The result is an output rate that appears as a saw-tooth with crests

and troughs


Policing vs. Shaping


Shaping Policing

ObjectiveBuffer and queue excess packets above the committed

rates.

Drop (or remark) excess packets above the

committed rates. Does not buffer.*

Token Refresh RateIncremented at the start of a time interval. (Minimum

number of intervals is required.)

Continuous based on formula: 1 / committed

information rate

Token Values Configured in bits per second. Configured in bytes.

Configuration Options

•shape command in the modular quality of service

command-line interface (MQC) to implement class-

based shaping.

•frame-relay traffic-shape command to implement

Frame Relay Traffic Shaping (FRTS).

•traffic-shape command to implement Generic Traffic

Shaping (GTS).

•police command in the MQC to implement class-

based policing.

•rate-limit command to implement committed access

rate (CAR).

Applicable on Inbound No Yes

Applicable on Outbound Yes Yes

Bursts

Controls bursts by smoothing the output rate over at

least eight time intervals. Uses a leaky bucket to delay

traffic, which achieves a smoothing effect.

Propagates bursts. Does no smoothing.

Advantages

Less likely to drop excess packets since excess packets

are buffered. (Buffers packets up to the length of the

queue. Drops may occur if excess traffic is sustained at

high rates.) Typically avoids retransmissions due to

dropped packets.

Controls the output rate through packet drops.

Avoids delays due to queuing.

DisadvantagesCan introduce delay due to queuing, particularly deep

queues.

Drops excess packets (when configured), throttling

TCP window sizes and reducing the overall output

rate of affected traffic streams. Overly aggressive

burst sizes may lead to excess packet drops and

throttle the overall output rate, particularly with

TCP-based flows.

Optional Packet Remarking No Yes (with legacy CAR feature).


Two Main Shaping Methods

Leaky Bucket Algorithm

Regulate output flow

Packets lost if buffer is full

Token Bucket Algorithm

Buffer filled with tokens

transmit ONLY if tokens available


The Leaky Bucket

Main Idea:

Keep a single server queuing system with constant service time

Allow one packet per clock tick onto the network

Old packets are discarded

Selected packets are discarded when bucket is full


Leaky Bucket

To understand the leaky bucket model, consider a bucket with a small hole

at the bottom. Three parameters define the bucket:

The capacity (B)

The rate at which water flows out of the bucket (R)

The initial fullness of the bucket (F)


Leaky Bucket

If water is poured into the bucket at exactly rate R, the bucket will remain at

F, because the input rate equals the output rate.

If the input rate increases while R remains constant, the bucket accumulates

water.

If the input rate is larger than R for a sustained period, eventually the bucket

overflows.

However, the input rate can vary around R without overflowing the bucket, as

long as the average input rate does not exceed the capacity of the bucket.

The larger the capacity, the more the input rate can vary within a given

window of time.


The Leaky Bucket Algorithm

Queue full - packet discarded.

What if packets are different size and fixed bytes/ unit time.


Leaky Bucket Example

A source generates data in terms of bursts: 3 MB bursts lasting 2 msec once every 100 msec.

The network offers a bandwidth of 60 MB/sec.

The leaky bucket has a capacity of 4 MB. How does the output look like?

Input: 0-2 msec: 1500 MB/sec; 100-102 msec: 1500 MB/sec; 200-202 msec: 1500 MB/sec; …

Output: 0-50 msec: 60 MB/sec; 100-150 msec: 60 MB/sec; ….


Leaky Bucket Example

What should be the capacity of the leaky bucket to avoid loss?

During the burst, data inflow is at the rate of 1.5 MB/msec and the outflow is at the rate of 0.06 MB/msec.

So accumulation is at the rate of 1.44 MB/msec. So at the end of 2 msec, there will be an accumulation of 2.88 MB. This is the minimum leaky bucket capacity to avoid buffer overflow and hence data loss.


Leaky Bucket Issues

After 500 ms, the bucket is discharged!

Drops packets

Does not allow host to save permission to transmit large burst later


The Token Bucket

In contrast to the LB, the Token Bucket (TB) algorithm, allows the output rate to vary,

depending on the size of the burst.

Packet gets tokens and only then transmitted

In the TB algorithm, the bucket holds tokens. To transmit a packet, the host must capture and

destroy one token

A variant – packets sent only if enough token available - token - fixed byte size

Token bucket holds up n tokens

Host captures tokens

Each token can hold some bytes

Token generated every T seconds

Allows bursts of packets to be sent - max n

Responds fast to sudden bursts

If bucket full – thrown token packets not lost


Token Bucket Algorithm


Token Bucket Example

Bucket capacity: 1 MB

Token arrival rate: 2 MB/sec

Network capacity: 10 MB/sec

Application produces 0.5 MB burst every 250 msec For 3 seconds

The bucket is full of tokens



Initially, output can be at the rate of 10 MB/s. But how long can the bucket sustain this?

First, 1MB can be sent

From then on, for X seconds, the token input rate is 2MB/s, the output traffic rate is 10MB/s

1 + 2X = 10X 8X = 1 X = 1/8 sec =125 ms

The bucket can transmit 1.25 MB in this time > 0.5MB the application produces

Output: 0-50 ms: 10 MB/s

50-250 ms: None



At the end of this period, the amount of tokens in the

bucket is:

1MB+250ms*2MB/s-0.5MB=1MB

So the bucket is full again!

Repeat for 3 seconds

How will the traffic look with Bucket Size = 200K? 0.2+2X=10X X=0.2/8=0.025s=25ms

0-25ms : 10 MB/s = 0.25MB. 0.25MB left

0.25MB/(2MB/s) = 125ms

25-150ms: 2MB/s

150-250ms: None


Dual Buckets

Mechanism

Consists of a two token buckets

Allows for policing on average rate, peak rate, and burst size

Parameters of buckets are set based on flow requirements


Summary

Leaky and Token bucket, both are designed for controlling average rate.

Token bucket used by IETF

Leaky bucket used by ATM

On bursty arrivals after a long idle:

Token bucket results in bursty departure

Leaky bucket results in smooth departure


Documents

Traffic Access Control - Sharif University of Technologyce.sharif.edu/courses/90-91/2/ce873-1/resources/root/Class Notes/MM… · The components of traffic access control are directly