Techniques To Achieve Good QOSBy Roshan Shrestha

Approaches To Congestion ControlO Open Loop/Traffic Shaping O Closed Loop

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1.

Open Loop/Traffic Shaping

O Leaky Bucket O Token Bucket

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2. Closed LoopO Admission Control O Load shedding

O Jitter ControlO Fair Queuing O Weighted Fair Queuing O Choke Packet O Hop by Hop Choke Packet

O Random Early Detection

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a. Load SheddingO Discard those packets which cant handle

by routers O Which packet is chosen to be the victim depends on the application and on the error strategy used in the data link layer. O Former policy (old is better than new) wine O Latter policy (new is better than old) milk5

b. Jitter ControlO Helps to minimize the jitter O Check the position of packet by router O Ahead packet get slow down and behind

packet get speed up

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c. Fair QueuingO Allocating bandwidth fairly to users O Achieving promptness (less delay) O Allocating buffer space properly

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c. Fair QueuingSOURCE

5 DESTINATIO N1 DESTINATIO N2 DESTINATIO N3

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d. Weighted Fair QueuingO Allocating bandwidth according to priority

of user O Allocating buffer space according to priority of user

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d. Weighted Fair QueuingSOURCE 3 DESTINATIO N1 DESTINATIO N2 DESTINATIO N310

e. Choke PacketO A more direct way of telling the source to

slow down. O control packet generated at a congested node and transmitted to restrict traffic flow. O The source must reduce its transmission rate by a certain percentage.11

e. Choke PacketSOURCE

DESTINATIO N

Congestion Point of failure12

f.

Hop by Hop Choke Packet

O A more efficient method is to send to

choke packets hop-by-hop. O This requires each hop to reduce its transmission even before the choke packet arrive at the source.

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f.

Hop by Hop Choke PacketDESTINATIO N

SOURCE

Congestion Point of failure

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g. RED (Random Early Detection)O Dealing with congestion after it is first

detected which is more effective O This is a proactive approach in which the router discards one or more packets before the buffer becomes completely full. O Each time a packet arrives, the RED algorithm computes the average queue length, avg. and check threshold (lower and upper)

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Techniques To Achieve Good QOS1. Overprovisioning 2. Buffering 3. Scheduling

4. Traffic Shaping5. Admission Control 6. Resource Reservation Protocol (RSVP)16

1. OverprovisioningO Use extra/additional resources O Too many ideal resources

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2. BufferingO Flows can be stored in the buffer on the

receiving end before being delivered. O Small buffers: O often drop packets due to bursts O but have small delays O Large buffers: O reduce number of packet drops (due to bursts) O but increase delays

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3. SchedulingO mechanism that selects a packet for

transmission from the packets waiting in the transmission queue O Types FIFO Queuing Priority Queuing Weighted Fair Queuing19

i. FIFO QueuingO First packet server first. O If queue is full then it discards the new

packets.N Arrival Full? Y Discard Queue20

Processor

Departure

ii. Priority QueuingO Packets are assigned to priority class O Each priority has own class. O Highest priority queue First

O Lowest priority queue LastO Starvation low priority queue will never

have chance to be processed21

ii. Priority QueuingN Full ? Higher Priority Queue Processo r N

YArrival Discard

Full ?Y Discard Lower Priority Queue

Departure22

iii. Weighted Fair QueuingO Give higher weight to higher priority O Use round robin technique to process

packet

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4. Traffic ShapingO Maintain uniform rate at which packets are

sent. O Traffic policing is the mechanism that monitors the admitted sessions traffic O Two traffic shaping algorithms are: Leaky Bucket Token Bucket24

Traffic Shaping Vs. Traffic Policing

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a. Leaky BucketO Rigid output pattern at constant rate O Cant support burst traffic O Bucket remains idle when no data to send

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a. Leaky Bucket

(a) A leaky bucket with water. packets.

(b) a leaky bucket with27

a. Leaky BucketO Case 1: Incoming traffic with rate R which is less

than the bucket rate r. The outgoing traffic rate is equal to R. O Case 2: Incoming traffic with rate R which is greater than the bucket rate r. The outgoing traffic rate is equal to r (bucket rate). O Case 3: Same as (B) but the bucket is full. Nonconformant traffic is either dropped or sent as best effort traffic.

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b. Token BucketO The bucket holds tokens. O To transmit a packet, we use one token. O Support burst traffic

O Generate tokens when resource is idle

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b. Token BucketOne token added per tick token

Packet Arrival

Full?Y Discard

N Queue

Processor

Departure31

b. Token BucketO Case 1: The incoming traffic rate (R) is less than

the token arrival rate (r). In this case the outgoing traffic rate is equal to the incoming traffic rate. O Case 2: The incoming traffic rate is greater than the token arrival rate. In case there are still tokens in the bucket, the outgoing traffic rate is equal to the incoming traffic rate. O Case 3: If the incoming traffic rate is still greater than the token arrival rate (e.g., long traffic burst), eventually all the tokens will be exhausted.32

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Leaky Vs. TokenLeaky BucketO Discard: Packets O

Token BucketDiscard: Tokens Packet management separate O Rate: Average rate Bursts allowed O Arriving Burst: Can be sent immediately

O Rate: fixed rate (perfect)

O Arriving Burst: Waits in bucket

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5. Admission ControlO mechanism that makes the decision

whether to allow a new session to join the network O This mechanism will ensure that existing sessions QOS will not be degraded and the new session will be provided QOS support35

6. Resource Reservation ProtocolO Congestion control for multicasting. O Each network device will reserve the proper

resources and configure its traffic handling mechanisms in order to provide the required QOS service. O Once the reservation process is complete, the sender host is allowed to transmit data with an agreed traffic profile36