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When one part of the subnet (e.g. one or morerouters in an area) becomes overloaded,congestion results.

Because routers are receiving packets fasterthan they can forward them, one of two thingsmust happen: The subnet must prevent additional packets from

entering the congested region until those alreadypresent can be processed.

The congested routers can discard queued packets tomake room for those that are arriving.

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Packet arrival rate exceeds the outgoing link capacity.

Insufficient memory to store arriving packets

Bursty traffic Slow processor

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Traffic Shaping

Anothermethod of congestion control is to shapethe traffic before it enters the network.

At connection set-up time, the sender and carrier

negotiate a traffic pattern (shape). Traffic shaping controls the rate at which packets

are sent (not just how many). Used in ATM andIntegrated Services networks.

Two traffic shaping algorithms are: Leaky Bucket

Token Bucket

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Token Bucket Token Bucket with Leaky Bucket Rate Control

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In contrast to the LB, the Token BucketAlgorithm, allows the output rate to vary,depending on the size of the burst.

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

Tokens are generated by a clock at the rate ofone token everyt sec.

Idle hosts can capture and save up tokens (up tothe max. size of the bucket) in order to sendlarger bursts later.

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A token is added to the bucket every 1/r sec.

The bucket can hold at the most b tokens.

If a token arrives when the bucket is full, it isdiscarded.

When a packet of n bytes arrives, n tokens are removedfrom the bucket, and the packet is sent to the network.

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Implementers of this algo. on platforms lacking theclock resolution necessary to add a single token to thebucket every 1/r sec. may want to consider analternative formulation.

No. of tokens to add every S ms

= (r*s)/1000

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Average Rate

Over the long run the output of conformant

packets is limited by the token rate r.

Burst Size

Let M be the max. possible transmission rate inbytes/sec.

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The max burst size is Lmax=Tmax*M

Tmax =

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LB discards packets; TB does not. TB discards tokens.

With TB, a packet can only be transmitted if there areenough tokens to cover its length in bytes.

LB sends packets at an average rate. TB allows for largebursts to be sent faster by speeding up the output.

TB allows saving up tokens (permissions) to send largebursts. LB does not allow saving.

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More accurate description of flows rate help networkto effectively manage its resources

Simplest shaping - leaky bucket - for fixed data rates token bucket (with leaky bucket) -> more diverse

traffic patterns

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