Router-Based Congestion Control: Queuing Disciplines and Fair Queuing, Slides of Computer Networks

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2

Lecture No. 35

Router-Based

Congestion Control Solution

3

5

Router Solutions: Queuing

Disciplines

• Router defines policies on each outgoing

link

• Allocates buffer space:

Which packets are discarded?

• Allocates bandwidth:

Which packets are transmitted?

• Affects packet latency:

When are packets transmitted?

6

More Fairness Choices

• First In, First Out (FIFO)
  • Fairness for latency– Minimizes per-packet delay– Bandwidth not considered (not good for

congestion)

• Fair queuing
  • Fairness for bandwidth– Provides equal bandwidths (possibly weighted)– Delay not considered

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15

20

10

Fair Queuing Example

A

B

C

10

10

20

15

S

A

S

B

S

C

10

15

20

10

15

20

10

9

Fair Queuing Example

Compare S

i

or S

i

+ P?

15

20

10

A

B

C

10

10

20

15

S

A

S

B

S

C

20

15

10

20

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10

Another detail: update counter at start or end of transmission?

11

Fair Queuing

• How might we fix this problem?
  • Don’t allow

inactive

flows to retain

service rates below that of any active flow

i.e.

after updating some flow’s S

i

• For each flow j with no packets in its

queue

• Set S

j

to the minimum S

k

for all active

flows k

(or 0 if no flows are active)

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20

10

Fair Queuing Example

A

B

C

10

20

15

S

A

S

B

S

C

10

20

15

20

10

14

10

15

20

10

Weighted Fair Queuing Example

A (1)

B (2)

C (1)

10

10

20 15

S

A

S

B

S

C

20

10

10

10

15

20

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Weighted Fair Queuing

• What makes up a flow for fair queuing in the

Internet

• Too many resources to have separate

queues/variables for host-to-host flows

Scale down

number of flows

• Typically just based on inputs•

e.g.

share outgoing STS-12 between incoming

ISP’s