11. One queue per flow
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12. Drop what/when? !
Drop arriving packets from "
flow i when queue i is full
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13. Send what? "
Each flow takes turn -- send the
packet at the head of the queues in
a round robin manner.
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14. Advantages of "
FIFO and Drop Tail
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16. Scale well"
(no per-connection states)
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17. Reduce delay for a bursty
connection"
(e.g. VoIP)
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18. Problems with "
FIFO and Drop Tail
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19. Problem 1!
Bias against bursty traffic"
burstiness increases chances that the queue will
overflow
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20. Problem 2!
Global synchronization"
connections reduce their windows simultaneously,
lowering utilization.
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21. Problem 3!
Queue size"
higher bandwidth needs longer "
queue, increasing delay. "
TCP tries to keep the queue full.
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22. Problem 4!
No isolation against "
unresponsive flows
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32. Drop what/when? "
Drop arriving packet randomly
when queue is longer than a
threshold
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33. Early random drop avoids
congestion (full queue) by
dropping packets before queue is
full.
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34. RED"
Random Early Detection
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35. Drop what/when? !
Drop arriving packet randomly
when average queue length "
is above a threshold
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36. Differences: Use average queue
length instead of instantaneous
length to absorb transient
congestion.
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38. Dropping Probability
1
Average Queue Length
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39. foreach incoming packet X"
calc average queue length"
if minth < average < maxth"
calc p "
drop X with probability p"
else if average > maxth"
drop X
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40. (Instead of dropping packets, we
can also mark a packet to indicate
congestion)
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41. How to calculate average queue
length?"
How to calculate drop probability?"
How to set thresholds?
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42. We can use exponentially weighted
average. On every packet arrival:
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43. Large wq : A burst of packets will
cause avg to increase too fast, hit
the max threshold"
Small wq : avg increases too slowly
and we are unable to detect initial
stage of congestions.
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52. How to calculate average queue
length?"
How to calculate drop probability?"
How to set thresholds?
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53. maxth - minth should be sufficiently"
large otherwise average queue size
can oscillate beyond maxth"
“need more research” for optimal
value.
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67. Conclusion:"
RED increases throughput, reduces
delay, controls average queue
sizes, reduces global sync, and is
fairer to bursty traffic. It is deployed
in routers today."
But careful tuning of parameters is
needed.
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