Sally Floyd and Van Jacobson
“Random Early Detection
Gateway for Congestion
Avoidance” "
TON, 1993
11 September 2009
CS5229, Semester 1 2009/10
1

Router’s "
Queue Management
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CS5229, Semester 1 2009/10
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Manages sharing of "
(i) buffer space"
(ii) bandwidth
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Q1: Which packet to drop "
(and when to drop) ?
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Q2: Which packet to send next?
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FIFO + Drop Tail
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Keep a single queue
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Drop what/when? "
Drop arriving packets "
when queue is full
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Send what? "
Send the packet at "
head of queue
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Round Robin
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One queue per flow
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Drop what/when? !
Drop arriving packets from "
flow i when queue i is full
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Send what? "
Each flow takes turn -- send the
packet at the head of the queues in
a round robin manner.
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Advantages of "
FIFO and Drop Tail
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Simple to implement
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Scale well"
(no per-connection states)
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Reduce delay for a bursty
connection"
(e.g. VoIP)
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Problems with "
FIFO and Drop Tail
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Problem 1!
Bias against bursty traffic"
burstiness increases chances that the queue will
overflow
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Problem 2!
Global synchronization"
connections reduce their windows simultaneously,
lowering utilization.
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Problem 3!
Queue size"
higher bandwidth needs longer "
queue, increasing delay. "
TCP tries to keep the queue full.
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Problem 4!
No isolation against "
unresponsive flows
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Random Drop
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Keep a single queue
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Drop what/when? "
Drop random packet in the queue "
when queue is full
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CS5229, Semester 1 2009/10
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Send what? "
Send the packet at "
head of queue
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No bias against bursty traffic -- "
bursty arrival causes random packets to
be dropped.
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Flows with higher rate occupy
more buffer spaces, have higher
chance to be dropped.
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Signal flows that is congesting the
network to slow down.
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Random drop recovers from
congestion (full queue) by
dropping packets.
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Early Random Drop
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Drop what/when? "
Drop arriving packet randomly
when queue is longer than a
threshold
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Early random drop avoids
congestion (full queue) by
dropping packets before queue is
full.
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RED"
Random Early Detection
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Drop what/when? !
Drop arriving packet randomly
when average queue length "
is above a threshold
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Differences: Use average queue
length instead of instantaneous
length to absorb transient
congestion.
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Differences: Dropping probability
changes dynamically depending
on queue length.
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Dropping Probability
1
Average Queue Length
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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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(Instead of dropping packets, we
can also mark a packet to indicate
congestion)
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How to calculate average queue
length?"
How to calculate drop probability?"
How to set thresholds?
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We can use exponentially weighted
average. On every packet arrival:
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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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We can use exponentially weighted
average. On every packet arrival:
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What if q drops to zero and no
packet arrives?
m is a function of period when
queue is empty
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How to calculate average queue
length?"
How to calculate drop probability"
How to set thresholds?
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Dropping Probability
1
pmax
Average
Queue Length
minth maxth
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How to calculate average queue
length?"
How to calculate drop probability?"
How to set thresholds?
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maxth - minth should be sufficiently"
large otherwise average queue size
can oscillate beyond maxth"
“need more research” for optimal
value.
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Advantages of RED
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No bias against bursty flows"
Less global synchronization"
Control average queue length
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RED does not deal with
unresponsive flows
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ns-2 demo
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Variations of RED
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RED biases against flow with small
packet size
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We can fix this by weighting drop
probability to packet size
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A router can keep one queue per
flow and apply RED to each one.
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Drop probability can be weighted
with the priority of the flow.
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This is known as WRED and is
implemented in some Cisco routers.
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Simulation"
Results
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Four TCP flows starting at time 0.2, 0.4, 0.6 and 0.8
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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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